Editing Light-emitting diode (section)

== Considerations for use ==

* Efficiency: LEDs emit more lumens per watt than incandescent light bulbs.<ref>{{cite web|url=http://www1.eere.energy.gov/buildings/ssl/comparing.html|archive-url=https://web.archive.org/web/20090505080533/http://www1.eere.energy.gov/buildings/ssl/comparing.html |archive-date=2009-05-05|title=Solid-State Lighting: Comparing LEDs to Traditional Light Sources|website=eere.energy.gov}}</ref> 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&nbsp;mm<sup>2</sup><ref>{{cite web|url=http://www.dialight.com/Assets/Brochures_And_Catalogs/Indication/MDEI5980603.pdf|archive-url=https://web.archive.org/web/20090205040334/http://www.dialight.com/Assets/Brochures_And_Catalogs/Indication/MDEI5980603.pdf |archive-date=2009-02-05|title=Dialight Micro LED SMD LED "598 SERIES" Datasheet|website=Dialight.com}}
</ref>) and are easily attached to printed circuit boards.

=== Power sources ===
{{Main|LED power sources}}
[[File:LED circuit.svg|class=skin-invert-image|thumb|upright|Simple LED circuit with resistor for current limiting]]

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|date=October 2020}}

LEDs are sensitive to voltage. They must be supplied with a voltage above their [[P–n junction#Forward bias|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).<ref>[http://www.ledmuseum.org/ The LED Museum]. Retrieved on March 16, 2012.</ref>

[[LED droop|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.<ref name="stevenson">Stevenson, Richard (August 2009), "{{usurped|1=[https://web.archive.org/web/20090805082614/http://www.spectrum.ieee.org/semiconductors/optoelectronics/the-leds-dark-secret The LED's Dark Secret: Solid-state lighting will not supplant the lightbulb until it can overcome the mysterious malady known as droop]}}". ''IEEE Spectrum''.</ref>

=== Electrical polarity ===
{{Main|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 [[Hardware random number generator|noise diode]].{{citation needed|date=October 2020}}

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|date=December 2022}}

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.

===Appearance===
* 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-[[#Other white LEDs|white LEDs]] have spectra that differ significantly from a [[black body]] radiator like the sun or an incandescent light. The spike at 460&nbsp;nm and dip at 500&nbsp;nm can make the color of objects [[color vision|appear differently]] under cool-white LED illumination than sunlight or incandescent sources, due to [[metamerism (color)|metamerism]],<ref>{{cite web|url = http://www.jimworthey.com/jimtalk2006feb.html|title = How White Light Works|author = Worthey, James A. |website = LRO Lighting Research Symposium, Light and Color|access-date = October 6, 2007}}</ref> 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|Color Rendering Index (CRI)]].
* Dimming: LEDs can be [[Dimmer|dimmed]] either by [[pulse-width modulation]] or lowering the forward current.<ref>{{Cite book |last1=Narra |first1=Prathyusha |last2=Zinger |first2=D.S. |title=Conference Record of the 2004 IEEE Industry Applications Conference, 2004. 39th IAS Annual Meeting |chapter=An effective LED dimming approach |year=2004|volume=3 |pages= 1671–1676 |doi=10.1109/IAS.2004.1348695 |isbn=978-0-7803-8486-6 |s2cid=16372401 }}</ref> 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]].

===Light properties===
* Switch on time: LEDs light up extremely quickly. A typical red indicator LED achieves full brightness in under a [[microsecond]].<ref>{{cite web|url=http://www.avagotech.com/docs/AV02-1555EN|title=Data Sheet&nbsp;— HLMP-1301, T-1 (3 mm) Diffused LED Lamps |publisher=Avago Technologies |access-date=May 30, 2010}}</ref> LEDs used in communications devices can have even faster response times.
* Focus: The solid package of the LED can be designed to [[focus (optics)|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 [[Lambert's cosine law|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.<ref>{{Cite book|author=Hecht, E. |title=Optics|url=https://archive.org/details/optics00ehec |url-access=limited |edition=4|page=[https://archive.org/details/optics00ehec/page/n596 591]|publisher=Addison Wesley|year= 2002|isbn=978-0-19-510818-7}}</ref>

===Reliability===
* Shock resistance: LEDs, being solid-state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile.<ref>{{cite web|url=https://www.larsonelectronics.com/a-5-led-light-bars-for-off-road-illumination.aspx|title=LED Light Bars For Off Road Illumination|website=Larson Electronics}}</ref>
* 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.<ref>{{cite web |url=https://www.ledsmagazine.com/articles/print/volume-6/issue-2/features/led-design-forum-avoiding-thermal-runaway-when-driving-multiple-led-strings-magazine.html |title=LED Design Forum: Avoiding thermal runaway when driving multiple LED strings |work=LEDs Magazine |date=20 April 2009 |access-date=17 January 2019 }}</ref>
* Slow failure: LEDs mainly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.<ref name=eere>{{cite web|url=http://www1.eere.energy.gov/buildings/ssl/lifetime.html |title=Lifetime of White LEDs |access-date=2009-04-10 |url-status=dead |archive-url=https://web.archive.org/web/20090410145015/http://www1.eere.energy.gov/buildings/ssl/lifetime.html |archive-date=April 10, 2009 |df=mdy }}, US Department of Energy</ref>
* Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life for white LEDs, though time to complete failure may be shorter or longer.<ref>[http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/lifetime_white_leds_aug16_r1.pdf Lifetime of White LEDs] {{Webarchive|url=https://web.archive.org/web/20160528075610/http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/lifetime_white_leds_aug16_r1.pdf |date=May 28, 2016 }}. US Department of Energy. (PDF). Retrieved on March 16, 2012.</ref> 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 [[United States Department of Energy|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.<ref>{{cite web|url=http://energy.ltgovernors.com/in-depth-advantages-of-led-lighting.html|title=In depth: Advantages of LED Lighting|website=energy.ltgovernors.com|access-date=July 27, 2012|archive-date=November 14, 2017|archive-url=https://web.archive.org/web/20171114184333/http://energy.ltgovernors.com/in-depth-advantages-of-led-lighting.html|url-status=dead}}</ref>
* 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 lamp]]s (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&nbsp;– 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.