LCOS

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 referred to as a spatial light modulator. LCoS was initially developed for projection televisions but is now used for wavelength selective switching, structured illumination, near-eye displays and optical pulse shaping. By way of comparison, some LCD projectors use transmissive LCD, allowing light to pass through the liquid crystal.

In an LCoS display, a CMOS chip controls the voltage on square reflective aluminium electrodes buried just below the chip surface, each controlling one pixel. For example, a chip with XGA resolution will have 1024x768 plates, each with an independently addressable voltage. Typical cells are about 1–3 centimeters square and about 2 mm thick, with pixel pitch as small as 2.79 μm. 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

General Electric first demonstrated a low-resolution LCoS display in the late 1970s. Starting in the late 1990s a number of companies attempted to develop products for both near-eye and projection applications.

At the 2004 CES, Intel announced plans for the large scale production of inexpensive LCoS chips for use in flat panel displays. These plans were cancelled in October 2004. Sony has made it to market (December 2005) with the Sony-VPL-VW100 or "Ruby" projector, using SXRD, 3 LCoS chips each with a native resolution of 1920×1080, with a stated contrast ratio of 15,000:1 using a dynamic iris.

Whilst LCoS technology was initially touted as a technology to enable large-screen, high-definition, rear-projection televisions with very high picture quality at relatively low cost, the development of large-screen LCD and plasma flat panel displays obsoleted rear projection televisions. As of October 2013, LCoS-based rear-projection televisions are no longer produced.

Commercial implementations of LCoS technology include Sony's Silicon X-tal Reflective Display (SXRD) and JVC's Digital Direct Drive Image Light Amplifier (D-ILA/). Every company which produces and markets LCoS rear-projection televisions uses three-panel LCoS technology,. Sony and JVC both produce and market front-projection displays that use three LCoS panels, as well as Canon, with XEED and REALiS projectors.

Developers and manufacturers who have left the LCoS imaging market include: Intel, Philips, MicroDisplay Corporation (the only company to successfully bring to market a single-panel LCoS television ), S-Vision, Colorado Microdisplay, Spatialight, Syntax-Brillian.

Display system architectures

There are two broad categories of LCoS displays: three-panel and single-panel. In three-panel designs, there is one display chip per color, and the images are combined optically. In single-panel designs, one display chip shows the red, green, and blue components in succession with the observer's eyes relied upon to combine the color stream. As each color is presented, a color wheel (or an RGB LED array) illuminates the display with only red, green or blue light. If the frequency of the color fields is lower than about 540 Hz, an effect called color breakup is seen, where false colors are briefly perceived when either the image or the observer's eye is in motion. While less expensive, single-panel projectors require higher-speed display elements to process all three colors during a single frame time, and the need to avoid color breakup makes further demands on the speed of the display technology.

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 polarized by beam splitters.

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. There were single-panel LCoS displays in production: One by Philips and one by Microdisplay Corporation. Forth Dimension Displays continues to offer a Ferroelectric LCoS display technology (known as Time Domain Imaging) available in QXGA, SXGA and WXGA resolutions which today is used for high resolution near-eye applications such as Training & Simulation, structured light pattern projection for AOI. Citizen Finedevice (CFD) also continues to manufacturer single panel RGB displays using FLCoS technology (Ferroelectric Liquid Crystals). They manufacture displays in multiple resolutions and sizes that are currently used in pico-projectors, electronic viewfinders for high end digital cameras, and head-mounted displays.

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 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 viewfinders for digital cameras, film cameras, and 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. Google's initial foray into wearable computing, Google glass, also uses a near-eye LCoS display.

At CES 2018, Hong Kong Applied Science and Technology Research Institute Company Limited ( ASTRI) and 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.

Wavelength-selective switches

LCoS is particularly attractive as a switching mechanism in a wavelength-selective switch (WSS). LCoS-based WSS were initially developed by Australian company Engana, now part of Finisar. The LCoS can be employed to control the phase of light at each pixel to produce beam-steering where the large number of pixels allow a near continuous addressing capability. Typically, a la