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Light-emitting diode

Blue, green, and red LEDs in 5 mm diffused cases. There are many different variants of LEDs.
Working principle‍	Electroluminescence
Inventor	H. J. Round (1907)[1] Oleg Losev (1927)[2] James R. Biard (1961)[3] Nick Holonyak (1962)[4]
First production	October 1962; 62 years ago (1962-10)
Pin names	Anode and cathode
Electronic symbol

A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (corresponding to the energy of the photons) is determined by the energy required for electrons to cross the band gap of the semiconductor.^[5] White light is obtained by using multiple semiconductors or a layer of light-emitting phosphor on the semiconductor device.^[6]

Appearing as practical electronic components in 1962, the earliest LEDs emitted low-intensity infrared (IR) light.^[7] Infrared LEDs are used in remote-control circuits, such as those used with a wide variety of consumer electronics. The first visible-light LEDs were of low intensity and limited to red.

Early LEDs were often used as indicator lamps, replacing small incandescent bulbs, and in seven-segment displays. Later developments produced LEDs available in visible, ultraviolet (UV), and infrared wavelengths with high, low, or intermediate light output, for instance, white LEDs suitable for room and outdoor lighting. LEDs have also given rise to new types of displays and sensors, while their high switching rates are useful in advanced communications technology with applications as diverse as aviation lighting, fairy lights, strip lights, automotive headlamps, advertising, general lighting, traffic signals, camera flashes, lighted wallpaper, horticultural grow lights, and medical devices.^[8]

LEDs have many advantages over incandescent light sources, including lower power consumption, a longer lifetime, improved physical robustness, smaller sizes, and faster switching. In exchange for these generally favorable attributes, disadvantages of LEDs include electrical limitations to low voltage and generally to DC (not AC) power, the inability to provide steady illumination from a pulsing DC or an AC electrical supply source, and a lesser maximum operating temperature and storage temperature.

LEDs are transducers of electricity into light. They operate in reverse of photodiodes, which convert light into electricity.

The first LED was created by Soviet inventor Oleg Losev^[9] in 1927, but electroluminescence was already known for 20 years, and relied on a diode made of silicon carbide.

Commercially viable LEDs only became available after Texas Instruments engineers patented efficient near-infrared emission from a diode based on GaAs in 1962.

From 1968, commercial LEDs were extremely costly and saw no practical use. Monsanto and Hewlett-Packard led the development of LEDs to the point where, in the 1970s, a unit cost less than five cents.^[10]

In a light-emitting diode, the recombination of electrons and electron holes in a semiconductor produces light (be it infrared, visible or UV), a process called "electroluminescence". The wavelength of the light depends on the energy band gap of the semiconductors used. Since these materials have a high index of refraction, design features of the devices such as special optical coatings and die shape are required to efficiently emit light.^[11]

Unlike a laser, the light emitted from an LED is neither spectrally coherent nor even highly monochromatic. Its spectrum is sufficiently narrow that it appears to the human eye as a pure (saturated) color.^[12][13] Also unlike most lasers, its radiation is not spatially coherent, so it cannot approach the very high intensity characteristic of lasers.

External videos
"The Original Blue LED", Science History Institute

By selection of different semiconductor materials, single-color LEDs can be made that emit light in a narrow band of wavelengths from near-infrared through the visible spectrum and into the ultraviolet range. The required operating voltages of LEDs increase as the emitted wavelengths become shorter (higher energy, red to blue), because of their increasing semiconductor band gap.

Blue LEDs have an active region consisting of one or more InGaN quantum wells sandwiched between thicker layers of GaN, called cladding layers. By varying the relative In/Ga fraction in the InGaN quantum wells, the light emission can in theory be varied from violet to amber.

Aluminium gallium nitride (AlGaN) of varying Al/Ga fraction can be used to manufacture the cladding and quantum well layers for ultraviolet LEDs, but these devices have not yet reached the level of efficiency and technological maturity of InGaN/GaN blue/green devices. If unalloyed GaN is used in this case to form the active quantum well layers, the device emits near-ultraviolet light with a peak wavelength centred around 365 nm. Green LEDs manufactured from the InGaN/GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems, but practical devices still exhibit efficiency too low for high-brightness applications.^{[citation needed]}

With AlGaN and AlGaInN, even shorter wavelengths are achievable. Near-UV emitters at wavelengths around 360–395 nm are already cheap and often encountered, for example, as black light lamp replacements for inspection of anti-counterfeiting UV watermarks in documents and bank notes, and for UV curing. Substantially more expensive, shorter-wavelength diodes are commercially available for wavelengths down to 240 nm.^[14] As the photosensitivity of microorganisms approximately matches the absorption spectrum of DNA, with a peak at about 260 nm, UV LED emitting at 250–270 nm are expected in prospective disinfection and sterilization devices. Recent research has shown that commercially available UVA LEDs (365 nm) are already effective disinfection and sterilization devices.^[15] UV-C wavelengths were obtained in laboratories using aluminium nitride (210 nm),^[16] boron nitride (215 nm)^[17][18] and diamond (235 nm).^[19]

There are two primary ways of producing white light-emitting diodes. One is to use individual LEDs that emit three primary colors—red, green and blue—and then mix all the colors to form white light. The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, similar to a fluorescent lamp. The yellow phosphor is cerium-doped YAG crystals suspended in the package or coated on the LED. This YAG phosphor causes white LEDs to appear yellow when off, and the space between the crystals allow some blue light to pass through in LEDs with partial phosphor conversion. Alternatively, white LEDs may use other phosphors like manganese(IV)-doped potassium fluorosilicate (PFS) or other engineered phosphors. PFS assists in red light generation, and is used in conjunction with conventional Ce:YAG phosphor.

In LEDs with PFS phosphor, some blue light passes through the phosphors, the Ce:YAG phosphor converts blue light to green and red (yellow) light, and the PFS phosphor converts blue light to red light. The color, emission spectrum or color temperature of white phosphor converted and other phosphor converted LEDs can be controlled by changing the concentration of several phosphors that form a phosphor blend used in an LED package.^{[20][21][22][23]}

The 'whiteness' of the light produced is engineered to suit the human eye. Because of metamerism, it is possible to have quite different spectra that appear white. The appearance of objects illuminated by that light may vary as the spectrum varies. This is the issue of color rendition, quite separate from color temperature. An orange or cyan object could appear with the wrong color and much darker as the LED or phosphor does not emit the wavelength it reflects. The best color rendition LEDs use a mix of phosphors, resulting in less efficiency and better color rendering.^{[citation needed]}

The first white light-emitting diodes (LEDs) were offered for sale in the autumn of 1996.^[24] Nichia made some of the first white LEDs which were based on blue LEDs with Ce:YAG phosphor.^[25] Ce:YAG is often grown using the Czochralski method.^[26]

Mixing red, green, and blue sources to produce white light needs electronic circuits to control the blending of the colors. Since LEDs have slightly different emission patterns, the color balance may change depending on the angle of view, even if the RGB sources are in a single package, so RGB diodes are seldom used to produce white lighting. Nonetheless, this method has many applications because of the flexibility of mixing different colors,^[27] and in principle, this mechanism also has higher quantum efficiency in producing white light.^[28]

There are several types of multicolor white LEDs: di-, tri-, and tetrachromatic white LEDs. Several key factors that play among these different methods include color stability, color rendering capability, and luminous efficacy. Often, higher efficiency means lower color rendering, presenting a trade-off between the luminous efficacy and color rendering. For example, the dichromatic white LEDs have the best luminous efficacy (120 lm/W), but the lowest color rendering capability. Although tetrachromatic white LEDs have excellent color rendering capability, they often have poor luminous efficacy. Trichromatic white LEDs are in between, having both good luminous efficacy (>70 lm/W) and fair color rendering capability.^[29]

One of the challenges is the development of more efficient green LEDs. The theoretical maximum for green LEDs is 683 lumens per watt but as of 2010 few green LEDs exceed even 100 lumens per watt. The blue and red LEDs approach their theoretical limits.^{[citation needed]}

Multicolor LEDs offer a means to form light of different colors. Most perceivable colors can be formed by mixing different amounts of three primary colors. This allows precise dynamic color control. Their emission power decays exponentially with rising temperature,^[30] resulting in a substantial change in color stability. Such problems inhibit industrial use. Multicolor LEDs without phosphors cannot provide good color rendering because each LED is a narrowband source. LEDs without phosphor, while a poorer solution for general lighting, are the best solution for displays, either backlight of LCD, or direct LED based pixels.

