Editing Backlight (section)

==Usage==
Colored LED backlighting is most commonly used in small, inexpensive LCD panels.  White LED backlighting is becoming dominant. ELP backlighting is often used for larger displays or when even backlighting is important; it can also be either colored or white. An ELP must be driven by relatively high{{Specify|date=August 2014}} voltage [[Alternating current|AC]] power, which is provided by an [[power inverter|inverter]] circuit. [[CCFL]] backlights are used on larger displays such as computer monitors, and are typically white in color; these also require the use of an inverter and diffuser. Incandescent backlighting was used by early LCD panels to achieve high brightness,<ref>https://www.mouser.com/datasheet/2/208/GF%20STYLE%20T-2-4786.pdf. JKL components T-2 GF series fuse style lamps datasheet</ref><ref>{{Cite web|url=https://archive.org/details/manual_RX1001VBK_SM_JVC/page/n99/mode/2up?q=fw852|title=Manual: RX1001VBK SM JVC|via=Internet Archive}}</ref> but the limited life and excess heat produced by incandescent bulbs were severe limitations. The heat generated by incandescent bulbs typically requires the bulbs to be mounted away from the display to prevent damage.

===CCFL backlights===
[[File:LCD-TV Backlight with CCFL.jpg|right|thumb|220px| 18 parallel CCFLs as backlight for an LCD TV]]
[[File:lcd-ccfl.svg|right|thumb|LCD with edge-lit CCFL backlight]]
For several years (until about 2010), the preferred backlight for matrix-addressed large LCD panels such as in monitors and TVs was based on a [[cold-cathode fluorescent lamp (CCFL)]] by using two CCFLs at opposite edges of the LCD or by an array of CCFLs behind the LCD (see picture of an array with 18 CCFLs for a 40-inch LCD TV). Due to the disadvantages in comparison with LED illumination (higher voltage and power needed, thicker panel design, no high-speed switching, faster aging), LED backlighting is becoming more popular.{{citation needed|date=March 2023}}

Many LCD models, from cheap TN-displays to color proofing S-IPS or S-PVA panels, have wide gamut CCFLs representing more than 95% of the [[NTSC]] color specification.

===LED backlights===
{{See also|LED-backlit LCD}}
[[File:lcd-led.svg|right|thumb|LCD with LED matrix backlight]]

LED backlighting in color screens comes in two varieties: white [[LED]] backlights and RGB LED backlights.<ref>{{cite web |url=http://ledtele.co.uk/whatisledtv.html |title=What is LED TV? |publisher=Ledtele.co.uk |access-date=19 February 2012 |url-status=live |archive-url=https://web.archive.org/web/20120211233152/http://www.ledtele.co.uk/whatisledtv.html |archive-date=11 February 2012 }}</ref> White LEDs are used most often in [[notebook computer]]s and desktop screens, and make up virtually all mobile LCD screens. A white LED is typically a [[LED#Phosphor-based LEDs|blue LED]] with broad spectrum yellow [[phosphor]] to result in the emission of white light. However, because the spectral curve peaks at yellow, it is a poor match to the transmission peaks of the red and green color filters of the LCD.  This causes the red and green primaries to shift toward yellow, reducing the color gamut of the display.<ref>The Evolution of LED Backlights; Adam Simmons; PCM PC monitors, Monitor articles, 12 November 2017; {{cite web |url=https://pcmonitors.info/articles/the-evolution-of-led-backlights/ |title=The Evolution of LED Backlights &#124; PC Monitors |access-date=27 November 2017 |url-status=live |archive-url=https://web.archive.org/web/20171201032305/https://pcmonitors.info/articles/the-evolution-of-led-backlights/ |archive-date=1 December 2017 }}</ref> RGB LEDs consist of [[LED#RGB systems|a red, a blue, and a green LED]] and can be controlled to produce different color temperatures of white. RGB LEDs for backlighting are found in high end color proofing displays such as the HP DreamColor LP2480zx monitor or selected [[HP EliteBook]] notebooks, as well as more recent consumer-grade displays such as Dell's Studio series laptops which have an optional RGB LED display.

