Индикаторы Phase Led

No sellers found for this product.

&#x;&#x;&#x;&#x;о&#x;ы&#x; &#x;н&#x;&#x;к&#x;&#x;о&#x;ы

&#x;н&#x;&#x;к&#x;&#x;о&#x; &#x;&#x;&#x; CPU (CPU Phase LED)

Э&#x;&#x; &#x;н&#x;&#x;к&#x;&#x;о&#x;ы пок&#x;&#x;ы&#x;&#x;ю&#x; &#x;&#x;&#x;&#x;м &#x;&#x;&#x;о&#x;ы &#x;&#x;&#x;о&#x;н&#x;к&#x; п&#x;&#x;&#x;н&#x;я CPU. &#x;н&#x;о&#x;м&#x;&#x;&#x;я

о &#x;о&#x;&#x;оян&#x;&#x; &#x;н&#x;&#x;к&#x;&#x;о&#x;о&#x; п&#x;&#x;&#x;&#x;&#x;&#x;н&#x; &#x; &#x;&#x;&#x;л&#x;&#x;&#x;.

&#x;КЛЮ&#x;&#x;Н

CPU Phase LED

&#x;ЫКЛЮ&#x;&#x;Н

CPU &#x;&#x;поль&#x;&#x;&#x;&#x; 1 &#x;&#x;&#x;&#x; п&#x;&#x;&#x;н&#x;я.

CPU &#x;&#x;поль&#x;&#x;&#x;&#x; 2 &#x;&#x;&#x;ы п&#x;&#x;&#x;н&#x;я.

CPU &#x;&#x;поль&#x;&#x;&#x;&#x; 3 &#x;&#x;&#x;ы п&#x;&#x;&#x;н&#x;я.

CPU &#x;&#x;поль&#x;&#x;&#x;&#x; 4 &#x;&#x;&#x;ы п&#x;&#x;&#x;н&#x;я.

Ru

&#x;&#x;&#x;&#x;о&#x;ы&#x; &#x;н&#x;&#x;к&#x;&#x;о&#x;ы

&#x;н&#x;&#x;к&#x;&#x;о&#x;ы &#x;о&#x;&#x;оян&#x;я APS

Э&#x;&#x; &#x;н&#x;&#x;к&#x;&#x;о&#x;ы APS (Active Phase Switching) пок&#x;&#x;ы&#x;&#x;ю&#x; &#x;&#x;&#x;&#x;м &#x;&#x;&#x;о&#x;ы

&#x;&#x;&#x;о&#x;н&#x;к&#x; п&#x;&#x;&#x;н&#x;я п&#x;о&#x;&#x;&#x;&#x;о&#x;&#x;. &#x;н&#x;о&#x;м&#x;&#x;&#x;я о &#x;о&#x;&#x;оян&#x;&#x; &#x;н&#x;&#x;к&#x;&#x;о&#x;о&#x; п&#x;&#x;&#x;&#x;&#x;&#x;н&#x;

&#x; &#x;&#x;&#x;л&#x;&#x;&#x;.

: &#x;КЛЮ&#x;&#x;Н,

: &#x;ЫКЛЮ&#x;&#x;Н

4 &#x;н&#x;&#x;к&#x;&#x;о&#x;&#x; &#x;о&#x;я&#x; &#x;ол&#x;&#x;ым п&#x;&#x; &#x;&#x;&#x;о&#x;&#x; 4 &#x;&#x;&#x; п&#x;&#x;&#x;н&#x;я CPU.

3 &#x;н&#x;&#x;к&#x;&#x;о&#x;&#x; &#x;о&#x;я&#x; &#x;ол&#x;&#x;ым п&#x;&#x; &#x;&#x;&#x;о&#x;&#x; 3 &#x;&#x;&#x; п&#x;&#x;&#x;н&#x;я CPU.

2 &#x;н&#x;&#x;к&#x;&#x;о&#x;&#x; &#x;о&#x;я&#x; &#x;ол&#x;&#x;ым п&#x;&#x; &#x;&#x;&#x;о&#x;&#x; 2 &#x;&#x;&#x; п&#x;&#x;&#x;н&#x;я CPU.

1 &#x;н&#x;&#x;к&#x;&#x;о&#x; &#x;о&#x;&#x;&#x; &#x;ол&#x;&#x;ым п&#x;&#x; &#x;&#x;&#x;о&#x;&#x; 1 &#x;&#x;&#x;ы п&#x;&#x;&#x;н&#x;я CPU.

