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{{Short description|
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'''Liquid crystal on silicon''' ('''LCoS''' or '''LCOS''') is a miniaturized reflective [[active-matrix liquid-crystal display]] or "microdisplay" using a [[liquid crystal]] layer on top of a silicon backplane. It is also known as a [[spatial light modulator]]. LCoS initially was developed for [[projection television]]s, but has since found additional uses in [[wavelength selective switching]], [[structured illumination]], near-eye displays and optical pulse shaping.
LCoS is distinct from other [[LCD projector]] technologies which use transmissive [[LCD]], allowing light to pass through the
==Technology==
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The history of LCoS projectors dates back to June 1972, when LCLV technology was first developed by scientists at [[Hughes Research Laboratories]] working on an internal research and development project.<ref>{{cite report |url=https://apps.dtic.mil/sti/citations/ADA010553 |title=Development of a Reflective Mode Liquid Crystal Light Valve |author1=Jacobson, A.D. |publisher=Hughes Research Labs |date=May 1975 |access-date=16 January 2024}}</ref> General Electric demonstrated a low-resolution LCoS display in the late 1970s.<ref>{{cite book |author1=Armitage, David |author2=Underwood, Ian |author3=Wu, Shin-Tson |date=2006 |title=Introduction to Microdisplays |publisher=Wiley |isbn=978-0-470-85281-1}}</ref> LCLV projectors were used primarily for military [[flight simulator]]s due to their large and bulky size.<ref>{{cite report |url=https://apps.dtic.mil/sti/citations/ADA209580 |title=Display Characteristics of Example Light-Valve Projectors |author=Howard, Celeste M. |date=June 1989 |publisher=University of Dayton Research Institute |access-date=17 January 2024}}</ref> A joint venture between [[Hughes Electronics]] and [[Japan Victor Corporation|JVC]] (Hughes-JVC) was founded in 1992<ref name=JVCPro-pr>{{cite press release |url=http://pro.jvc.com/pro/vsd/jvchjt.htm |title=JVC consolidates projector operations with absorption of Hughes-JVC |date=December 16, 1999 |publisher=JVC Professional |access-date=16 January 2024}}</ref> to develop LCLV technology for commercial movie theaters under the branding ILA (Image Light Amplifer).<ref>{{cite web |url=http://pro.jvc.com/pro/hjt/technology/download/sid99.pdf |title=Electronic Cinema Using ILA Projector Technology |author1=Sterling, R.D. |author2=Bleha, W.P. |publisher=Hughes-JVC Technology Corporation |access-date=16 January 2024}}</ref> One example was {{cvt|72.5|in}} tall and weighed {{cvt|1670|lb}}, using a 7 kW [[Xenon arc lamp]].<ref>{{cite web |url=http://pro.jvc.com/pro/hjt/products/ila12k.html |title=ILA-12K Projector |website=JVC Professional |access-date=16 January 2024}}</ref>
[[File:Lcos.svg|thumb|right|upright=1.5|Conceptual diagram of an LCoS projector
In 1997, engineers at JVC developed the D-ILA (Direct-Drive Image Light Amplifier) from the Hughes LCLV,<ref name=JVCPro-pr/><ref>{{cite conference |doi=10.1117/12.305518 |title=Reflective active-matrix LCD: D-ILA |author1=Nakano, Atsushi |author2=Honma, Akira |author3=Nakagaki, Shintaro |author4=Doi, Keiichiro |date=1998 |conference=Photonics West / Electronic Imaging |location=San Jose, California |publisher=Society of Photo-Optical Instrumentation Engineers}}</ref> which led to smaller and more affordable digital LCoS projectors, using three-chip D-ILA devices.<ref>{{cite conference |doi=10.1117/12.497532 |title=D-ILA technology for high-resolution projection displays |author1=Bleha, William P. |author2=Sterling, Rodney D. |publisher=Society of Photo-Optical Instrumentation Engineers |conference=AeroSense |date=2003 |location=Orlando, Florida}}</ref> Although these were not as bright and had less resolution than the cinema ILA projectors, they were more portable, starting at {{cvt|33|lb}}.<ref>{{cite web |url=http://pro.jvc.com/pro/special/dila/pdf_u/DLA_G11_U.pdf |title=D-ILA Projector: DLA-G11 |publisher=JVC Professional |date=November 1999 |access-date=16 January 2024}}</ref>
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LCoS projectors have continued to evolve, with manufacturers introducing features like [[4K resolution]] and HDR ([[High Dynamic Range]]) support. LCoS projectors are now available at a range of price points, from affordable models for home theater use to high-end professional models used in commercial installations.
===Display system architectures===
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The optical system is responsible for directing the light from the light source onto the LCos panel and projecting the resulting image onto a screen or other surface. The optical system consists of a number of lenses, mirrors, and other optical components that are carefully designed and calibrated to provide the necessary magnification, focus, and color correction for the display system.
====Three-panel designs====
The white light is separated into three components (red, green and blue) and then combined back after modulation by the 3 LCoS devices. The light is additionally [[Polarization (waves)|polarized]] by [[beam splitter]]s.