RGB LEDs can deliver an enormous color [[gamut]] to screens.<ref>Competing display technologies for the best image performance; A.J.S.M. de Vaan; Journal of the society of information displays, Volume 15, Issue 9 September 2007 Pages 657–666; http://onlinelibrary.wiley.com/doi/10.1889/1.2785199/abstract?</ref> When using three separate LEDs ([[additive color]]) the backlight can produce a color spectrum that closely matches the color filters in the LCD [[pixel]]s themselves. In this way, the filter [[passband]] can be narrowed so that each color component lets only a very narrow band of spectrum through the LCD. This improves the efficiency of the display since less light is blocked when white is displayed. The actual [[RGB color model|red, green, and blue]] points can be moved farther out so that the display is capable of reproducing more vivid colors.

A method to further improve the color gamut of LED-backlit LCD panels is based on blue LEDs (such as [[gallium nitride]] (GaN) LEDs) that illuminate a layer of nanocrystal phosphors, called [[quantum dot]]s (QDs).<ref>{{cite web|url=http://www.nanosysinc.com/what-we-do/display-backlighting/qdef/|title=QDEF|website=Quantum Dot Pioneers|url-status=live|archive-url=https://web.archive.org/web/20140529101034/http://www.nanosysinc.com/what-we-do/display-backlighting/qdef/|archive-date=29 May 2014}}</ref> The quantum dots convert the blue wavelengths to the desired longer wavelengths as narrow-bandwidth green and red colors for optimal illumination of the LCD from behind. The manufacturer, [[Nanosys]], claims that the color output of the dots can be tuned precisely by controlling the size of the nanocrystals. Other companies pursuing this method are [[Nanoco]] Group PLC (UK), [[QD Vision]], [[3M]] a licensee of Nanosys and Avantama of [[Switzerland]].<ref>[https://avantama.com/markets/cadmium-free-quantum-dot-display ''Cadmium-free quantum dot display.''] avantama.com. Retrieved 17 August 2019</ref><ref>IEEE Spectrum, 2012, 8, p.11-12, ''Quantum Dots Are Behind New Displays''</ref>
[[Sony]] has adapted quantum dot technology from the US company QD Vision<ref>{{cite web |url=http://www.qdvision.com/displays |title=QD Vision Displays |access-date=23 July 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130902215454/http://www.qdvision.com/displays |archive-date=2 September 2013 }}</ref> to introduce LCD TVs with an improved ''edge-lit'' LED backlight marketed under the term ''Triluminos'' in 2013. With a blue LED and optimized nanocrystals for green and red colors in front of it, the resulting combined white light allows for an equivalent or better color gamut than that emitted by a more expensive set of three RGB LEDs. At the [[Consumer Electronics Show]] 2015, a number of companies showed QD-enhanced LED-backlighting of LCD TVs, including [[Samsung Electronics]], [[LG Electronics]], and the Chinese [[TCL Corporation]].<ref>{{Cite web|url=https://spectrum.ieee.org/what-the-heck-are-quantum-dots|title=What the Heck Are Quantum Dots? - IEEE Spectrum|website=spectrum.ieee.org}}</ref><ref>{{Cite web|url=https://spectrum.ieee.org/ces-2015-placing-bets-on-the-new-tv-technologies|title=CES 2015: Placing Bets on the New TV Technologies - IEEE Spectrum|website=spectrum.ieee.org}}</ref>

