The LCoS Microdevice
Instead of using liquid crystal between two polarized panels like an LCD, an LCoS microdevice has a liquid crystal layer between one transparent thin-film transistor (TFT) and one silicon semiconductor. The semiconductor has a reflective, pixilated surface. The lamp shines light through a polarizing filter and onto the device, and the liquid crystals act like gates or valves, controlling the amount of light that reaches the reflective surface. The more voltage a particular pixel's crystal receives, the more light the crystal allows to pass. It takes several layers of different materials to do this.
From the bottom to the top, here are the components of an LCoS microdevice and what they do:
- Printed circuit board (PCB): carries instructions and electricity from the television to the device
- Silicon (a chip or sensor): controls the liquid crystal, generally with one transistor per pixel, using data from the television's pixel drivers
- Reflective coating: reflects light to create a picture
- Liquid crystal: controls the amount of light that reaches and leaves the reflective coating
- Alignment layer: keeps the liquid crystals properly aligned so they can direct the light accurately
- Transparent electrode: completes the circuit with the silicon and the liquid crystal
- Glass cover: Protects and seals the system
The exact materials and configurations differ from manufacturer to manufacturer. Some use nematic liquid crystals and others use ferroelectric crystals. Some use organic alignment layers, which can break down through use and exposure to the high-intensity light from the lamp. Others use photosensitive materials and light to control the impulses to the liquid crystal.
In general, LCoS devices have only a very small gap between pixels. The pixel pitch -- the horizontal distance between one pixel and the next pixel of the same color -- is between 8 and 20 microns (10-6). This reduces or eliminates the "screen door" effect found on some DLP televisions and helps keep the image smooth and uniform.
The system generally creates a good picture, but it does have some pros and cons. We'll look at those next.