====One-panel designs====
Both Toshiba's and Intel's single-panel LCOS display program were discontinued in 2004 before any units reached final-stage prototype.<ref>{{cite web|last=Hachman|first=Mark|title=Update: Intel Cancels LCOS Chip Plans|url=http://www.extremetech.com/extreme/73648-update-intel-cancels-lcos-chip-plans|work=415.992.5910|publisher=Extreme Tech|access-date=June 17, 2011}}</ref> There were single-panel LCoS displays in production: One by [[Philips]] and one by Microdisplay Corporation. [[Forth Dimension Displays]] continues to offer a [[Ferroelectricity|Ferroelectric]] LCoS display technology (known as Time Domain Imaging) available in [[graphic display resolutions#Extended Graphics Array|QXGA]], [[graphic display resolutions#Extended Graphics Array|SXGA]] and [[graphic display resolutions#Extended Graphics Array|WXGA]] resolutions which today is used for high resolution near-eye applications such as Training & Simulation, structured light pattern projection for [[Automated optical inspection|AOI]]. Citizen Finedevice (CFD) also continues to manufacturer single panel RGB displays using FLCoS technology (Ferroelectric Liquid Crystals).
===Pico projectors, near-eye and head-mounted displays===
Whilst initially developed for large-screen projectors, LCoS displays have found a consumer niche in the area of [[Handheld projectors|pico-projectors]], where their small size and low power consumption are well-matched to the constraints of such devices.
LCoS devices are also used in near-eye applications such as [[electronic viewfinder]]s for digital cameras, film cameras, and [[Head-mounted display|head-mounted displays (HMDs)]]. These devices are made using ferroelectric liquid crystals (so the technology is named FLCoS) which are inherently faster than other types of liquid crystals to produce high quality images.<ref>{{cite journal|author=Collings, N.|title=The Applications and Technology of Phase-Only Liquid Crystal on Silicon Devices|doi=10.1109/JDT.2010.2049337|journal=IEEE Journal of Display Technology|volume= 7|issue= 3|pages=112–119|year=2011|bibcode=2011JDisT...7..112C |s2cid=34118772 }}</ref>
At [[Consumer Electronics Show|CES]] 2018, Hong Kong Applied Science and Technology Research Institute Company Limited ([[Hong Kong Applied Science and Technology Research Institute|ASTRI]]) and [[OmniVision Technologies|OmniVision]] showcased a [[reference design]] for a wireless augmented reality headset that could achieve 60 degree [[field of view]] (FoV). It combined a single-chip 1080p LCOS display and image sensor from OmniVision with ASTRI's optics and electronics. The headset is said to be smaller and lighter than others because of its single-chip design with integrated driver and memory buffer.<ref>{{Cite web|title=This AR Headset Surpasses the Field of View of HoloLens, but You Still Won't Wear It in Public|url=https://augmented.reality.news/news/ar-headset-surpasses-field-view-hololens-but-you-still-wont-wear-public-0182110/|access-date=2020-06-23|website=Next Reality|date=January 11, 2018 |language=en}}</ref>
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==Wavelength-selective switches==
LCoS is particularly attractive as a switching mechanism in a [[Wavelength selective switching|wavelength-selective switch]] (WSS).
</ref>
In operation, the light passes from a fibre array through the polarisation imaging optics which separates physically and aligns orthogonal polarisation states to be in the high efficiency s-polarisation state of the diffraction grating.
WSS based on [[MEMS]]<ref>Marom, D. M.
LCoS-based WSS, however, permit dynamic control of channel centre frequency and bandwidth through on-the-fly modification of the pixel arrays via embedded software.
==Other LCoS applications==
===Optical pulse shaping===
The ability of an LCoS-based WSS to independently control both the amplitude and phase of the transmitted signal leads to the more general ability to manipulate the amplitude and/or phase of an optical pulse through a process known as Fourier-domain pulse shaping.<ref>{{cite journal|author= Weiner, A.M.|url=https://engineering.purdue.edu/~fsoptics/articles/Femtosecond_pulse_shaping-Weiner.pdf|doi=10.1063/1.1150614|title=Femtosecond pulse shaping using spatial light modulators|journal= Rev. Sci. Instrum. |volume=71|issue=5|pages= 1929–1960 |year=2000|bibcode=2000RScI...71.1929W }}</ref>
As an example, an LCoS-based Programmable Optical Processor (POP) has been used to broaden a mode-locked laser output into a 20 nm supercontinuum source whilst a second such device was used to compress the output to 400 fs, transform-limited pulses.<ref>A. M. Clarke, D. G. Williams, M. A. F. Roelens, M. R. E. Lamont, and B. J. Eggleton, "Parabolic pulse shaping for enhanced continuum generation using an LCoS-based wavelength selective switch," in 14th OptoElectronics and Communications Conference (OECC) 2009.</ref> Passive mode-locking of fiber lasers has been demonstrated at high repetition rates, but inclusion of an LCoS-based POP allowed the phase content of the spectrum to be changed to flip the pulse train of a passively mode-locked laser from bright to dark pulses.<ref>{{cite journal|author=Schroeder, Jochen B. |title=Dark and Bright Pulse Passive Mode-locked Laser with In-cavity Pulse-shaper|journal=Optics Express |volume=18|issue= 22 |year= 2010|pages=22715–22721|pmid=21164610|doi=10.1364/OE.18.022715|bibcode=2010OExpr..1822715S |doi-access=free}}</ref> A similar approach uses spectral shaping of optical frequency combs to create multiple pulse trains.
=== Light structuring===
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