Dimming a multicolor LED source to match the characteristics of incandescent lamps is difficult because manufacturing variations, age, and temperature change the actual color value output. To emulate the appearance of dimming incandescent lamps may require a feedback system with color sensor to actively monitor and control the color.^[31]

This method involves coating LEDs of one color (mostly blue LEDs made of InGaN) with phosphors of different colors to form white light; the resultant LEDs are called phosphor-based or phosphor-converted white LEDs (pcLEDs).^[32] A fraction of the blue light undergoes the Stokes shift, which transforms it from shorter wavelengths to longer. Depending on the original LED's color, various color phosphors are used. Using several phosphor layers of distinct colors broadens the emitted spectrum, effectively raising the color rendering index (CRI).^[33]

Phosphor-based LEDs have efficiency losses due to heat loss from the Stokes shift and also other phosphor-related issues. Their luminous efficacies compared to normal LEDs depend on the spectral distribution of the resultant light output and the original wavelength of the LED itself. For example, the luminous efficacy of a typical YAG yellow phosphor based white LED ranges from 3 to 5 times the luminous efficacy of the original blue LED because of the human eye's greater sensitivity to yellow than to blue (as modeled in the luminosity function).

Due to the simplicity of manufacturing, the phosphor method is still the most popular method for making high-intensity white LEDs. The design and production of a light source or light fixture using a monochrome emitter with phosphor conversion is simpler and cheaper than a complex RGB system, and the majority of high-intensity white LEDs presently on the market are manufactured using phosphor light conversion.^{[citation needed]}

Among the challenges being faced to improve the efficiency of LED-based white light sources is the development of more efficient phosphors. As of 2010, the most efficient yellow phosphor is still the YAG phosphor, with less than 10% Stokes shift loss. Losses attributable to internal optical losses due to re-absorption in the LED chip and in the LED packaging itself account typically for another 10% to 30% of efficiency loss. Currently, in the area of phosphor LED development, much effort is being spent on optimizing these devices to higher light output and higher operation temperatures. For instance, the efficiency can be raised by adapting better package design or by using a more suitable type of phosphor. Conformal coating process is frequently used to address the issue of varying phosphor thickness.^{[citation needed]}

Some phosphor-based white LEDs encapsulate InGaN blue LEDs inside phosphor-coated epoxy. Alternatively, the LED might be paired with a remote phosphor, a preformed polycarbonate piece coated with the phosphor material. Remote phosphors provide more diffuse light, which is desirable for many applications. Remote phosphor designs are also more tolerant of variations in the LED emissions spectrum. A common yellow phosphor material is cerium-doped yttrium aluminium garnet (Ce³⁺:YAG).^{[citation needed]}

White LEDs can also be made by coating near-ultraviolet (NUV) LEDs with a mixture of high-efficiency europium-based phosphors that emit red and blue, plus copper and aluminium-doped zinc sulfide (ZnS:Cu, Al) that emits green. This is a method analogous to the way fluorescent lamps work. This method is less efficient than blue LEDs with YAG:Ce phosphor, as the Stokes shift is larger, so more energy is converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both methods offer comparable brightness. A concern is that UV light may leak from a malfunctioning light source and cause harm to human eyes or skin.^{[citation needed]}

A new style of wafers composed of gallium-nitride-on-silicon (GaN-on-Si) is being used to produce white LEDs using 200-mm silicon wafers. This avoids the typical costly sapphire substrate in relatively small 100- or 150-mm wafer sizes.^[34] The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted. It was predicted that since 2020, 40% of all GaN LEDs are made with GaN-on-Si. Manufacturing large sapphire material is difficult, while large silicon material is cheaper and more abundant. LED companies shifting from using sapphire to silicon should be a minimal investment.^[35]

There are RGBW LEDs that combine RGB units with a phosphor white LED on the market. Doing so retains the extremely tunable color of RGB LED, but allows color rendering and efficiency to be optimized when a color close to white is selected.^[36]

Some phosphor white LED units are "tunable white", blending two extremes of color temperatures (commonly 2700K and 6500K) to produce intermediate values. This feature allows users to change the lighting to suit the current use of a multifunction room.^[37] As illustrated by a straight line on the chromaticity diagram, simple two-white blends will have a pink bias, becoming most severe in the middle. A small amount of green light, provided by another LED, could correct the problem.^[38] Some products are RGBWW, i.e. RGBW with tunable white.^[39]

A final class of white LED with mixed light is dim-to-warm. These are ordinary 2700K white LED bulbs with a small red LED that turns on when the bulb is dimmed. Doing so makes the color warmer, emulating an incandescent light bulb.^[39]

Another method used to produce experimental white light LEDs used no phosphors at all and was based on homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate that simultaneously emitted blue light from its active region and yellow light from the substrate.^[40]

In an organic light-emitting diode (OLED), the electroluminescent material composing the emissive layer of the diode is an organic compound. The organic material is electrically conductive due to the delocalization of pi electrons caused by conjugation over all or part of the molecule, and the material therefore functions as an organic semiconductor.^[41] The organic materials can be small organic molecules in a crystalline phase, or polymers.^[42]

The potential advantages of OLEDs include thin, low-cost displays with a low driving voltage, wide viewing angle, and high contrast and color gamut.^[43] Polymer LEDs have the added benefit of printable and flexible displays.^[44][45][46] OLEDs have been used to make visual displays for portable electronic devices such as cellphones, digital cameras, lighting and televisions.^[42][43]

LEDs are made in different packages for different applications. A single or a few LED junctions may be packed in one miniature device for use as an indicator or pilot lamp. An LED array may include controlling circuits within the same package, which may range from a simple resistor, blinking or color changing control, or an addressable controller for RGB devices. Higher-powered white-emitting devices will be mounted on heat sinks and will be used for illumination. Alphanumeric displays in dot matrix or bar formats are widely available. Special packages permit connection of LEDs to optical fibers for high-speed data communication links.

These are mostly single-die LEDs used as indicators, and they come in various sizes from 1.8 mm to 10 mm, through-hole and surface mount packages.^[47] Typical current ratings range from around 1 mA to above 20 mA. LED's can be soldered to a flexible PCB strip to form LED tape popularly used for decoration.

Common package shapes include round, with a domed or flat top, rectangular with a flat top (as used in bar-graph displays), and triangular or square with a flat top. The encapsulation may also be clear or tinted to improve contrast and viewing angle. Infrared devices may have a black tint to block visible light while passing infrared radiation, such as the Osram SFH 4546.^[48]

5 V and 12 V LEDs are ordinary miniature LEDs that have a series resistor for direct connection to a 5 V or 12 V supply.^[49]

High-power LEDs (HP-LEDs) or high-output LEDs (HO-LEDs) can be driven at currents from hundreds of mA to more than an ampere, compared with the tens of mA for other LEDs. Some can emit over a thousand lumens.^[50][51] LED power densities up to 300 W/cm² have been achieved. Since overheating is destructive, the HP-LEDs must be mounted on a heat sink to allow for heat dissipation. If the heat from an HP-LED is not removed, the device fails in seconds. One HP-LED can often replace an incandescent bulb in a flashlight, or be set in an array to form a powerful LED lamp.

Some HP-LEDs in this category are the Nichia 19 series, Lumileds Rebel Led, Osram Opto Semiconductors Golden Dragon, and Cree X-lamp. As of September 2009, some HP-LEDs manufactured by Cree exceed 105 lm/W.^[52]

Examples for Haitz's law—which predicts an exponential rise in light output and efficacy of LEDs over time—are the CREE XP-G series LED, which achieved 105 lm/W in 2009^[52] and the Nichia 19 series with a typical efficacy of 140 lm/W, released in 2010.^[53]

LEDs developed by Seoul Semiconductor can operate on AC power without a DC converter. For each half-cycle, part of the LED emits light and part is dark, and this is reversed during the next half-cycle. The efficiency of this type of HP-LED is typically 40 lm/W.^[54] A large number of LED elements in series may be able to operate directly from line voltage. In 2009, Seoul Semiconductor released a high DC voltage LED, named 'Acrich MJT', capable of being driven from AC power with a simple controlling circuit. The low-power dissipation of these LEDs affords them more flexibility than the original AC LED design.^[55]

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Flashing

Flashing LEDs are used as attention seeking indicators without requiring external electronics. Flashing LEDs resemble standard LEDs but they contain an integrated voltage regulator and a multivibrator circuit that causes the LED to flash with a typical period of one second. In diffused lens LEDs, this circuit is visible as a small black dot. Most flashing LEDs emit light of one color, but more sophisticated devices can flash between multiple colors and even fade through a color sequence using RGB color mixing. Flashing SMD LEDs in the 0805 and other size formats have been available since early 2019.

