Backlight

Source: Wikipedia, the free encyclopedia.

This article is about backlights in liquid crystal displays. For the rear window of an automobile, see
Car glass. For the lighting design practice, see Backlighting (lighting design). For other uses, see Backlight (disambiguation)
.
Views of a liquid-crystal display, both with electroluminescent backlight switched on (top) and switched off (bottom)

A backlight is a form of illumination used in

cold cathode fluorescent lamps
(CCFLs).

Simple types of LCDs such as those used in

polarizing filter
and a switching one, to block the undesired light.

Many types of displays other than LCD generate their own light and do not require a backlight, for example,

cathode ray tube
(CRT), and plasma (PDP) displays.

A similar type of technology is called a frontlight, which illuminates an LCD from the front.

A review of some early backlighting schemes for LCDs is given in a report Engineering and Technology History by Peter J. Wild.[2]

Light source types

The light source can be made up of:

An ELP gives off uniform light over its entire surface, but other backlights frequently employ a diffuser to provide even lighting from an uneven source.

Backlights come in many colors.

color spectrum
.

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[

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, 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

18 parallel CCFLs as backlight for an LCD TV
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
]

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
LCD with LED matrix backlight

LED backlighting in color screens comes in two varieties: white

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.[5] When using three separate LEDs (additive color) the backlight can produce a color spectrum that closely matches the color filters in the LCD pixels 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 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 dots (QDs).[6] 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.[7][8]

TCL Corporation.[10][11]

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

Benq G2420HDB consumer display has a 49W consumption compared to the 24W of the LED version of the same display (G2420HDBL
).

To overcome the aforementioned challenges with RGB and white LED backlights an 'advanced remote phosphor' [13] LED technology has been developed by NDF Special Light Products, specifically for high-end and long-life LCD applications such as cockpit displays,[14] 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.

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. 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[15] (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[16][17][18]).

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[19]), 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

flicker of CRT displays.[20][21]
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.

Diffusers

For a non-ELP backlight to produce even lighting, which is critical for displays, the light is first passed through a lightguide (Light guide plate, LGP) - a specially designed layer of

aluminum foil
or a simple white-pigmented surface.

Reflective polarizers

The LCD backlight systems are made highly efficient by applying optical films such as prismatic structure to gain the light into the desired viewer directions and reflective polarizing films that recycle the polarized light that was formerly absorbed by the first polarizer of the LCD (invented by Philips researchers Adrianus de Vaan and Paulus Schaareman),[22] generally achieved using so called DBEF films manufactured and supplied by 3M.[23] These polarizers consist of a large stack of uniaxial oriented birefringent films that reflect the former absorbed polarization mode of the light.[24] Such reflective polarizers using uniaxial oriented polymerized liquid crystals (birefringent polymers or birefringent glue) are invented in 1989 by Philips researchers Dirk Broer, Adrianus de Vaan and Joerg Brambring.[25] The combination of such reflective polarizers, and LED dynamic backlight control[16] make today's LCD televisions far more efficient than the CRT-based sets, leading to a worldwide energy saving of 600 TWh (2017), equal to 10% of the electricity consumption of all households worldwide or equal to 2 times the energy production of all solar cells in the world.[26][27]

Power consumption

The evolution of energy standards and the increasing public expectations regarding power consumption have made it necessary for backlight systems to manage their power. As for other consumer electronics products (e.g., fridges or light bulbs), energy consumption categories are enforced for television sets.[28] Standards for power ratings for TV sets have been introduced, e.g., in the USA, EU, and Australia[29] as well as in China.[30] Moreover, a 2008 study[31] showed that among European countries, power consumption is one of the most important criteria for consumers when they choose a television, as important as the screen size.[32]

