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 televisions, but has since found additional uses in wavelength selective switching, structured illumination, near-eye displays and optical pulse shaping. JVC's development of LCoS is branded as Direct-drive Image Light Amplifier (D-ILA), while Sony's own development is branded as Silicon X-tal Reflective Display (SXRD). LCoS is distinct from LCD projection technology, which uses transmissive LCD panel(s) that block or allow light through. LCoS is more similar to DLP displays.
Technology
The Hughes liquid crystal light valve (LCLV) was designed to modulate a high-intensity light beam using a weaker light source, conceptually similar to how an amplifier increases the amplitude of an electrical signal; LCLV was named after the common name for the triode vacuum tube. A high-resolution, low-intensity light source (typically a CRT) was used to "write" an image in the CdS photosensor layer, which is energized by a transparent indium tin oxide electrode, driven by an alternating current source at approximately 10 mV. A CdTe light-blocking layer prevents the low-intensity writing light from shining through the device; the photosensor and light-blocking layer together form a rectifying junction, producing a DC voltage bias across the liquid crystal layer, transferring the image to the reflecting side by changing the rotation of polarization in the twisted nematic liquid crystal. On the reflecting side, a high-intensity, polarized projection light source reflects selectively from the dielectric mirror based on the polarization within the liquid crystal being controlled by the photosensor. The dielectric mirror is formed by sputtering alternating layers of TiO2 and SiO2, with the final SiO2 layer etched to align the liquid crystal material. Later development of the LCLV used similar semiconductor materials arranged in the same basic structures.
The LCLV principle is carried forward in a digital LCoS display device, which features an array of pixels, each equivalent to the reflecting side of a single LCLV. These pixels on the LCoS device are driven directly by signals to modulate the intensity of reflected light, rather than a low intensity "writing light" source in the LCLV. For example, a chip with XGA resolution has an array of 1024×768 pixels, each with an independently addressable transistor. In the LCoS device, a complementary metal–oxide–semiconductor (CMOS) chip controls the voltage on square reflective aluminium electrodes buried just below the chip surface, each controlling one pixel. Typical chips are approximately 1–3 cm (0.39–1.18 in) square and approximately 2 mm (0.079 in) thick, with pixel pitch as small as 2.79 μm (0.110 mils). A common voltage for all the pixels is supplied by a transparent conductive layer made of indium tin oxide on the cover glass.
Displays
History 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. General Electric demonstrated a low-resolution LCoS display in the late 1970s. LCLV projectors were used primarily for military flight simulators due to their large and bulky size. A joint venture between Hughes Electronics and JVC (Hughes-JVC) was founded in 1992 to develop LCLV technology for commercial movie theaters under the branding ILA (Image Light Amplifer). One example was 72.5 in (1,840 mm) tall and weighed 1,670 lb (760 kg), using a 7 kW Xenon arc lamp.
In 1997, engineers at JVC developed the D-ILA (Direct-drive Image Light Amplifier) from the Hughes LCLV, which led to smaller and more affordable digital LCoS projectors, using three-chip D-ILA devices. Although these were not as bright and had less resolution than the cinema ILA projectors, they were more portable, starting at 33 lb (15 kg). The early LCoS projectors had their challenges. They suffered from a phenomenon called "image sticking," where the image would remain on the screen after it was supposed to be gone. This was due to the mirrors sticking in their positions, which resulted in ghosting on the screen. However, manufacturers continued to refine the technology, and today's LCoS projectors have largely overcome this issue. Sony introduced its SXRD (Silicon X-tal Reflective Display) technology in 2003, with Full HD resolution. SXRD was an evolution of LCoS technology that used even smaller pixels and a higher resolution, resulting in an even more accurate image. 4K SXRD panels were developed the following year. The SXRD technology was used in Sony's high-end home theater projectors, and it quickly gained a reputation for its exceptional picture quality.
JVC introduced an updated D-ILA technology in 2006, which eliminated the need for a polarizing filter, resulting in a brighter and more vibrant image. The D-ILA technology has since become a popular choice for home theater enthusiasts. 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.
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