Flickering

Integrated electronics Simple electronic circuits integrated into the LED package have been around since at least 2011 which produce a random LED intensity pattern reminiscent of a flickering candle.^[56] Reverse engineering in 2024 has suggested that some flickering LEDs with automatic sleep and wake modes might be using an integrated 8-bit microcontroller for such functionally.^[57]

Bi-color

Bi-color LEDs contain two different LED emitters in one case. There are two types of these. One type consists of two dies connected to the same two leads antiparallel to each other. Current flow in one direction emits one color, and current in the opposite direction emits the other color. The other type consists of two dies with separate leads for both dies and another lead for common anode or cathode so that they can be controlled independently. The most common bi-color combination is red/traditional green. Others include amber/traditional green, red/pure green, red/blue, and blue/pure green.

RGB tri-color

Tri-color LEDs contain three different LED emitters in one case. Each emitter is connected to a separate lead so they can be controlled independently. A four-lead arrangement is typical with one common lead (anode or cathode) and an additional lead for each color. Others have only two leads (positive and negative) and have a built-in electronic controller. RGB LEDs consist of one red, one green, and one blue LED.^[58] By independently adjusting each of the three, RGB LEDs are capable of producing a wide color gamut. Unlike dedicated-color LEDs, these do not produce pure wavelengths. Modules may not be optimized for smooth color mixing.

Decorative-multicolor

Decorative-multicolor LEDs incorporate several emitters of different colors supplied by only two lead-out wires. Colors are switched internally by varying the supply voltage.

Alphanumeric

Alphanumeric LEDs are available in seven-segment, starburst, and dot-matrix format. Seven-segment displays handle all numbers and a limited set of letters. Starburst displays can display all letters. Dot-matrix displays typically use 5×7 pixels per character. Seven-segment LED displays were in widespread use in the 1970s and 1980s, but rising use of liquid crystal displays, with their lower power needs and greater display flexibility, has reduced the popularity of numeric and alphanumeric LED displays.

Digital RGB

Digital RGB addressable LEDs contain their own "smart" control electronics. In addition to power and ground, these provide connections for data-in, data-out, clock and sometimes a strobe signal. These are connected in a daisy chain, which allows individual LEDs in a long LED strip light to be easily controlled by a microcontroller. Data sent to the first LED of the chain can control the brightness and color of each LED independently of the others. They are used where a combination of maximum control and minimum visible electronics are needed such as strings for Christmas and LED matrices. Some even have refresh rates in the kHz range, allowing for basic video applications. These devices are known by their part number (WS2812 being common) or a brand name such as NeoPixel.

Filament

An LED filament consists of multiple LED chips connected in series on a common longitudinal substrate that forms a thin rod reminiscent of a traditional incandescent filament.^[59] These are being used as a low-cost decorative alternative for traditional light bulbs that are being phased out in many countries. The filaments use a rather high voltage, allowing them to work efficiently with mains voltages. Often a simple rectifier and capacitive current limiting are employed to create a low-cost replacement for a traditional light bulb without the complexity of the low voltage, high current converter that single die LEDs need.^[60] Usually, they are packaged in bulb similar to the lamps they were designed to replace, and filled with inert gas at slightly lower than ambient pressure to remove heat efficiently and prevent corrosion.

Chip-on-board arrays

Surface-mounted LEDs are frequently produced in chip on board (COB) arrays, allowing better heat dissipation than with a single LED of comparable luminous output.^[61] The LEDs can be arranged around a cylinder, and are called "corn cob lights" because of the rows of yellow LEDs.^[62]

• Efficiency: LEDs emit more lumens per watt than incandescent light bulbs.^[63] The efficiency of LED lighting fixtures is not affected by shape and size, unlike fluorescent light bulbs or tubes.
• Size: LEDs can be very small (smaller than 2 mm^2[64]) and are easily attached to printed circuit boards.

The current in an LED or other diodes rises exponentially with the applied voltage (see Shockley diode equation), so a small change in voltage can cause a large change in current. Current through the LED must be regulated by an external circuit such as a constant current source to prevent damage. Since most common power supplies are (nearly) constant-voltage sources, LED fixtures must include a power converter, or at least a current-limiting resistor. In some applications, the internal resistance of small batteries is sufficient to keep current within the LED rating.^{[citation needed]}

LEDs are sensitive to voltage. They must be supplied with a voltage above their threshold voltage and a current below their rating. Current and lifetime change greatly with a small change in applied voltage. They thus require a current-regulated supply (usually just a series resistor for indicator LEDs).^[65]

Efficiency droop: The efficiency of LEDs decreases as the electric current increases. Heating also increases with higher currents, which compromises LED lifetime. These effects put practical limits on the current through an LED in high power applications.^[66]

Unlike a traditional incandescent lamp, an LED will light only when voltage is applied in the forward direction of the diode. No current flows and no light is emitted if voltage is applied in the reverse direction. If the reverse voltage exceeds the breakdown voltage, which is typically about five volts, a large current flows and the LED will be damaged. If the reverse current is sufficiently limited to avoid damage, the reverse-conducting LED is a useful noise diode.^{[citation needed]}

By definition, the energy band gap of any diode is higher when reverse-biased than when forward-biased. Because the band gap energy determines the wavelength of the light emitted, the color cannot be the same when reverse-biased. The reverse breakdown voltage is sufficiently high that the emitted wavelength cannot be similar enough to still be visible. Though dual-LED packages exist that contain a different color LED in each direction, it is not expected that any single LED element can emit visible light when reverse-biased.^{[citation needed]}

It is not known if any zener diode could exist that emits light only in reverse-bias mode. Uniquely, this type of LED would conduct when connected backwards.

• Color: LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs.
• Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
• Color rendition: Most cool-white LEDs have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can make the color of objects appear differently under cool-white LED illumination than sunlight or incandescent sources, due to metamerism,^[67] red surfaces being rendered particularly poorly by typical phosphor-based cool-white LEDs. The same is true with green surfaces. The quality of color rendition of an LED is measured by the Color Rendering Index (CRI).
• Dimming: LEDs can be dimmed either by pulse-width modulation or lowering the forward current.^[68] This pulse-width modulation is why LED lights, particularly headlights on cars, when viewed on camera or by some people, seem to flash or flicker. This is a type of stroboscopic effect.

• Switch on time: LEDs light up extremely quickly. A typical red indicator LED achieves full brightness in under a microsecond.^[69] LEDs used in communications devices can have even faster response times.
• Focus: The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner. For larger LED packages total internal reflection (TIR) lenses are often used to the same effect. When large quantities of light are needed, many light sources such as LED chips are usually deployed, which are difficult to focus or collimate on the same target.
• Area light source: Single LEDs do not approximate a point source of light giving a spherical light distribution, but rather a lambertian distribution. So, LEDs are difficult to apply to uses needing a spherical light field. Different fields of light can be manipulated by the application of different optics or "lenses". LEDs cannot provide divergence below a few degrees.^[70]

• Shock resistance: LEDs, being solid-state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile.^[71]
• Thermal runaway: Parallel strings of LEDs will not share current evenly due to the manufacturing tolerances in their forward voltage. Running two or more strings from a single current source may result in LED failure as the devices warm up. If forward voltage binning is not possible, a circuit is required to ensure even distribution of current between parallel strands.^[72]
• Slow failure: LEDs mainly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.^[73]
• Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be shorter or longer.^[74] Fluorescent tubes typically are rated at about 10,000 to 25,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000 to 2,000 hours. Several DOE demonstrations have shown that reduced maintenance costs from this extended lifetime, rather than energy savings, is the primary factor in determining the payback period for an LED product.^[75]
• Cycling: LEDs are ideal for uses subject to frequent on-off cycling, unlike incandescent and fluorescent lamps that fail faster when cycled often, or high-intensity discharge lamps (HID lamps) that require a long time to warm up to full output and to cool down before they can be lighted again if they are being restarted.
• Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment – or thermal management properties. Overdriving an LED in high ambient temperatures may result in overheating the LED package, eventually leading to device failure. An adequate heat sink is needed to maintain long life. This is especially important in automotive, medical, and military uses where devices must operate over a wide range of temperatures, and require low failure rates.

