Editing Light-emitting diode (section)

=== RGB systems ===
[[File:RGB_LED_Spectrum.svg|thumb|Combined spectral curves for blue, yellow-green, and high-brightness red solid-state semiconductor LEDs. [[Full width at half maximum|FWHM]] spectral bandwidth is approximately 24–27 nm for all three colors.]]

[[File:RGB-Led-projection.jpg|thumb|An RGB LED projecting red, green, and blue onto a surface]]

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,<ref>{{Cite journal | doi = 10.1364/OE.15.003607 | last1 = Moreno | first1 = I. | last2 = Contreras | first2 = U. | title = Color distribution from multicolor LED arrays | journal = Optics Express | volume = 15 | issue = 6 | pages = 3607–3618 | year = 2007 | pmid = 19532605| bibcode = 2007OExpr..15.3607M | s2cid = 35468615 | doi-access = free }}</ref> and in principle, this mechanism also has higher [[quantum efficiency]] in producing white light.<ref>{{Cite web|url=http://spie.org/newsroom/1069-making-white-light-emitting-diodes-without-phosphors?SSO=1|title=Making white-light-emitting diodes without phosphors {{!}} SPIE Homepage: SPIE|last1=Yeh|first1=Dong-Ming|last2=Huang|first2=Chi-Feng|website=spie.org|access-date=2019-04-07|last3=Lu|first3=Chih-Feng|last4=Yang|first4=Chih-Chung}}</ref>

There are several types of multicolor white LEDs: [[:wiktionary:dichromatic|di-]], [[trichromatic|tri-]], and [[tetrachromatic]] white LEDs. Several key factors that play among these different methods include color stability, [[color rendering index|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.<ref>{{cite book |last1=Cabrera |first1=Rowan |title=Electronic Devices and Circuits |date=2019 |publisher=EDTECH |isbn=978-1839473838}}</ref>

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|date=October 2020}}

Multicolor LEDs offer a means to form light of different colors. Most [[color#Perception|perceivable colors]] can be formed by mixing different amounts of three primary colors. This allows precise dynamic color control. Their emission power [[exponential decay|decays exponentially]] with rising temperature,<ref>{{Cite journal |last1=Schubert |first1=E. Fred |last2=Kim |first2=Jong Kyu |journal=Science |volume=308 |issue=5726 |doi=10.1126/science.1108712 |pmid=15919985 |pages=1274–1278 |year=2005 |title=Solid-State Light Sources Getting Smart |bibcode=2005Sci...308.1274S |s2cid=6354382 |url=https://www.ecse.rpi.edu/~schubert/Reprints/2005%20Schubert%20and%20Kim%20(Science)%20Solid-state%20light%20sources%20getting%20smart.pdf|archive-url=https://web.archive.org/web/20160205165109/https://www.ecse.rpi.edu/~schubert/Reprints/2005%20Schubert%20and%20Kim%20(Science)%20Solid-state%20light%20sources%20getting%20smart.pdf |archive-date=February 5, 2016 }}</ref>
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.<ref>{{cite journal | title = Sensors and Feedback Control of Multicolor LED Systems | format = PDF | first1 = Thomas | last1 = Nimz | first2 = Fredrik | last2 = Hailer | first3 = Kevin | last3 = Jensen | journal = Led Professional Review: Trends & Technologie for Future Lighting Solutions | publisher = LED Professional | date = November 2012 | issue = 34 | issn = 1993-890X | pages = 2–5 | url = http://www.mazet.de/en/english-documents/english/featured-articles/sensors-and-feedback-control-of-multi-color-led-systems-1/download#.UX7VXYIcUZI | archive-url = https://web.archive.org/web/20140429162806/http://www.mazet.de/en/english-documents/english/featured-articles/sensors-and-feedback-control-of-multi-color-led-systems-1/download#.UX7VXYIcUZI | url-status = dead | archive-date = 2014-04-29 }}</ref>