&#x;н&#x;&#x;к&#x;&#x;о&#x; &#x;&#x;&#x; LED

&#x;о&#x;&#x;&#x; &#x;ол&#x;&#x;ым п&#x;&#x; &#x;&#x;&#x;о&#x;&#x; NB.

APS LEDs

NB Phase LED

Clr

RESET

CMOS

Ru

Semiconductor and solid-state light source

This article is about the electronic device. For specific use in lighting, see LED lamp.

"LED" redirects here. For other uses, see LED (disambiguation).

Not to be confused with Liquid-crystal display or photodiode.

A light-emitting diode (LED) is a semiconductordevice 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 , 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.

History[edit]

Discoveries and early devices[edit]

Electroluminescence as a phenomenon was discovered in by the English experimenter H. J. Round of Marconi Labs, using a crystal of silicon carbide and a cat's-whisker detector.^[9][10] Russian inventor Oleg Losev reported the creation of the first LED in ^[11] His research was distributed in Soviet, German and British scientific journals, but no practical use was made of the discovery for several decades, partly due to the very inefficient light-producing properties of silicon carbide, the semiconductor Losev used.^[12][13]

In , Georges Destriau observed that electroluminescence could be produced when zinc sulphide (ZnS) powder is suspended in an insulator and an alternating electrical field is applied to it. In his publications, Destriau often referred to luminescence as Losev-Light. Destriau worked in the laboratories of Madame Marie Curie, also an early pioneer in the field of luminescence with research on radium.^[14][15]

Hungarian Zoltán Bay together with György Szigeti pre-empted LED lighting in Hungary in by patenting a lighting device based on silicon carbide, with an option on boron carbide, that emitted white, yellowish white, or greenish white depending on impurities present.^[16]Kurt Lehovec, Carl Accardo, and Edward Jamgochian explained these first LEDs in using an apparatus employing SiC crystals with a current source of a battery or a pulse generator and with a comparison to a variant, pure, crystal in ^[17][18]

Rubin Braunstein^[19] of the Radio Corporation of America reported on infrared emission from gallium arsenide (GaAs) and other semiconductor alloys in ^[20] Braunstein observed infrared emission generated by simple diode structures using gallium antimonide (GaSb), GaAs, indium phosphide (InP), and silicon-germanium (SiGe) alloys at room temperature and at 77&#;kelvins. In , Braunstein further demonstrated that the rudimentary devices could be used for non-radio communication across a short distance. As noted by Kroemer^[21] Braunstein "…had set up a simple optical communications link: Music emerging from a record player was used via suitable electronics to modulate the forward current of a GaAs diode. The emitted light was detected by a PbS diode some distance away. This signal was fed into an audio amplifier and played back by a loudspeaker. Intercepting the beam stopped the music. We had a great deal of fun playing with this setup." This setup presaged the use of LEDs for optical communication applications.

In September , while working at Texas Instruments in Dallas, Texas, James R. Biard and Gary Pittman discovered near-infrared (&#;nm) light emission from a tunnel diode they had constructed on a GaAs substrate.^[7] By October , they had demonstrated efficient light emission and signal coupling between a GaAs p-n junction light emitter and an electrically isolated semiconductor photodetector.^[22] On August 8, , Biard and Pittman filed a patent titled "Semiconductor Radiant Diode" based on their findings, which described a zinc-diffused p–n junction LED with a spaced cathode contact to allow for efficient emission of infrared light under forward bias. After establishing the priority of their work based on engineering notebooks predating submissions from G.E. Labs, RCA Research Labs, IBM Research Labs, Bell Labs, and Lincoln Lab at MIT, the U.S. patent office issued the two inventors the patent for the GaAs infrared light-emitting diode (U.S. Patent US), the first practical LED.^[7] Immediately after filing the patent, Texas Instruments (TI) began a project to manufacture infrared diodes. In October , TI announced the first commercial LED product (the SNX), which employed a pure GaAs crystal to emit an &#;nm light output.^[7] In October , TI announced the first commercial hemispherical LED, the SNX^[23]

In the s, several laboratories focused on LEDs that would emit visible light. A particularly important device was demonstrated by Nick Holonyak on October 9, , while he was working for General Electric in Syracuse, New York. The device used the semiconducting alloy gallium phosphide arsenide (GaAsP). It was the first semiconductor laser to emit visible light, albeit at low temperatures. At room temperature it still functioned as a red light-emitting diode. GaAsP was the basis for the first wave of commercial LEDs emitting visible light. It was mass produced by the Monsanto and Hewlett-Packard companies and used widely for displays in calculators and wrist watches.^[24][25][26]