LED manufacturing involves multiple steps, including epitaxy, chip processing, chip separation, and packaging.^[76]

In a typical LED manufacturing process, encapsulation is performed after probing, dicing, die transfer from wafer to package, and wire bonding or flip chip mounting,^[77] perhaps using indium tin oxide, a transparent electrical conductor. In this case, the bond wire(s) are attached to the ITO film that has been deposited in the LEDs.

Flip chip circuit on board (COB) is a technique that can be used to manufacture LEDs.^[78]

Conventional LEDs are made from a variety of inorganic semiconductor materials. The following table shows the available colors with wavelength range, voltage drop and material:

	Color	Wavelength (nm)	Voltage (V)	Semiconductor material
	Infrared	λ > 760	ΔV < 1.9	Gallium arsenide (GaAs) Aluminium gallium arsenide (AlGaAs)
	Red	610 < λ < 760	1.63 < ΔV < 2.03	Aluminium gallium arsenide (AlGaAs) Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)
	Orange	590 < λ < 610	2.03 < ΔV < 2.10	Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)
	Yellow	570 < λ < 590	2.10 < ΔV < 2.18	Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)
	Green	500 < λ < 570	1.9[79] < ΔV < 4.0	Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN) Gallium(III) phosphide (GaP) Aluminium gallium indium phosphide (AlGaInP) Aluminium gallium phosphide (AlGaP)
	Blue	450 < λ < 500	2.48 < ΔV < 3.7	Zinc selenide (ZnSe) Indium gallium nitride (InGaN) Silicon carbide (SiC) as substrate Silicon (Si) as substrate — (under development)
	Violet	400 < λ < 450	2.76 < ΔV < 4.0	Indium gallium nitride (InGaN)
	Purple	multiple types	2.48 < ΔV < 3.7	Dual blue/red LEDs, blue with red phosphor, or white with purple plastic
	Ultraviolet	λ < 400	3.1 < ΔV < 4.4	Diamond (235 nm)[80] Boron nitride (215 nm)[81][82] Aluminium nitride (AlN) (210 nm)[16] Aluminium gallium nitride (AlGaN) Aluminium gallium indium nitride (AlGaInN) — (down to 210 nm)[83]
	White	Broad spectrum	2.7 < ΔV < 3.5	Blue diode with yellow phosphor or violet/UV diode with multi-color phosphor

LED uses fall into five major categories:

• Visual signals where light goes more or less directly from the source to the human eye, to convey a message or meaning
• Illumination where light is reflected from objects to give visual response of these objects
• Measuring and interacting with processes involving no human vision^[84]
• Narrow band light sensors where LEDs operate in a reverse-bias mode and respond to incident light, instead of emitting light^{[85][86][87][88]}
• Indoor cultivation, including cannabis.^[89]

The application of LEDs in horticulture has revolutionized plant cultivation by providing energy-efficient, customizable lighting solutions that optimize plant growth and development.^[90] LEDs offer precise control over light spectra, intensity, and photoperiods, enabling growers to tailor lighting conditions to the specific needs of different plant species and growth stages. This technology enhances photosynthesis, improves crop yields, and reduces energy costs compared to traditional lighting systems. Additionally, LEDs generate less heat, allowing closer placement to plants without risking thermal damage, and contribute to sustainable farming practices by lowering carbon footprints and extending growing seasons in controlled environments.^[91] Light spectrum affects growth, metabolite profile, and resistance against fungal phytopathogens of Solanum lycopersicum seedlings.^[92] LEDs can also be used in micropropagation.^[93]

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The low energy consumption, low maintenance and small size of LEDs has led to uses as status indicators and displays on a variety of equipment and installations. Large-area LED displays are used as stadium displays, dynamic decorative displays, and dynamic message signs on freeways. Thin, lightweight message displays are used at airports and railway stations, and as destination displays for trains, buses, trams, and ferries.

One-color light is well suited for traffic lights and signals, exit signs, emergency vehicle lighting, ships' navigation lights, and LED-based Christmas lights

Because of their long life, fast switching times, and visibility in broad daylight due to their high output and focus, LEDs have been used in automotive brake lights and turn signals. The use in brakes improves safety, due to a great reduction in the time needed to light fully, or faster rise time, about 0.1 second faster^{[citation needed]} than an incandescent bulb. This gives drivers behind more time to react. In a dual intensity circuit (rear markers and brakes) if the LEDs are not pulsed at a fast enough frequency, they can create a phantom array, where ghost images of the LED appear if the eyes quickly scan across the array. White LED headlamps are beginning to appear. Using LEDs has styling advantages because LEDs can form much thinner lights than incandescent lamps with parabolic reflectors.

Due to the relative cheapness of low output LEDs, they are also used in many temporary uses such as glowsticks and throwies. Artists have also used LEDs for LED art.

With the development of high-efficiency and high-power LEDs, it has become possible to use LEDs in lighting and illumination. To encourage the shift to LED lamps and other high-efficiency lighting, in 2008 the US Department of Energy created the L Prize competition. The Philips Lighting North America LED bulb won the first competition on August 3, 2011, after successfully completing 18 months of intensive field, lab, and product testing.^[94]

Efficient lighting is needed for sustainable architecture. As of 2011, some LED bulbs provide up to 150 lm/W and even inexpensive low-end models typically exceed 50 lm/W, so that a 6-watt LED could achieve the same results as a standard 40-watt incandescent bulb. The lower heat output of LEDs also reduces demand on air conditioning systems. Worldwide, LEDs are rapidly adopted to displace less effective sources such as incandescent lamps and CFLs and reduce electrical energy consumption and its associated emissions. Solar powered LEDs are used as street lights and in architectural lighting.

The mechanical robustness and long lifetime are used in automotive lighting on cars, motorcycles, and bicycle lights. LED street lights are employed on poles and in parking garages. In 2007, the Italian village of Torraca was the first place to convert its street lighting to LEDs.^[95]

Cabin lighting on recent^[when?] Airbus and Boeing jetliners uses LED lighting. LEDs are also being used in airport and heliport lighting. LED airport fixtures currently include medium-intensity runway lights, runway centerline lights, taxiway centerline and edge lights, guidance signs, and obstruction lighting.

LEDs are also used as a light source for DLP projectors, and to backlight newer LCD television (referred to as LED TV), computer monitor (including laptop) and handheld device LCDs, succeeding older CCFL-backlit LCDs although being superseded by OLED screens. RGB LEDs raise the color gamut by as much as 45%. Screens for TV and computer displays can be made thinner using LEDs for backlighting.^[96]

LEDs are small, durable and need little power, so they are used in handheld devices such as flashlights. LED strobe lights or camera flashes operate at a safe, low voltage, instead of the 250+ volts commonly found in xenon flashlamp-based lighting. This is especially useful in cameras on mobile phones, where space is at a premium and bulky voltage-raising circuitry is undesirable.

LEDs are used for infrared illumination in night vision uses including security cameras. A ring of LEDs around a video camera, aimed forward into a retroreflective background, allows chroma keying in video productions.

LEDs are used in mining operations, as cap lamps to provide light for miners. Research has been done to improve LEDs for mining, to reduce glare and to increase illumination, reducing risk of injury to the miners.^[97]

LEDs are increasingly finding uses in medical and educational applications, for example as mood enhancement.^[98] NASA has even sponsored research for the use of LEDs to promote health for astronauts.^[99]

Light can be used to transmit data and analog signals. For example, lighting white LEDs can be used in systems assisting people to navigate in closed spaces while searching necessary rooms or objects.^[100]

Assistive listening devices in many theaters and similar spaces use arrays of infrared LEDs to send sound to listeners' receivers. Light-emitting diodes (as well as semiconductor lasers) are used to send data over many types of fiber optic cable, from digital audio over TOSLINK cables to the very high bandwidth fiber links that form the Internet backbone. For some time, computers were commonly equipped with IrDA interfaces, which allowed them to send and receive data to nearby machines via infrared.

Because LEDs can cycle on and off millions of times per second, very high data bandwidth can be achieved.^[101] For that reason, visible light communication (VLC) has been proposed as an alternative to the increasingly competitive radio bandwidth.^[102] VLC operates in the visible part of the electromagnetic spectrum, so data can be transmitted without occupying the frequencies of radio communications.

Machine vision systems often require bright and homogeneous illumination, so features of interest are easier to process. LEDs are often used.

Barcode scanners are the most common example of machine vision applications, and many of those scanners use red LEDs instead of lasers. Optical computer mice use LEDs as a light source for the miniature camera within the mouse.

