Date: Thu, 18 Dec 1997 03:37:48 GMT Server: BESTWWWD/2.1 MIME-version: 1.0 Content-Type: text/html Content-Length: 6135 Last-modified: Tue, 24 Sep 1996 16:58:10 GMT Accept-Ranges: bytes Spatialight Inc. Applications

Spatialight Incorporated -- Applications


SLM M4-704X512 Spatial light Modulator

The Spatialight, Inc. SLM M4-704X512 is a high performance spatial light modulator suitable for display, projection, and coherent light modulation applications. The device is based on active matrix liquid crystal display technology. It is made of liquid crystal sandwiched between an electrode coated top glass and a processed silicon chip. The silicon chip is cut from a wafer, made using standard semiconductor technology. The silicon chip contains the pixel elements as well as the drive circuitry. The device is packaged on an aluminum carrier suitable for optical bench, head mount or OEM applications.

The performance of the device is outstanding for many reasons. The silicon substrate allows for a great deal of integration at very low cost. The polysilicon on glass technologies are large format and much more expensive. It is also much more difficult to produce high performance row and column circuitry. The cost of the high temperature glass thin film transistor process is much higher than that of CMOS silicon, making the silicon much more attractive.

The reflective design, a near necessity of the monocrystaline silicon technology, offers additional technical advantages. Producing the pixel elements on silicon allows the pixel transistors to be located underneath the pixels themselves. This means that the design rules are the only limit of the pixel aperture ratio, which is the ratio of total area to active pixel area. Because our design rules easily accommodate 1 micron or so, our pixel aperture ratio is 76%. That figure multiplied by the reflectivity of the pixel give an approximation of the efficiency. The efficiency of our 360,448 pixel device is over 60%. For a comparison, the Sony 103,114 pixel polysilicon device has an efficiency around 2%. One of the reasons that the efficiency is so low is that the competition must put the transistors for each pixel adjacent to the pixel. Transmissive devices can not utilize any space under the pixel, as this would block the light path. This low efficiency directly effects the lighting requirements, which will determine the power source. This is critical for projection applications. In head mount display applications, the aperture ration will limit the number of pixels, or make the device too large.

The device pixels are on a 20 micron pitch. This means that the size of the display is 14.08mm by 10.24 mm. That is very close to a standard 16mm projector format. This format is excellent for projection as well as head mount displays. It also makes the device the highest pixel count small format SLM in production today.

The coherent applications for the SLM, which include optical computing, holographic data storage, and beam steering, are the most demanding for any liquid crystal device. The wave front distortion is an important figure of merit for laser applications. Any deviations from flatness will cause interference of the reflected waves, resulting in fringe patterns, the symptom of poor wave front distortion. With flatness better than 1/4 wave, the SPATIALIGHT, INC. SLM is a compact, high performance solution to many problems. The small format of the device couples well with popular CCD formats, making system design easier. The device, being reflective is ideally suited to folded or solid optics.

The reflected light from surface of the SLM has two components, the scattered and the specular reflection. An image may be obtained from each of these sources. The zero order reflection, or the specular component may be projected without any lens when using coherent light. Most of the light reflected is in the zero order. A lens can be used to capture light in the other orders. This is the most light efficient method. The most serious problems in coherent systems are interference. This occurs when wave fronts interfere either constructively or destructively, creating bright or dark "fringes". The fringes may be eliminated by using very flat and parallel surfaces in the system. It is also important that the silicon backplane, the reflector, of the SLM is very flat across the field. The use of a wave plate, such as a 1/8 or 1/4 wave plate, is very effective at reducing or eliminating fringe patterns in liquid crystal systems.


Reflective SLMs can be used in a variety of different applications. These applications range from sunlight readable displays to high power projection systems. The reflective SLM also has uses as key elements in optical computing, hologrphic data storage, holographic reproduction, and high speed laser printing applications. All of the above uses have been demonstrated for the Spatialight, Inc. LCD Chip SLM. The use of a 1/8 wave plate is highly recommended in some applications, particularly when using coherent light.



Reflective mode SLMs require a different approach to application than transmissive type displays. These uses may not be readily apparent because of the lack of high volume use of active matrix reflective LCDs. The back surface is a specular reflector that offers a wide viewing angle. The SPATIALIGHT, INC. LCD chip SLM offers the benefits of a scatter mode direct view system as well as a specular reflective device for high power projection type systems.

SPATIALIGHT, INC. does not assume any responsibility for the use of the SLM described, no applications patent licenses are implied, and SPATIALIGHT, INC. reserves the right, without notice, to change said SLM or specifications.