There are several challenges with LED backlights. Uniformity is hard to achieve, especially as the LEDs age, with each LED aging at a different rate. The use of three separate light sources for red, green, and blue means that the [[white point]] of the display can move as the LEDs age at different rates; white LEDs are affected by this phenomenon, with changes of several hundred [[kelvin]]s of [[color temperature]] being recorded. White LEDs suffer from blue shifts at higher temperatures varying from 3141K to 3222K for 10&nbsp;°C to 80&nbsp;°C respectively.<ref>{{cite web |url=http://www.instrumentsystems.com/fileadmin/editors/downloads/Products/LpR10_Instrument_Systems_web.pdf |title=White Light LEDs - Importance of measurement standards |access-date=19 February 2012 |url-status=live |archive-url=https://web.archive.org/web/20120225034846/http://www.instrumentsystems.com/fileadmin/editors/downloads/Products/LpR10_Instrument_Systems_web.pdf |archive-date=25 February 2012 }}</ref> Power efficiency may be a challenge; first generation implementations could potentially use more power than their CCFL counterparts, though it is possible for an LED display to be more power efficient.{{Citation needed|date=November 2008}} In 2010, current generation LED displays can have significant power consumption advantages. For example, the non-LED version of the 24" [[Benq]] [https://web.archive.org/web/20110707223724/http://www.benq.com/products/product_detail.cfm?product=1515&pltag=3&ptag=10 G2420HDB] consumer display has a 49W consumption compared to the 24W of the LED version of the same display ([https://web.archive.org/web/20110707223744/http://www.benq.com/products/product_detail.cfm?product=1622&pltag=3&ptag=10 G2420HDBL]).

To overcome the aforementioned challenges with RGB and white LED backlights an 'advanced remote phosphor' <ref>{{cite web |url=http://ndf.eu/display/technology-roadmap/arphos/ |title=ARPHOS®, a revolution in LCD backlights |url-status=dead |archive-url=https://web.archive.org/web/20160919164133/http://ndf.eu/display/technology-roadmap/arphos/ |archive-date=19 September 2016 |access-date=29 July 2016 }}</ref> LED technology has been developed by NDF Special Light Products, specifically for high-end and long-life LCD applications such as [[cockpit]] displays,<ref>{{cite web |url=http://www.transport-research.info/project/technology-development-remote-phosphor-avionic-cockpit-displays |title=Technology Development of Remote Phosphor for Avionic Cockpit Displays |url-status=usurped |archive-url=https://web.archive.org/web/20160815092423/http://www.transport-research.info/project/technology-development-remote-phosphor-avionic-cockpit-displays |archive-date=15 August 2016 }}</ref> [[air traffic control]] displays, and medical displays. This technology uses blue pump LEDs in combination with a sheet on which phosphorous luminescent materials are printed for colour conversion. The principle is similar to quantum dots, but the phosphors applied are much more robust than the quantum dot nano-particles for applications that require long lifetime in more demanding operational conditions. Because the phosphor sheet is placed at a distance (remote) of the LED it experiences much less temperature stress than phosphors in white LEDs. As a result, the white point is less dependent on individual LEDs, and degrading of individual LEDs over lifetime, leading to a more homogenous backlight with improved colour consistency and lower lumen depreciation.

The use of LED backlights in notebook computers has been growing. [[Sony]] has used LED backlights in some of its higher-end slim [[VAIO]] notebooks since 2005, and [[Fujitsu]] introduced notebooks with LED backlights in 2006. In 2007, [[Asus]], [[Dell]], and [[Apple Inc.|Apple]] introduced LED backlights into some of their notebook models. {{As of|2008}}, [[Lenovo]] has announced LED-backlit notebooks. In October 2008, Apple announced that it would be using LED backlights for all of its notebooks and new 24-inch [[Apple Cinema Display]], and one year later it introduced a new LED [[iMac]], meaning all of Apple's new computer screens became LED-backlit displays. Almost every laptop with a [[16:9|16:9 display]] introduced since September 2009 uses LED-backlit panels. This is also the case for most LCD television sets, which are marketed in some countries under the misleading name ''LED TV'', although the image is still generated by an LCD panel.

Most LED backlights for LCDs are ''edge-lit'', i.e. several LEDs are placed at the edges of a lightguide (Light guide plate, LGP), which distributes the light behind the LC panel. Advantages of this technique are the very thin flat-panel construction and low cost. A more expensive version is called ''full-array'' or ''direct'' LED and consists of many LEDs placed behind the LC panel (an ''array'' of LEDs), such that large panels can be evenly illuminated. Full-array local dimming is often abbreviated as "FALD". This arrangement allows for ''local dimming'' to obtain darker ''black'' pixels depending on the image displayed.