LEDs are useful for machine vision because they provide a compact, reliable source of light. LED lamps can be turned on and off to suit the needs of the vision system, and the shape of the beam produced can be tailored to match the system's requirements.

The discovery of radiative recombination in aluminum gallium nitride (AlGaN) alloys by U.S. Army Research Laboratory (ARL) led to the conceptualization of UV light-emitting diodes (LEDs) to be incorporated in light-induced fluorescence sensors used for biological agent detection.^{[103][104][105]} In 2004, the Edgewood Chemical Biological Center (ECBC) initiated the effort to create a biological detector named TAC-BIO. The program capitalized on semiconductor UV optical sources (SUVOS) developed by the Defense Advanced Research Projects Agency (DARPA).^[105]

UV-induced fluorescence is one of the most robust techniques used for rapid real-time detection of biological aerosols.^[105] The first UV sensors were lasers lacking in-field-use practicality. In order to address this, DARPA incorporated SUVOS technology to create a low-cost, small, lightweight, low-power device. The TAC-BIO detector's response time was one minute from when it sensed a biological agent. It was also demonstrated that the detector could be operated unattended indoors and outdoors for weeks at a time.^[105]

Aerosolized biological particles fluoresce and scatter light under a UV light beam. Observed fluorescence is dependent on the applied wavelength and the biochemical fluorophores within the biological agent. UV induced fluorescence offers a rapid, accurate, efficient and logistically practical way for biological agent detection. This is because the use of UV fluorescence is reagentless, or a process that does not require an added chemical to produce a reaction, with no consumables, or produces no chemical byproducts.^[105]

Additionally, TAC-BIO can reliably discriminate between threat and non-threat aerosols. It was claimed to be sensitive enough to detect low concentrations, but not so sensitive that it would cause false positives. The particle-counting algorithm used in the device converted raw data into information by counting the photon pulses per unit of time from the fluorescence and scattering detectors, and comparing the value to a set threshold.^[106]

The original TAC-BIO was introduced in 2010, while the second-generation TAC-BIO GEN II, was designed in 2015 to be more cost-efficient, as plastic parts were used. Its small, light-weight design allows it to be mounted to vehicles, robots, and unmanned aerial vehicles. The second-generation device could also be utilized as an environmental detector to monitor air quality in hospitals, airplanes, or even in households to detect fungus and mold.^[107][108]

The light from LEDs can be modulated very quickly so they are used extensively in optical fiber and free space optics communications. This includes remote controls, such as for television sets, where infrared LEDs are often used. Opto-isolators use an LED combined with a photodiode or phototransistor to provide a signal path with electrical isolation between two circuits. This is especially useful in medical equipment where the signals from a low-voltage sensor circuit (usually battery-powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a recording or monitoring device operating at potentially dangerous voltages. An optoisolator also lets information be transferred between circuits that do not share a common ground potential.

Many sensor systems rely on light as the signal source. LEDs are often ideal as a light source due to the requirements of the sensors. The Nintendo Wii's sensor bar uses infrared LEDs. Pulse oximeters use them for measuring oxygen saturation. Some flatbed scanners use arrays of RGB LEDs rather than the typical cold-cathode fluorescent lamp as the light source. Having independent control of three illuminated colors allows the scanner to calibrate itself for more accurate color balance, and there is no need for warm-up. Further, its sensors only need be monochromatic, since at any one time the page being scanned is only lit by one color of light.

Since LEDs can also be used as photodiodes, they can be used for both photo emission and detection. This could be used, for example, in a touchscreen that registers reflected light from a finger or stylus.^[109] Many materials and biological systems are sensitive to, or dependent on, light. Grow lights use LEDs to increase photosynthesis in plants,^[110] and bacteria and viruses can be removed from water and other substances using UV LEDs for sterilization.^[15] LEDs of certain wavelengths have also been used for light therapy treatment of neonatal jaundice and acne.^[111]

UV LEDs, with spectra range of 220 nm to 395 nm, have other applications, such as water/air purification, surface disinfection, glue curing, free-space non-line-of-sight communication, high performance liquid chromatography, UV curing dye printing, phototherapy (295nm Vitamin D, 308nm Excimer lamp or laser replacement), medical/ analytical instrumentation, and DNA absorption.^[104][112]

LEDs have also been used as a medium-quality voltage reference in electronic circuits. The forward voltage drop (about 1.7 V for a red LED or 1.2V for an infrared) can be used instead of a Zener diode in low-voltage regulators. Red LEDs have the flattest I/V curve above the knee. Nitride-based LEDs have a fairly steep I/V curve and are useless for this purpose. Although LED forward voltage is far more current-dependent than a Zener diode, Zener diodes with breakdown voltages below 3 V are not widely available.

The progressive miniaturization of low-voltage lighting technology, such as LEDs and OLEDs, suitable to incorporate into low-thickness materials has fostered experimentation in combining light sources and wall covering surfaces for interior walls in the form of LED wallpaper.

LEDs require optimized efficiency to hinge on ongoing improvements such as phosphor materials and quantum dots.^[113]

The process of down-conversion (the method by which materials convert more-energetic photons to different, less energetic colors) also needs improvement. For example, the red phosphors that are used today are thermally sensitive and need to be improved in that aspect so that they do not color shift and experience efficiency drop-off with temperature. Red phosphors could also benefit from a narrower spectral width to emit more lumens and becoming more efficient at converting photons.^[114]

In addition, work remains to be done in the realms of current efficiency droop, color shift, system reliability, light distribution, dimming, thermal management, and power supply performance.^[113]

Early suspicions were that the LED droop was caused by elevated temperatures. Scientists showed that temperature was not the root cause of efficiency droop.^[115] The mechanism causing efficiency droop was identified in 2007 as Auger recombination, which was taken with mixed reaction.^[66] A 2013 study conclusively identified Auger recombination as the cause.^[116]

A new family of LEDs are based on the semiconductors called perovskites. In 2018, less than four years after their discovery, the ability of perovskite LEDs (PLEDs) to produce light from electrons already rivaled those of the best performing OLEDs.^[117] They have a potential for cost-effectiveness as they can be processed from solution, a low-cost and low-tech method, which might allow perovskite-based devices that have large areas to be made with extremely low cost. Their efficiency is superior by eliminating non-radiative losses, in other words, elimination of recombination pathways that do not produce photons; or by solving outcoupling problem (prevalent for thin-film LEDs) or balancing charge carrier injection to increase the EQE (external quantum efficiency). The most up-to-date PLED devices have broken the performance barrier by shooting the EQE above 20%.^[118]

In 2018, Cao et al. and Lin et al. independently published two papers on developing perovskite LEDs with EQE greater than 20%, which made these two papers a mile-stone in PLED development. Their device have similar planar structure, i.e. the active layer (perovskite) is sandwiched between two electrodes. To achieve a high EQE, they not only reduced non-radiative recombination, but also utilized their own, subtly different methods to improve the EQE.^[118]

In the work of Cao et al.,^[119] researchers targeted the outcoupling problem, which is that the optical physics of thin-film LEDs causes the majority of light generated by the semiconductor to be trapped in the device.^[120] To achieve this goal, they demonstrated that solution-processed perovskites can spontaneously form submicrometre-scale crystal platelets, which can efficiently extract light from the device. These perovskites are formed via the introduction of amino acid additives into the perovskite precursor solutions. In addition, their method is able to passivate perovskite surface defects and reduce nonradiative recombination. Therefore, by improving the light outcoupling and reducing nonradiative losses, Cao and his colleagues successfully achieved PLED with EQE up to 20.7%.^[119]

Lin and his colleague used a different approach to generate high EQE. Instead of modifying the microstructure of perovskite layer, they chose to adopt a new strategy for managing the compositional distribution in the device—an approach that simultaneously provides high luminescence and balanced charge injection. In other words, they still used flat emissive layer, but tried to optimize the balance of electrons and holes injected into the perovskite, so as to make the most efficient use of the charge carriers. Moreover, in the perovskite layer, the crystals are perfectly enclosed by MABr additive (where MA is CH₃NH₃). The MABr shell passivates the nonradiative defects that would otherwise be present perovskite crystals, resulting in reduction of the nonradiative recombination. Therefore, by balancing charge injection and decreasing nonradiative losses, Lin and his colleagues developed PLED with EQE up to 20.3%.^[121]