M. George Craford,^[27] a former graduate student of Holonyak, invented the first yellow LED and improved the brightness of red and red-orange LEDs by a factor of ten in ^[28] In , T. P. Pearsall designed the first high-brightness, high-efficiency LEDs for optical fiber telecommunications by inventing new semiconductor materials specifically adapted to optical fiber transmission wavelengths.^[29]

Initial commercial development[edit]

Until , visible and infrared LEDs were extremely costly, on the order of US$ per unit, and so had little practical use.^[30] The first commercial visible-wavelength LEDs used GaAsP semiconductors and were commonly used as replacements for incandescent and neon indicator lamps, and in seven-segment displays, first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as calculators, TVs, radios, telephones, as well as watches.^[31]

The Hewlett-Packard company (HP) was engaged in research and development (R&D) on practical LEDs between and , by a research team under Howard C. Borden, Gerald P. Pighini at HP Associates and HP Labs.^[32] During this time HP collaborated with Monsanto Company on developing the first usable LED products.^[33] The first usable LED products were HP's LED display and Monsanto's LED indicator lamp, both launched in ^[33]

Monsanto was the first organization to mass-produce visible LEDs, using Gallium arsenide phosphide (GaAsP) in to produce red LEDs suitable for indicators.^[30] Monsanto had previously offered to supply HP with GaAsP, but HP decided to grow its own GaAsP.^[30] In February , Hewlett-Packard introduced the HP Model Numeric Indicator, the first LED device to use integrated circuit (integrated LED circuit) technology.^[32] It was the first intelligent LED display, and was a revolution in digital display technology, replacing the Nixie tube and becoming the basis for later LED displays.^[34]

In the s, commercially successful LED devices at less than five cents each were produced by Fairchild Optoelectronics. These devices employed compound semiconductor chips fabricated with the planar process (developed by Jean Hoerni,^[35][36] ). The combination of planar processing for chip fabrication and innovative packaging methods enabled the team at Fairchild led by optoelectronics pioneer Thomas Brandt to achieve the needed cost reductions.^[37] LED producers continue to use these methods.^[38]

The early red LEDs were bright enough for use as indicators, as the light output was not enough to illuminate an area. Readouts in calculators were so small that plastic lenses were built over each digit to make them legible. Later, other colors became widely available and appeared in appliances and equipment.

Early LEDs were packaged in metal cases similar to those of transistors, with a glass window or lens to let the light out. Modern indicator LEDs are packed in transparent molded plastic cases, tubular or rectangular in shape, and often tinted to match the device color. Infrared devices may be dyed, to block visible light. More complex packages have been adapted for efficient heat dissipation in high-power LEDs. Surface-mounted LEDs further reduce the package size. LEDs intended for use with fiber optics cables may be provided with an optical connector.

Blue LED[edit]

The first blue-violet LED using magnesium-doped gallium nitride was made at Stanford University in by Herb Maruska and Wally Rhines, doctoral students in materials science and engineering.^[39][40] At the time Maruska was on leave from RCA Laboratories, where he collaborated with Jacques Pankove on related work. In , the year after Maruska left for Stanford, his RCA colleagues Pankove and Ed Miller demonstrated the first blue electroluminescence from zinc-doped gallium nitride, though the subsequent device Pankove and Miller built, the first actual gallium nitride light-emitting diode, emitted green light.^[41][42] In the U.S. Patent Office awarded Maruska, Rhines, and Stanford professor David Stevenson a patent for their work in (U.S. Patent US A). Today, magnesium-doping of gallium nitride remains the basis for all commercial blue LEDs and laser diodes. In the early s, these devices were too dim for practical use, and research into gallium nitride devices slowed.

