25-08-2017, 09:32 PM
How LCoS Works
Introduction to How LCoS Works3.docx (Size: 223.27 KB / Downloads: 26)
Introduction to How LCoS Works :-
Most people grew up watching a cathode ray tube (CRT) television. These televisions, while bulky and heavy, had a great picture as long as they got a clear signal. CRT sets are still what a lot of people think of when they think of TVs.
But if you've shopped around for a TV recently, you've seen that now there are a lot more options. CRT still works well for screen sizes up to 40 inches. But if you want a larger screen, a flat panel TV, widescreen model or HDTVcompatibility, you'll have to choose from several types of sets, including liquid crystal display(LCD), digital light processing(DLP) and liquid crystal over silicon (LCoS).
LCoS isn't particularly new technology, but it wasn't readily available until recently. In this article, we'll look at the technology behind LCoS, how it provides a clear picture and how manufacturers have addressed issues with black and contrast.
Review of LCD and DLP:-
The most common use for LCoS is front- and rear-projection televisions. The setup is a lot like what you find in a DLP system. DLP uses a digital micromirror device(DMD) to create a picture using a process that's like making a mosaic out of small, square tiles. The DMD contains millions of microscopic mirrors that reflect light from a lamp. Each mirror creates onepixel of the final image.
The mirrors flip back and forth between their "on" and "off" positions very rapidly. When mirrors are on, they point toward a projection lens. The longer a mirror is in the on position, the brighter the pixel it creates. Mirrors creating black pixels remain off. In most DLP televisions, a color wheel spins between the lamp and the DMD, adding red, green and blue light to the picture. The viewer's eyes combine these colors to create the finished image.
LCoS uses a very similar idea. As with DMDs, LCoS devices are tiny -- most are less than one inch square. Both technologies are also reflective -- the devices reflect light from a source to a lens or prism that collects the light and displays the image. But instead of tiny mirrors that turn on and off, LCoS uses liquid crystals to control the amount of reflected light.
Projection and Color:-
It takes several steps to create a picture in an LCoS television. The process includes a high-intensity lamp, a series of mirrors and microdevices arranged into a cube, a prism and a projection lens. From beginning to end, here's what happens:
1. The lamp creates a beam of white light.
2. The beam passes through acondenser lens that focuses and directs the light. It also passes through a filter that only allows visible light, which helps protect the other components.
3. The white light is separated into red, green and blue light in one of two ways: The beam passes through a polarizing beam splitter (PBS), which divides the light into three beams, and those beams pass through filters that add red, green and blue. The beam passes through a series of dichroic mirrorsthat reflect some wavelengths while allowing the rest of the light to pass through. For example, the dichroic mirror can separate red light from the white light, leaving blue and green, and a second mirror can separate the green light, leaving only blue.
4. The newly created beams of colored light simultaneously come into contact with one of three LCoS microdevices - one each for red, green and blue. We'll look at exactly what happens in the devices in the next section.
5. The reflected light from the microdevices passes through a prism that combines the light.
6. Prism directs the light - which now creates a full-color image - into a projection lens, which magnifies the image and displays it on the screen.
Most rear-projection LCoS televisions use this process. Some projectors use a linear setup rather than a cube, and the white light strikes surfaces that color it red, green and blue before reaching the microdevices. A very few systems use only one microdevice along with other methods for adding color. Some examples are color wheels like those found in DLP systems or transmissive dyes on the microdevices themselves. Some systems use additional polarizers or filters to further improve picture quality and contrast.
Without the projection lens, the picture created in this process would be too small to see clearly. That's why LCoS technology falls into the category of microdisplays -- displays that are too small to see without some kind of magnification.
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 siliconsemiconductor. 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.