Certain blue LEDs and cool-white LEDs can exceed safe limits of the so-called blue-light hazard as defined in eye safety specifications such as "ANSI/IESNA RP-27.1–05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems".^[122] One study showed no evidence of a risk in normal use at domestic illuminance,^[123] and that caution is only needed for particular occupational situations or for specific populations.^[124] In 2006, the International Electrotechnical Commission published IEC 62471 Photobiological safety of lamps and lamp systems, replacing the application of early laser-oriented standards for classification of LED sources.^[125]

While LEDs have the advantage over fluorescent lamps, in that they do not contain mercury, they may contain other hazardous metals such as lead and arsenic.^[126]

In 2016 the American Medical Association (AMA) issued a statement concerning the possible adverse influence of blueish street lighting on the sleep-wake cycle of city-dwellers. Critics in the industry claim exposure levels are not high enough to have a noticeable effect.^[127]

• Light pollution: Because white LEDs emit more short wavelength light than sources such as high-pressure sodium vapor lamps, the increased blue and green sensitivity of scotopic vision means that white LEDs used in outdoor lighting cause substantially more sky glow.^[55]
• Impact on wildlife: LEDs are much more attractive to insects than sodium-vapor lights, so much so that there has been speculative concern about the possibility of disruption to food webs.^[128][129] LED lighting near beaches, particularly intense blue and white colors, can disorient turtle hatchlings and make them wander inland instead.^[130] The use of "turtle-safe lighting" LEDs that emit only at narrow portions of the visible spectrum is encouraged by conservancy groups in order to reduce harm.^[131]
• Use in winter conditions: Since they do not give off much heat in comparison to incandescent lights, LED lights used for traffic control can have snow obscuring them, leading to accidents.^[132][133]

• LED tattoo
• High-CRI LED lighting
• List of light sources
• MicroLED
• Superluminescent diode
• Perovskite light-emitting diode

• David L. Heiserman (1968). Light -Emitting Diodes (PDF). Electronics World.
• Shuji Nakamura; Gerhard Fasol; Stephen J Pearton (2000). The Blue Laser Diode: The Complete Story. Springer Verlag. ISBN 978-3-540-66505-2.

• Building a do-it-yourself LED
• Color cycling LED in a single two pin package,
• Educational video on LEDs on YouTube

 

 

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The various U.S. appraisal groups and international professional appraisal organizations have started collaborating in recent years towards the development of International Valuation Standards. This will facilitate global real estate appraisal standards, a much-needed adjunct to real estate investment portfolios which cross national boundaries. Some appraisal groups are already international organizations and thus, to some extent, already incorporate some level of global standards.

The International Valuation Standards Council (IVSC) is a non-governmental organization (NGO) member of the United Nations with membership that encompasses all the major national valuation standard-setters and professional associations from 150 different countries (including the Appraisal Institute, the American Society of Appraisers, the RICS, the [Practising Valuers Association of India] and the Appraisal Institute of Canada). IVSC publishes the International Valuation Standards (IVS), now in its 12th edition.

In Germany, real estate appraisal is known as real estate valuation (Immobilienbewertung). Real estate appraisers (Immobilienbewerter or Gutachter) can qualify to become a Öffentlich bestellter und vereidigter Sachverständiger (officially appointed and sworn expert). However, this formerly very important title has lost a lot of its importance over the past years, but still is of some value in court procedures. The title is not generally required for appraisals.

Real estate appraisal in Germany is partly codified by law. The federal Baugesetzbuch (abbr. BauGB, "German statutory code on building and construction'") contains guidelines on governing authorities, defines the term market value and refers to continuative rules (chapter 3, articles 192 ff.). Each municipality (city or administrative district) must form a Gutachterausschuss (appraisal committee), consisting of a chairman and honorary members.^[30] The committee gathers information on all real estate deals (it is mandatory to send a copy of each notarial purchase contract to the Gutachterausschuss) and includes it in the Kaufpreissammlung (purchase price database). Most committees publish an official real estate market report every two years, in which besides other information on comparables the land value is determined. The committees also perform appraisals on behalf of public authorities.

The BauGB defines the Verkehrswert or Marktwert (market value, both terms with identical meaning) as follows: "The market value is determined by the price that can be realized at the date of valuation, in an arm's length transaction, with due regard to the legal situation and the effective characteristics, the nature and lay of the premises or any other subject of the valuation"^[31] (non-official translation). The intention, as in other countries, is to include all objective influences and to exclude all influences resulting from the subjective circumstances of the involved parties.

This federal law is supported by the Wertermittlungsverordnung (abbr. WertV, "regulation on the determination of value").^[32] The WertV defines the codified valuation approaches and the general valuation technique. German codified valuation approaches (other approaches such as DCF or residual approach are also permitted, but not codified) are the:

• Vergleichswertverfahren (sales comparison approach) – used where good evidence of previous sales is available and for owner-occupied assets, especially condominiums and single-family houses;
• Ertragswertverfahren (German income approach) – standard procedure for property that produces future cash flows from the letting of the property;
• Sachwertverfahren (German cost approach) – used for specialised property where none of the above approaches applies, e. g. public buildings.

WertV's general regulations are further supported by the Wertermittlungsrichtlinie (abbr. WertR, "directive on the determination of value").^[33] The WertR provides templates for calculations, tables (e.g., economic depreciation) and guidelines for the consideration of different influences. WertV and WertR are not binding for appraisals for nonofficial use, nonetheless, they should be regarded as best practice or Generally Accepted (German) Valuation Practice (GAVP).

In most regards Generally Accepted (German) Valuation Principles is consistent with international practice. The investment market weighs the income approach most heavily. However, there are some important differences:

• Land and improvements are treated separately. German GAVP assumes that the land can be used indefinitely, but the buildings have a limited lifespan; This coincides with the balancing of the assets. The value of the land is determined by the sales comparison approach in both the income and cost approaches, using the data accumulated by the Gutachterausschuss which is then added to the building value.
• In order to account for the usage of the land, the net operating income is reduced by the Liegenschaftszins (interest paid to the land-owner by the owner of the building, i.e., ground rent). The Liegenschaftszins is the product of the land value and the Liegenschaftszinssatz (interest rate for land use). The Liegenschaftszinssatz is the equivalent of the yield—with some important differences—and is also determined by the Gutachterausschuss.
• Unlike the All Risks Yield (ARY) in UK practice, the Liegenschaftszinssatz (abbr. LZ) does not include an allowance for default (not to be confused with a structural vacancy), therefore this needs to be subtracted from gross operating income. As a result, the Liegenschaftszinssatz will usually be lower than the All Risks Yield.
• Based on the assumption that the economic life of the improvements is limited, the yield and remaining economic life determine the building value from the net operating income.
• Contracts in Germany generally prescribe that the landlord bears a higher portion of maintenance and operating costs than their counterparts in the United States and the UK.

Mathematically the distinction between land and improvements in the income approach will have no impact on the overall value when the remaining economic life is more than thirty years. For this reason, it has become quite common to use the Vereinfachtes Ertragswertverfahren (simplified income approach), omitting the land value and the Liegenschaftszins. However, the separate treatment of land and buildings leads to more precise results for older buildings, especially for commercial buildings, which typically have a shorter economic life than residential buildings.

An advantage of the comparatively high degree of standardization practiced by professional appraisers is the greater ability to check an appraisal for inconsistency, accuracy and transparency.

The Federal German Organisation of Appointed and Sworn Experts (Bundesverband Deutscher Sachverständiger und Fachgutachter, abbr. BDSF)^[34] is the main professional organization encompassing the majority of licensed appraisers in Germany. In recent years, with the move towards a more global outlook in the valuation profession, the RICS has gained a foothold in Germany, somewhat at the expense of the BDSF. Another German Organisation of Appointed and Sworn Experts is the Deutsche Sachverständigen Gesellschaft, abbr. DESAG.^[35] This organization also includes a large number of licensed appraisers in Germany.

With special focus on hypothetical value, in 1996, German banks with real estate financing activities formed the HypZert GmbH, an association for the certification of real estate valuers.^[36] A HypZert qualification is regarded as mandatory by many German banks.

In Israel, the real estate appraisal profession is regulated by the Council of Land Valuers, an organ of the Ministry of Justice; the largest professional organization, encompassing the majority of appraisers/land valuers is the Association of Land Valuers. Valuers must be registered with the Council, which is a statutory body set up by law, and which oversees the training and administers the national professional exams that are a prerequisite for attaining registration. In 2005 the Council set up a Valuation Standards Committee with the purpose of developing and promulgating standards that would reflect best practice; these have tended to follow a rules-based approach.

