Traditionally, liquid crystal display devices have been built as a sandwich between two glass plates, with liquid crystal in between. Virtually all LCDs sold today are built in this manner, and are placed between crossed polarizers and viewed directly. However, as faster computers facilitate the need for higher resolution, the difficulty of display fabrication is becoming an issue. Very high resolution displays are generally built using thin film transistor (TFT) technology, which involves deposition and patterning of a large number of layers of materials in order to build a dense array of transistors on one piece of glass. This process is very expensive.
An alternative way to create high-resolution images with liquid crystals involves the use of Liquid Crystal on Silicon (LCOS) devices. LCOS devices use only one glass substrate, and employ a silicon surface for the back of the display. Silicon processing technology is advanced to the point that patterning several million pixels and their related drivers on a 1-inch square section of crystal is easily done. The pixels are then generally coated with a reflective aluminum layer, and then a polyimide alignment layer. Thus, the liquid crystal industry can piggyback off of existing silicon technology to allow for a high resolution microdisplay that is easy and inexpensive to manufacture. A simple picture of the optics geometry for an LCOS system is shown in the figure below.
Figure 1. Projection geometry of an LCOS projector. The polarizers have different orientations, so they are at some distance from the LCOS cell rather than laminated to its surface as in a standard LCD.
Microdisplays are likely to be used in a wide range of applications. The two most likely uses involve virtual displays, in which a series of passive optical elements is used to project the image from the display into your eye, and in projectors. In all of these cases, color may be obtained by one of three methods:
The first method is generally not used because of the expense of patterning color filters onto such small displays. The second method is common in large projectors and projection monitors. The third technique is commonly used with virtual displays. Field sequential color (FSC) consists of separating color temporally rather than spatially. This necessitates extremely fast switching, at least 3 times as fast as a video frame – several hundred Hz. FSC is easily achieved with ferroelectric devices and micromirror devices, but these are more difficult and expensive to manufacture than nematics LCDs.
In our studies here, we have investigated the applicability of the pi-cell, a fast-switching nematic mode, to FSC LCOS displays. Results from this study may be found in the following reference:
Refrences:
Optimization of Bend Cells for Field-Sequential Color Microdisplay Applications. P. Watson, P. J. Bos, J. Gandhi, Y. Ji, M. Stefanov. Accepted, Society of Information Display Symposium (1999).