In August , Cree introduced the first commercially available blue LED based on the indirect bandgap semiconductor, silicon carbide (SiC).^[43] SiC LEDs had very low efficiency, no more than about %, but did emit in the blue portion of the visible light spectrum.^[44][45]

In the late s, key breakthroughs in GaN epitaxial growth and p-type doping^[46] ushered in the modern era of GaN-based optoelectronic devices. Building upon this foundation, Theodore Moustakas at Boston University patented a method for producing high-brightness blue LEDs using a new two-step process in ^[47] In , a US court ruled that three Taiwanese companies had infringed Moustakas's prior patent, and ordered them to pay licensing fees of not less than US$13 million.^[48]

Two years later, in , high-brightness blue LEDs were demonstrated by Shuji Nakamura of Nichia Corporation using a gallium nitride (GaN) growth process.^[49][50][51] These LEDs had efficiencies of 10%.^[52] In parallel, Isamu Akasaki and Hiroshi Amano of Nagoya University were working on developing the important GaN deposition on sapphire substrates and the demonstration of p-type doping of GaN. This new development revolutionized LED lighting, making high-power blue light sources practical, leading to the development of technologies like Blu-ray.^[53][54]

Nakamura was awarded the Millennium Technology Prize for his invention.^[55] Nakamura, Hiroshi Amano, and Isamu Akasaki were awarded the Nobel Prize in Physics in for "the invention of efficient blue light-emitting diodes, which has enabled bright and energy-saving white light sources."^[56]

In , Alberto Barbieri at the Cardiff University Laboratory (GB) investigated the efficiency and reliability of high-brightness LEDs and demonstrated a "transparent contact" LED using indium tin oxide (ITO) on (AlGaInP/GaAs).

In ^[57] and ,^[58] processes for growing gallium nitride (GaN) LEDs on silicon were successfully demonstrated. In January , Osram demonstrated high-power InGaN LEDs grown on silicon substrates commercially,^[59] and GaN-on-silicon LEDs are in production at Plessey Semiconductors. As of , some manufacturers are using SiC as the substrate for LED production, but sapphire is more common, as it has the most similar properties to that of gallium nitride, reducing the need for patterning the sapphire wafer (patterned wafers are known as epi wafers). Samsung, the University of Cambridge, and Toshiba are performing research into GaN on Si LEDs. Toshiba has stopped research, possibly due to low yields.^{[60][61][62][63][64][65][66]} Some opt for epitaxy, which is difficult on silicon, while others, like the University of Cambridge, choose a multi-layer structure, in order to reduce (crystal) lattice mismatch and different thermal expansion ratios, to avoid cracking of the LED chip at high temperatures (e.g. during manufacturing), reduce heat generation and increase luminous efficiency. Sapphire substrate patterning can be carried out with nanoimprint lithography.^{[67][68][69][70][71][72][73]}

GaN-on-Si is difficult but desirable since it takes advantage of existing semiconductor manufacturing infrastructure. It allows for the wafer-level packaging of LED dies resulting in extremely small LED packages.^[74]

GaN is often deposited using Metalorganic vapor-phase epitaxy (MOCVD),^[75] and it also utilizes Lift-off.

White LEDs and the illumination breakthrough[edit]

Even though white light can be created using individual red, green and blue LEDs, this results in poor color rendering, since only three narrow bands of wavelengths of light are being emitted. The attainment of high efficiency blue LEDs was quickly followed by the development of the first white LED. In this device a Y
₃Al
₅O
₁₂:Ce (known as "YAG" or Ce:YAG phosphor) cerium-doped phosphor coating produces yellow light through fluorescence. The combination of that yellow with remaining blue light appears white to the eye. Using different phosphors produces green and red light through fluorescence. The resulting mixture of red, green and blue is perceived as white light, with improved color rendering compared to wavelengths from the blue LED/YAG phosphor combination.^[76]

The first white LEDs were expensive and inefficient. The light output then increased exponentially. The latest research and development has been propagated by Japanese manufacturers such as Panasonic, and Nichia, and by Korean and Chinese manufacturers such as Samsung, Solstice, Kingsun, Hoyol and others. This trend in increased output has been called Haitz's law after Roland Haitz.^[77][78]

Light output and efficiency of blue and near-ultraviolet LEDs rose and the cost of reliable devices fell. This led to relatively high-power white-light LEDs for illumination, which are replacing incandescent and fluorescent lighting.^[79][80]

Experimental white LEDs were demonstrated in to produce lumens per watt of electricity (lm/W); some can last up to , hours.^[81][82] Commercially available LEDs have an efficiency of up to lm/W as of ^[83][84][85] A previous record of lm/W was achieved by Nichia in ^[86] Compared to incandescent bulbs, this is a huge increase in electrical efficiency, and even though LEDs are more expensive to purchase, overall lifetime cost is significantly cheaper than that of incandescent bulbs.^[87]