Historically, most valuations in Israel were statutory valuations (such as valuations performed for purposes of Betterment Tax, a tax administered on any gains accruing to the property by way of changes to the local planning) as well as valuations performed for purposes of bank lending. Since Israel implemented the International Financial Reporting Standards (IFRS) in 2008, the profession has been engaged in performing valuations for purposes of financial reporting.

In the UK, real estate appraisal is known as property valuation and a real estate appraiser is a land valuer or property valuer (usually a qualified chartered surveyor who specializes in property valuation).^[15] Property valuation in the UK is regulated by the Royal Institution of Chartered Surveyors (RICS), a professional body encompassing all of the building and property-related professions. The RICS professional guidelines for valuers are published in what is commonly known as the Red Book. The 2017 version was the RICS Valuation – Global Standards (1 July 2017),^[37] superseding an edition published in 2011. RICS Valuation Standards contains mandatory rules, best practice guidance and related commentary. The 2017 version adopts and applies the International Valuation Standards (IVS) published by the International Valuation Standards Council (IVSC). Changes to the standards are approved by the RICS Valuation Professional Group Board, and the Red Book is updated accordingly on a regular basis. While based in the UK, RICS is a global organization and has become very active in the United States in recent years through its affiliation with the Counselors of Real Estate, a division of the National Association of Realtors.

Appraisal practice in the United States is regulated by state. The Appraisal Foundation (TAF) is the primary standards body; its Appraisal Standards Board (ASB) promulgates and updates best practices as codified in the Uniform Standards of Professional Appraisal Practice (USPAP), while its Appraisal Qualifications Board (AQB) promulgates minimum standards for appraiser certification and licensing.

The federal government regulates appraisers indirectly because if the Appraisal Subcommittee (ASC) of the Federal Financial Institutions Examination Council (FFIEC) finds that a particular state's appraiser regulation and certification program is inadequate, then under federal regulations all appraisers in that state would no longer be eligible to conduct appraisals for federally chartered banks.^[38] The ASC oversees the TAF. Banks make widespread use of mortgage loans and mortgage-backed securities, and would be unable to do so without appraisals.

The Financial Institutions Reform, Recovery, and Enforcement Act of 1989 (FIRREA) demanded all the states to develop systems for licensing and certifying real estate appraisers.^[39] To accomplish this, the Appraisal Subcommittee (ASC) was formed within the FFIEC, with representatives from the various Federal mortgage regulatory agencies.^[40] Thus, currently all the real estate appraisers must be state-licensed and certified. But prior to the 1990s, there were no commonly accepted standards either for appraisal quality or for appraiser licensure. In the 1980s, an ad-hoc committee representing various appraisal professional organizations in the United States and Canada met to codify the best practices into what became known as the USPAP. The U.S. Savings and Loan Crisis resulted in increased federal regulation via FIRREA, which required federal lending regulators to adopt appraisal standards. A nonprofit organization, The Appraisal Foundation (TAF), was formed by the same organizations that had developed USPAP, and the copyright for USPAP was signed over to TAF. Federal oversight of TAF is provided by the Appraisal Subcommittee, made up of representatives of various federal lending regulators. TAF carries out its work through two boards: the Appraisal Standards Board promulgates and updates USPAP; the Appraisal Qualifications Board (AQB) promulgates minimum recommended standards for appraiser certification and licensure. During the 1990s, all of the states adopted USPAP as the governing standards within their states and developed licensure standards which met or exceeded the recommendations of TAF. Also, the various state and federal courts have adopted USPAP for real estate litigation and all of the federally lending regulators adopt USPAP for mortgage finance appraisal.^[40]

In addition, there are professional appraisal organizations, organized as private non-profit organizations that date to the Great Depression of the 1930s. One of the oldest in the United States is the American Society of Farm Managers and Rural Appraisers (ASFMRA), which was founded in 1929.^[41] Others were founded as needed and the opportunity arose in specialized fields, such as the Appraisal Institute (AI) and the American Society of Appraisers (ASA) founded in the 1930s, the International Right of Way Association and the National Association of Realtors which were founded after World War II. These organizations all existed to establish and enforce standards, but their influence waned with increasing government regulation. In March 2007, three of these organizations (ASFMRA, ASA, and AI) announced an agreement in principle to merge. NAIFA (National Association of Independent Fee Appraisers), a charter member of The Appraisal Foundation, helped to write Title XI, the Real Estate Appraisal Reform Amendments. It was founded in 1961.

One of the most recognized professional organizations of real estate appraisers in America is the Appraisal Institute (AI). It was formed from the merger of the American Institute of Real Estate Appraisers and the Society of Real Estate Appraisers. Founded along with others in the 1930s, the two organizations merged in the 1990s to form the AI. This group awards four professional designations: SRA, to residential appraisers, AI-RRS, to residential review appraisers, MAI, to commercial appraisers, and AI-GRS, to commercial review appraisers. The Institute has enacted rigorous regulations regarding the use and display of these designations. For example, contrary to popular belief, "MAI" does not stand for "Member, Appraisal Institute". According to the institute, the letters "do not represent specific words", and an MAI may not use the words "Member, Appraisal Institute" in lieu of the MAI mark. The primary motive for this rule is to prevent trademark dilution. These designations require attendance in appraisal technique classes, ethical training, exams, and a review of the candidate's work by designated appraisers.

The National Association of Appraisers (NAA) was formed with a purpose of uniting those engaged in the appraisal profession for the purpose of exerting a beneficial influence upon the profession and to advocate appraiser interests. The NAA has established an advisory group consisting of leadership at the state organizations and coalitions called the Board of Governors where those states can help guide the NAA in acting in the best interest of all appraisers. The NAA also has a designated membership, MNAA (Member of the National Association of Appraisers, who is an individual who holds an appraisal license, certification or similar appraisal credential issued by a governmental agency; and who accepts the membership requirements and objectives of the National Association of Appraisers.

Other leading appraisal organizations include the National Association of Independent Fee Appraisers and the National Association of Master Appraisers, which were also founding sponsor-members of the Appraisal Foundation.^[42] The Massachusetts Board of Real Estate Appraisers (MBREA), founded in 1934, is the only state appraisal association that has been named a sponsor of the Appraisal Foundation.^[43] In recent years, the Royal Institution of Chartered Surveyors (RICS) has become highly regarded in the United States, and has formed a collaboration with the Counselors of Real Estate, a division of the National Association of Realtors. RICS, which is headquartered in London, operates on a global scale and awards the designations MRICS and FRICS to Members and Fellows of RICS. The Real Estate Counseling Group of America is a small group of top U.S. appraisers and real estate analysts who have collectively authored a disproportionately large body of appraisal methodology and, the National Association of Real Estate Appraisers (NAREA), founded in 1966, with the goal to elevate the professionalism and success of the Appraisal Industry.

The leading appraisal organization for personal property valuation is the American Society of Appraisers which is a sponsor member of the Appraisal Foundation and awards the ASA (Accredited Senior Appraiser) designation to candidates who complete five years of documented appraisal experience, pass a comprehensive exam along with required commercial and/or residential appraisal coursework, and submit two appraisal reports for review.

Implicit bias and racial composition of neighborhoods have long been thought to impact on home appraisal values.^[44] Recent studies from Freddie Mac and other industry leaders have confirmed that traditional modelling based on comparable sales and a variety of other factors (income, credit score, etc.) cannot explain the appraisal value gap minorities face.^[45] Some would argue that these pricing disparities are partially explained by neighborhood quality, which opponents say is a byproduct of historical redlining.^[46]

In Russia, on par with many other former Soviet Union economies, the profession emerged in the first half of 1990, and represented a clean break with the former practice of industry-specific pricing specialists and with activities of statutory price-setting authorities in the Soviet Union. Currently, property valuation, as it is called, is a specialism within general-purpose "valuation profession", which functions in a self-regulatory mode overseen by "self-regulated professional organizations" of valuers (SROs), i.e. public supervisory entities established under provisions of special legislation (which very loosely can be likened to trade unions). The principal among those is Russian Society of Appraisers, established in 1993 and presently exercising oversight over about half of the valuation profession membership. Among its 6000+ members a sizeable majority are real property valuers, rubbing shoulders with business and intangible assets appraisers. The latter categories of valuers are also allowed to value property, though valuation professionals tend to specialize. In late 2016, it was mandated that valuers should pass through compulsory state-administered attestation process to verify their competence, the details of which as to breakdown in specialization or otherwise remain to be hammered out.