====Backlight dimming====
LED backlight are often dynamically controlled using the video information<ref>LED TVs: 10 things you need to know; David Carnoy, David Katzmaier; CNET.com/news; 3 June 2010; {{cite web |url=https://www.cnet.com/news/led-tvs-10-things-you-need-to-know/ |title=LED TVs: 10 things you need to know |access-date=22 November 2017 |url-status=live |archive-url=https://web.archive.org/web/20171201061250/https://www.cnet.com/news/led-tvs-10-things-you-need-to-know/ |archive-date=1 December 2017 }}</ref> (dynamic backlight control or dynamic "local dimming" LED backlight, also marketed as HDR, high dynamic range television, invented by Philips researchers Douglas Stanton, Martinus Stroomer and Adrianus de Vaan<ref name="USRE42428E">Method of and device for generating an image having a desired brightness; D.A. Stanton; M.V.C. Stroomer; A.J.S.M. de Vaan; US patent USRE42428E; 7 June 2011; https://worldwide.espacenet.com/publicationDetails/biblio?CC=US&NR=RE42428E</ref><ref>LED local dimming explained; G. Morrison; CNET.com/news; 26 March 2016; {{cite web |url=https://www.cnet.com/news/led-local-dimming-explained/ |title=LED local dimming explained |access-date=20 November 2017 |url-status=live |archive-url=https://web.archive.org/web/20171123124120/https://www.cnet.com/news/led-local-dimming-explained/ |archive-date=23 November 2017 }}</ref><ref>Pixel-by-pixel local dimming for high dynamic range liquid crystal displays; H. Chen; R. Zhu; M.C. Li; S.L. Lee and S.T. Wu; Vol. 25, No. 3; 6 February 2017; Optics Express 1973; https://www.osapublishing.org/oe/viewmedia.cfm?uri=oe-25-3-1973&seq=0</ref>).

Using PWM (pulse-width modulation, a technology where the intensity of the LEDs are kept constant, but the brightness adjustment is achieved by varying a time interval of flashing these constant light intensity light sources<ref>Dimming options for LCD brightness; J. Moronski; Electronicproducts.com; 3 Januari 2004; {{cite web |url=http://www.electronicproducts.com/Optoelectronics/Dimming_options_for_LCD_brightness_control.aspx |title=Dimming options for LCD brightness control |access-date=20 November 2017 |url-status=live |archive-url=https://web.archive.org/web/20170728120219/http://www.electronicproducts.com/Optoelectronics/Dimming_options_for_LCD_brightness_control.aspx |archive-date=28 July 2017 |date=March 2004 }}</ref>), the backlight is dimmed to the brightest color that appears on the screen while simultaneously boosting the LCD contrast to the maximum achievable levels

If the frequency of the pulse-width modulation is too low or the user is very sensitive to flicker, this may cause discomfort and eye-strain, similar to the [[Cathode-ray tube#Flicker|flicker of CRT displays]].<ref>[http://forums.lenovo.com/t5/X-Series-Tablet-ThinkPad-Laptops/Flickering-LED-Screen-on-my-X200-Tablet/m-p/111086 Flickering LED Screen on my X200 Tablet] {{webarchive|url=https://web.archive.org/web/20101129221157/http://forums.lenovo.com/t5/X-Series-Tablet-ThinkPad-Laptops/Flickering-LED-Screen-on-my-X200-Tablet/m-p/111086 |date=29 November 2010 }} Post on Lenovo's support forum, 17 March 2009</ref><ref>[http://forum.tabletpcreview.com/lenovo-ibm/22598-migraine-headaches-led-backlighting-x200t.html Migraine headaches from LED backlighting in x200t] {{webarchive|url=https://web.archive.org/web/20110716183008/http://forum.tabletpcreview.com/lenovo-ibm/22598-migraine-headaches-led-backlighting-x200t.html |date=16 July 2011 }} Post on Lenovo's support forum, 12 March 2008</ref>  This can be tested by a user simply by waving a hand or object in front of the screen.  If the object appears to have sharply defined edges as it moves, the backlight is strobing on and off at a fairly low frequency.  If the object appears blurry, the display either has a continuously illuminated backlight or is operating the backlight at a frequency higher than the brain can perceive.  The flicker can be reduced or eliminated by setting the display to full brightness, though this may have a negative impact on image quality and battery life due to increased power consumption.