The LED chip is encapsulated inside a small, plastic, white mold. It can be encapsulated using resin (polyurethane-based), silicone, or epoxy containing (powdered) Cerium-doped YAG phosphor. After allowing the solvents to evaporate, the LEDs are often tested, and placed on tapes for SMT placement equipment for use in LED light bulb production. Some "remote phosphor" LED light bulbs use a single plastic cover with YAG phosphor for several blue LEDs, instead of using phosphor coatings on single-chip white LEDs.^[88]

The temperature of the phosphor during operation and how it is applied limits the size of an LED die. Wafer-level packaged white LEDs allow for extremely small LEDs.^[74]

Physics of light production and emission[edit]

Main article: Light-emitting diode physics

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.^[89]

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.^[90][91] Also unlike most lasers, its radiation is not spatially coherent, so it cannot approach the very high intensity characteristic of lasers.

Single-color LEDs[edit]

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. As the wavelengths become shorter, because of the larger band gap of these semiconductors, the operating voltage of the LED increases.

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 &#;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 –&#;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 &#;nm.^[92] As the photosensitivity of microorganisms approximately matches the absorption spectrum of DNA, with a peak at about &#;nm, UV LED emitting at –&#;nm are expected in prospective disinfection and sterilization devices. Recent research has shown that commercially available UVA LEDs (&#;nm) are already effective disinfection and sterilization devices.^[93] UV-C wavelengths were obtained in laboratories using aluminium nitride (&#;nm),^[94]boron nitride (&#;nm)^[95][96] and diamond (&#;nm).^[97]

White LEDs[edit]

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.^[98][99][][]

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 ^[]

RGB systems[edit]

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,^[] and in principle, this mechanism also has higher quantum efficiency in producing white light.^[]

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 ( 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.^[]

One of the challenges is the development of more efficient green LEDs. The theoretical maximum for green LEDs is lumens per watt but as of few green LEDs exceed even 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,^[] 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.^[]

Phosphor-based LEDs[edit]

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).^[] 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).^[]

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 , 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-dopedyttrium 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 mm silicon wafers. This avoids the typical costly sapphiresubstrate in relatively small or mm wafer sizes.^[] The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted. It was predicted that since , 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.^[]

Mixed white LEDs[edit]

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.^[]

Some phosphor white LED units are "tunable white", blending two extremes of color temperatures (commonly K and K) to produce intermediate values. This feature allows users to change the lighting to suit the current use of a multifunction room.^[] 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.^[] Some products are RGBWW, i.e. RGBW with tunable white.^[]

A final class of white LED with mixed light is dim-to-warm. These are ordinary K 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.^[]

Other white LEDs[edit]

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.^[]

Organic light-emitting diodes (OLEDs)[edit]

Main article: OLED

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.^[] The organic materials can be small organic molecules in a crystallinephase, or polymers.^[]

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

Perovskite Light-emitting diodes (PeLEDs)[edit]

Perovskite light-emitting diodes (PeLEDs) have emerged as promising candidates for next-generation display and lighting technologies. In recent years, researchers have shown a growing interest in perovskite light-emitting diodes (PeLEDs) owing to their capacity for emitting light with narrow bandwidth, adjustable spectrum, ability to deliver high color purity, and cost-effective solution fabrication.^[][]

Green PeLEDs[edit]

When it comes to efficiency, PeLEDs have not surpassed commercial organic light-emitting diodes (OLEDs) because specific critical parameters, such as charge carrier transport and optical output coupling efficiency, have not been thoroughly optimized.^[]

In response to this challenge, the development of ultra-efficient green PeLEDs with a external quantum efficiency (EQE) exceeding the remarkable 30% milestone was reported by Bai and his colleagues on May 29, ^[] This achievement was made by strategic adjustments in charge carrier transport and the distribution of near-field light. These optimizations effectively reduced electron leakage and resulted in an exceptional light output coupling efficiency of %. A NiMgO_x film with high refractive index and increased hole carrier mobility was used as a hole injection layer to balance charge carrier injection, and a polyethylene glycol layer was inserted between the hole transport layer and the perovskite emission layer to prevent electron leakage and minimize photon loss.^[]

The modified structure of green PeLED enabled it to achieve a world-record external quantum efficiency of % (with a mean of ± %) at a brightness level of cd/m². This pioneering work introduces a compelling approach to building ultra-efficient PeLEDs by effectively balancing electron-hole recombination and enhancing light outcoupling.^[]