As of mid-2016, Valuers in Russia, including real property valuers, are deemed to be purposely-educated individuals maintaining their Valuation SRO membership and bearing unlimited property liability for the result of their services, that is their professional status is modeled on the organization of public notaries. Regardless of the fact, over 80% of valuers tend to be employed by valuation or consulting companies, and thus do not enter practice as stand-alone individual entrepreneurs. High-end appraisal services are principally represented by valuation arms of the International "Big-four" consultancies in the country, but there also exist reputable national corporate valuation brands.

The majority of property valuations in the country are typically conducted to meet legal requirements outlined in the Federal Valuation Law, with the most recent amendment taking place in 2016. Additionally, other related laws, such as the Joint Stock Companies Law, outline over 20 instances where valuations are mandated. These mandatory cases include valuations for purposes such as privatization, securing loans, handling bankruptcy and liquidation proceedings, among others.

Before the year 2000, valuations for corporate financial reporting held greater significance. However, this changed when the national accounting regulator discontinued its promotion of the accounting fair value option. Currently, the government is in the process of outsourcing the mass appraisal of properties for taxation purposes to professional valuation institutions.

Adjudication of valuer-certified estimates of value in case of the onset of disputes is conducted through the Experts Councils of valuers' SROs. Official courts tend to concur with the resolutions of such Councils. In some rare instances the imprimatur of SRO's Experts Councils is also required for a valuation done by a particular valuer to enter into effect.

The technical details of practice of real estate valuers in Russia are aligned with the international pattern. Members of the Russian Society of Appraisers formerly were bound by the observance of the International Valuation Standards. There also exists a set of 14 general-purpose government-developed "Federal Valuation Standards" (FSOs 1,2,3 --are the general valuation standards first adopted in 2007 (and revised 2015) and covering Terms of engagement and Valuation report content requirements, FSOs 7–11 are asset-specific standards adopted in 2015, while FSO 9 is currently the only purpose-specific standard in the set dealing with valuations of property for loan security purposes; the last two FSO standards adopted in 2016 cover determination of investment and liquidation values, however, they do not touch on the methodology for determining these values, only scraping the reporting requirements). In view of the international conformity drive in the latest round of FSO standards setting, general requirements in the new FSO standards are close to those in the International Valuation standards set, however they can be more specific on occasion and mandate compulsory disclosure of uncertainty in valuation reports using the interval/range format.

With effect from 1 August 2017, new amendments to the Federal Valuation Law came to impose the compulsory certification of valuers by a state-affiliated testing centre. Consequently, this two-hour written exam certification measure, aimed to counter a perception of wide-spread malpractice among the members of the national valuation profession, provides for three valuer-specializations: real estate valuers, plant and machinery valuers, and business and intangible asset valuers, with the exam content requirements varying substantially for each specialization. Valuers would lose a right to practice, unless they comply with the requirement to take this compulsory certification exam at or before 31 March 2018. A general assessment of this measure is that the numbers of certified valuers in Russia are set to dwindle down to some 2000–3000 valuers nationwide (across all the specialisms mentioned), i.e. decimating some 80% of the current Valuer SRO's membership, due to the complexity of the certification exams.

The Hong Kong Institute of Surveyors (HKIS) regulates property surveyors in Hong Kong. Established in 1984, Institute is the only professional organisation representing the surveying profession in Hong Kong. The HKIS was statutorily incorporated by virtue of the Hong Kong Institute of Surveyors Ordinance in January 1990 (Cap. 1148). In July 1991, the Surveyors Registration Ordinance (Cap. 417) was passed to set up a Registration Board to administer the registration of surveyors. In May 2006, the number of members had reached 6,723. A general practice surveyor advises on the best use of the land, assesses the feasibility and viability of the proposed development project as well as the valuation, marketing, sale, leasing and management of completed developments. It also has a website to provide real-time properties' value estimate across whole Hong Kong.^[47]

The Australian Property Institute (API) was formed in 1926 as the Commonwealth Institute of Valuers. The Institute has undergone several name changes over the last century as the array of services offered by its members expanded. It serves to regulate the profession of property valuers throughout Australia.

Today the API represents the interests of more than 8,600 property professionals throughout Australia. API members include residential, commercial and plant and machinery valuers, property advisers, property analysts, property fund and asset managers, property facility managers, property lawyers and property researchers and academics. The Institute's primary role is to set and maintain the highest standards of professional practice, education, ethics and professional conduct for its members and the broader property profession.^[48]

Real estate valuation in New Zealand is regulated by the New Zealand Institute of Valuers ('NZIV') and the Valuers Registration Board of New Zealand ('VRB'), both of which are statutory bodies established under the Valuers Act 1948 (NZ). The NZIV remains the statutory professional body for valuers in New Zealand, with perpetual succession under the Act. The NZIV can make Rules as lower level legislation and has a Code of Ethics (reviewed in 2023). The NZIV Rules were last changed in 2012 and remain current. The VRB has jurisdiction in relation to serious matters affecting the registration of a valuer including discipline where a valuer has acted in such a way as to meet the threshold. The Valuers Act 1948 sets the threshold under s31 as matters where a valuer could be struck off the register of valuers. The NZIV has power for discipline for relatively more minor matters. The NZIV governs NZIV members and has power to discipline members and fine them up to $500, admonish members or terminate their membership. The designations "Registered Valuer" and "Public Valuer" are legally protected under the legislation, being reserved for Valuers Registered under the Act. The NZIV, under the Act, can admit non-valuer members (such as non-valuer land economists).

There are also voluntary professional bodies for real estate valuation such as the Royal Institute of Chartered Surveyors (RICS) and the Property Institute of New Zealand (PINZ). Both of these bodies have a wider membership, beyond real estate valuers. PINZ has around 1,700 members in New Zealand and overseas (such as ex-pats in the UK, Asia and Australia). PINZ has a service level agreement with the NZIV, whereby PINZ contracts to perform tasks for the statutory professional body, NZIV. PINZ was formed in 2000 to act as the voice of the property professions. There have been 'political divisions' within the valuation profession in New Zealand, expressed at AGMs and through 'proxy wars' over the last 20 years or so. Many valuers are supportive of amalgamation of the NZIV functions under the multi-disciplinary voluntary body PINZ, whilst many others wish to retain a separate statutory professional body for valuers (the NZIV). There are various reasons in the debate and the governing legislation is under review and amendments or repeal is being considered. At present, the Act remains in force and the NZIV is legally a distinct body with statutory functions, powers and duties.

PINZ incorporated much of the membership of the NZIV, the Institute of Plant & Machinery Valuers (IPMV) and the Property & Land Economy Institute of New Zealand (PLEINZ). PINZ now represents the interests of valuers, property and facilities managers, property advisors and plant and machinery valuers. PINZ has developed into one of the largest professional bodies for standards, qualifications and ethics across all facets of the property profession within New Zealand. It works with government, industry and other professional associations, education stakeholders and the media to promote its standards and views.^[49]

In New Zealand, the terms "valuation" and "valuer" usually relates to one who undertakes that professional role in terms of the Valuer Act 1948 requirements or the unregulated or voluntarily self-regulated (if members of PINZ) plant and machinery, marine or art valuers. Whereas, the term "appraisal" is usually related to an estimate by a real estate sales person or licensed agent under the Real Estate Agents Act 2008. The Real Estate Institute of New Zealand includes many valuer members, but the governing legislation for sales and agency (disposal of interests of land on behalf of others) does not extend to include provision for that role by valuers regardless of membership of NZIV, RICS or PINZ.

There exists a significant difference in the responsibilities of a real estate agent and a valuer. While a real estate agent is allowed to represent the interests of their client, a valuer is required to offer an unbiased and independent assessment of value. The legal framework governing these roles is distinct as well. Lawyers, Conveyancers, and Real Estate Agents operate under legislation separate from that which regulates valuers. Specifically, the legal provision outlining the responsibilities of Lawyers and Conveyancers is the Lawyers and Conveyancers Act of 2006..

The number of Registered Valuers in New Zealand has generally between 1,000 - 1,300. This is an ageing 'top heavy' professional with difficulty retaining new and young members due to pay, work stress and the recent advent of 'clearing houses' for banks to order valuations for mortgage purposes. The clearing houses have largely ended the long-standing local practice of members of the public seeking advice directly from a valuer. The use of electronic estimates based on Rating Values (Local Government mass appraisal for levies) is also leading to a reduction in standard valuation work and is significantly affecting the viability of small valuation businesses. The profession is in the process of a wider corporate re-structuring of the valuation market due to these factors with various perceptions within profession as to the merits of the events of the last five years.

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