See also

References

  1. ^ U.S. patent 4,096,550: W. Boller, M. Donati, J. Fingerle, P. Wild, Illuminating Arrangement for a Field-Effect Liquid-Crystal Display as well as Fabrication and Application of the Illuminating Arrangement, filed 15 October 1976.
  2. ^ "First-Hand Histories: Liquid Crystal Display Evolution - Swiss Contributions". Engineering and Technology History Wiki. Archived from the original on 3 July 2017. Retrieved 30 June 2017.
  3. ^ "What is LED TV?". Ledtele.co.uk. Archived from the original on 11 February 2012. Retrieved 19 February 2012.
  4. ^ The Evolution of LED Backlights; Adam Simmons; PCM PC monitors, Monitor articles, 12 November 2017; "The Evolution of LED Backlights | PC Monitors". Archived from the original on 1 December 2017. Retrieved 27 November 2017.
  5. ^ 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?
  6. ^ "QDEF". Quantum Dot Pioneers. Archived from the original on 29 May 2014.
  7. ^ Cadmium-free quantum dot display. avantama.com. Retrieved 17 August 2019
  8. ^ IEEE Spectrum, 2012, 8, p.11-12, Quantum Dots Are Behind New Displays
  9. ^ "QD Vision Displays". Archived from the original on 2 September 2013. Retrieved 23 July 2013.
  10. ^ IEEE Spectrum: CES 2015 - What the Heck are Quantum Dots?, 2 January 2015 Archived 13 January 2015 at the Wayback Machine
  11. ^ IEEE Spectrum: CES 2015 - Placing bets on the New TV Technologies. 7 January, 2015 Archived 28 January 2017 at the Wayback Machine
  12. ^ "White Light LEDs - Importance of measurement standards" (PDF). Archived (PDF) from the original on 25 February 2012. Retrieved 19 February 2012.
  13. ^ "ARPHOS®, a revolution in LCD backlights". Archived from the original on 19 September 2016. Retrieved 29 July 2016.
  14. ^ "Technology Development of Remote Phosphor for Avionic Cockpit Displays". Archived from the original on 15 August 2016.
  15. ^ LED TVs: 10 things you need to know; David Carnoy, David Katzmaier; CNET.com/news; 3 June 2010; "LED TVs: 10 things you need to know". Archived from the original on 1 December 2017. Retrieved 22 November 2017.
  16. ^ a b 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
  17. ^ LED local dimming explained; G. Morrison; CNET.com/news; 26 March 2016; "LED local dimming explained". Archived from the original on 23 November 2017. Retrieved 20 November 2017.
  18. ^ 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
  19. ^ Dimming options for LCD brightness; J. Moronski; Electronicproducts.com; 3 Januari 2004; "Dimming options for LCD brightness control". March 2004. Archived from the original on 28 July 2017. Retrieved 20 November 2017.
  20. ^ Flickering LED Screen on my X200 Tablet Archived 29 November 2010 at the Wayback Machine Post on Lenovo's support forum, 17 March 2009
  21. ^ Migraine headaches from LED backlighting in x200t Archived 16 July 2011 at the Wayback Machine Post on Lenovo's support forum, 12 March 2008
  22. ^ Illumination system and display device including such a system; A.J.S.M. de Vaan; P.B. Schaareman; European patent EP0606939B1; https://worldwide.espacenet.com/publicationDetails/biblio?CC=EP&NR=0606939B1&KC=B1&FT=D&ND=5&date=19980506&DB=EPODOC&locale=en_EP#
  23. ^ Brochure 3M Display Materials & Systems Division Solutions for Large Displays: The right look matters; "Archived copy" (PDF). Archived (PDF) from the original on 2 August 2017. Retrieved 20 November 2017.{{cite web}}: CS1 maint: archived copy as title (link)
  24. ^ Broadband reflective polarizers based on form birefringence for ultra-thin liquid crystal displays; S.U. Pan; L. Tan and H.S. Kwok; Vol. 25, No. 15; 24 July 2017; Optics Express 17499; https://www.osapublishing.org/oe/viewmedia.cfm?uri=oe-25-15-17499&seq=0
  25. ^ Polarisation-sensitive beam splitter; D.J. Broer; A.J.S.M. de Vaan; J. Brambring; European patent EP0428213B1; 27 July 1994; https://worldwide.espacenet.com/publicationDetails/biblio?CC=EP&NR=0428213B1&KC=B1&FT=D#
  26. ^ Energy Efficiency Success Story: TV Energy Consumption Shrinks as Screen Size and Performance Grow, Finds New CTA Study; Consumer Technology Association; press release 12 July 2017; "CTA - Energy Efficiency Success Story: TV Energy Consumption Shrinks as Screen Size and Performance Grow, Finds New CTA Study". Archived from the original on 4 November 2017. Retrieved 20 November 2017.
  27. ^ LCD Television Power Draw Trends from 2003 to 2015; B. Urban and K. Roth; Fraunhofer USA Center for Sustainable Energy Systems; Final Report to the Consumer Technology Association; May 2017; "Archived copy" (PDF). Archived from the original (PDF) on 1 August 2017. Retrieved 20 November 2017.{{cite web}}: CS1 maint: archived copy as title (link)
  28. ^ "Implementing directive 2005/32/EC of the European Parliament and of the Council with regard to ecodesign requirements for televisions", 2009; "EUR-Lex - 32009R0642 - EN - EUR-Lex". Archived from the original on 17 August 2017. Retrieved 22 November 2017.
  29. ^ "EU Australia and US regulation on energy consumption in TV sets", 2008
  30. ^ "China Regulation on Energy Consumption in TV Sets", 2010
  31. ^ "International survey on the importance of the energy efficiency of TV appliances", 2008
  32. S2CID 24082090
    .

External links

Look up backlight in Wiktionary, the free dictionary.
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