However, expanding the effective area of perovskite LEDs can lead to a significant drop in their performance. To address this issue, Sun eunic-brussels.eu^[] introduced L-methionine (NVAL) to construct an intermediate phase with low formation enthalpy and COO^- coordination. This new intermediate phase altered the crystallization pathway, effectively inhibiting phase segregation. Consequently, high-quality large-area quasi-2D perovskite films were achieved. They further fine-tuned the film's composite dynamics, leading to high-efficiency quasi-2D perovskite green LEDs with an effective area of cm². An external quantum efficiency (EQE) of % was attained at <n> = 3, making it the most efficient large-area perovskite LED. Moreover, a luminance of × cd/m² was achieved in the <n> = 10 films.^[]

Blue PeLEDs[edit]

On March 16, , Zhou et al.^[] published a study demonstrating their successful control of ion behavior to create highly efficient sky-blue perovskite light-emitting diodes. They achieved this by utilizing a bifunctional passivator, which consisted of Lewis base benzoic acid anions and alkali metal cations. This passivator had a dual role: it effectively passivated the deficient lead atom while inhibited the migration of halide ions. The outcome of this innovative approach was the realization of an efficient perovskite LED that emitted light at a stable wavelength of nm. The LED exhibited a commendable external quantum efficiency (EQE) of %, with a peak EQE reaching %. Through optical coupling enhancement, the EQE was further boosted to %.^[]

Red PeLEDs[edit]

One of the most crucial aspects of lighting and display technology is the efficient generation of red emission. Quasi-2D perovskites have demonstrated potential for high emission efficiency due to robust carrier confinement. However, the external quantum efficiencies (EQE) of most red quasi-2D PeLEDs are not optimal due to different n-value phases within complex quasi-2D perovskite films.

To address this challenge, Jiang eunic-brussels.eu^[] published their findings in Advanced Materials on July 20, Their research focused on strategically incorporating large cations to enhance the efficiency of red light perovskite LEDs. By introducing phenethylammonium iodide (PEAI)/3-fluorophenylethylammonium iodide (m-F-PEA) and 1-naphthylmethylammonium iodide (NMAI), they achieved precise control over the phase distribution of quasi-2D perovskite materials. This approach effectively reduced the prevalence of smaller n-index phases and concurrently addressed lead and halide defects in the perovskite films. The outcome of this research was the development of perovskite LEDs capable of achieving an EQE of % at nm, accompanied by a peak brightness of cd/m².^[]

White PeLEDs[edit]

High-performance white perovskite LED with high light extraction efficiency can be constructed through near-field optical coupling.^[] The near-field optical coupling between blue perovskite diode and red perovskite nanocrystal was achieved by a reasonably designed multi-layer translucent electrode (LiF/Al/Ag/LiF). The red perovskite nano-crystalline layer allows the waveguide mode and surface plasmon polarization mode captured in the blue perovskite diode to be extracted and converted into red light emission, increasing the light extraction efficiency by 50%. At the same time, the complementary emission spectra of blue photons and down-converted red photons contribute to the formation of white LEDs. Finally, the off-device quantum efficiency exceeds 12%, and the brightness exceeds cd/m², which are both the highest in white PeLEDs.^[]

Lifetime[edit]

Preparing high-quality all-inorganic perovskite films through solution-based methods remains a formidable challenge, primarily attributed to the rapid and uncontrollable crystallization of such materials. The key innovation involved controlling the crystal orientation of the all-inorganic perovskite along the () plane through a low-temperature annealing process (°C). This precise control led to the orderly stacking of crystals, which significantly increased surface coverage and reduced defects within the material. After thorough optimization, the well-oriented CsPbBr₃ perovskite LED achieved an external quantum efficiency (EQE) of up to %, a remarkable brightness of 79, cd/m², and a lifespan of hours when initially operated at a brightness level of cd/m².^[]

On September 20, , the team led by Sargent eunic-brussels.eu^[] from the University of Toronto published their research findings in the Journal of the American Chemical Society (JACS) on bright and stable light-emitting diodes (LEDs) based on perovskite quantum dots within a perovskite matrix. The research reported that perovskite quantum dots remain stable in a precursor solution thin film of perovskite and drive the uniform crystallization of the perovskite matrix using strain quantum dots as nucleation centers. The type I band alignment ensures that quantum dots act as charge acceptors and radiative emitters.^[]

The new material exhibits suppressed biexciton Auger recombination and bright luminescence even at high excitation ( W/cm²). The red LEDs based on the new material demonstrate an external quantum efficiency of 18% and maintain high performance at a brightness exceeding cd/m². The new material extends the LED's operating half-life to hours at an initial brightness of cd/m².^[]

Types[edit]

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.

Miniature[edit]

These are mostly single-die LEDs used as indicators, and they come in various sizes from 2&#;mm to 8&#;mm, through-hole and surface mount packages.^[] Typical current ratings range from around 1&#;mA to above 20&#;mA. Multiple LED dies attached to a flexible backing tape form an LED strip light.^{[citation needed]}

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.^{[citation needed]}

Ultra-high-output LEDs are designed for viewing in direct sunlight.^{[citation needed]}

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.^{[citation needed]}

High-power[edit]

See also: Solid-state lighting, LED lamp, and Thermal management of high-power LEDs

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.^[][] LED power densities up to 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 well-known 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 , some HP-LEDs manufactured by Cree now exceed &#;lm/W.^[]

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 &#;lm/W in ^[] and the Nichia 19 series with a typical efficacy of &#;lm/W, released in ^[]

AC-driven[edit]

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.^[] A large number of LED elements in series may be able to operate directly from line voltage. In , 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.^[]

Strip[edit]

This section is an excerpt from LED strip light.[edit]

An LED strip, tape, or ribbon light is a flexible circuit board populated by surface-mount light-emitting diodes (SMD LEDs) and other components that usually comes with an adhesive backing. Traditionally, strip lights had been used solely in accent lighting, backlighting, task lighting, and decorative lighting applications, such as cove lighting.

Since their emergence in the early s, increased luminous efficacyand higher-power SMDs have allowed LED strip lights to be used in applications such as high brightness task lighting, fluorescent and halogen lighting fixture replacements, indirect lighting applications, ultravioletinspection during manufacturing processes, set and costume design, and growing plants.

Application-specific[edit]

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 and other size formats have been available since early

Flickering

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

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.^[] 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 s and s, 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 (WS 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.^[] 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.^[] 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.^[] The LEDs can be arranged around a cylinder, and are called "corn cob lights" because of the rows of yellow LEDs.^[]

[edit]

Power sources[edit]

Main article: LED power sources

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]}

Electrical polarity[edit]

Main article: Electrical polarity of LEDs

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.

Safety and health[edit]

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– Recommended Practice for Photobiological Safety for Lamp and Lamp Systems".^[] One study showed no evidence of a risk in normal use at domestic illuminance,^[] and that caution is only needed for particular occupational situations or for specific populations.^[] In , the International Electrotechnical Commission published IEC Photobiological safety of lamps and lamp systems, replacing the application of early laser-oriented standards for classification of LED sources.^[]

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.^[]

In 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. Industry critics claim exposure levels are not high enough to have a noticeable effect.^[]

Advantages[edit]

• Efficiency: LEDs emit more lumens per watt than incandescent light bulbs.^[] The efficiency of LED lighting fixtures is not affected by shape and size, unlike fluorescent light bulbs or tubes.
• 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.
• Size: LEDs can be very small (smaller than 2&#;mm^2[]) and are easily attached to printed circuit boards.
• Switch on time: LEDs light up extremely quickly. A typical red indicator LED achieves full brightness in under a microsecond.^[] LEDs used in communications devices can have even faster response times.
• 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.
• Dimming: LEDs can very easily be dimmed either by pulse-width modulation or lowering the forward current.^[] 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.
• 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.
• Slow failure: LEDs mainly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.^[]
• Lifetime: LEDs can have a relatively long useful life. One report estimates 35, to 50, hours of useful life, though time to complete failure may be shorter or longer.^[] Fluorescent tubes typically are rated at about 10, to 25, hours, depending partly on the conditions of use, and incandescent light bulbs at 1, to 2, 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.^[]
• Shock resistance: LEDs, being solid-state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile.^[]
• 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 are usually deployed, which are difficult to focus or collimate on the same target.

Disadvantages[edit]

• 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.
• Voltage sensitivity: LEDs 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).^[]
• 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 &#;nm and dip at &#;nm can make the color of objects appear differently under cool-white LED illumination than sunlight or incandescent sources, due to metamerism,^[] 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).
• 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.^[]
• 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.^[]
• 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.^[]
• 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.^[][]