LCoS


(a.k.a D-ILA, SXRD and Reflective LCD)

Liquid crystal on silicone (LCoS) technology has a few variances that are in the market place; JVC calls its version D-ILA, while Sony has named its version SXRD. Nonetheless, all versions work in pretty much the same manner.

Think of LCoS technology as an LCD on a silicone wafer. To build on that, let's start with how a liquid crystal in a LCoS panel works.

Polarized light is first sent to the LCoS panel. Think of the polarization process as a filter that only accepts light waves that are all traveling on the same plane. In this way, if you were to put two polarized filters back-to-back that filtered the light in opposite planes, all of the light would be blocked from passing through.

This polarized light travels to the LCoS panel and passes through a pane of glass and transparent electrode to the liquid crystal inside.

Liquid crystals have the unique property of bending light as the light travels though them. In this manner, the polarized light leaves the liquid crystal, actually traveling on an entirely different plane that when it entered. In the case of an LCoS panel, the light is reflected off the silicone layer behind the liquid crystal and back out of the front of the LCoS panel.

If you apply current to liquid crystal, the crystals align themselves and will not bend the light. Instead, the light leaves the liquid crystal on the same plane that it entered.

If you align a second polarizing filter to filter the light that was reflected through the LCoS panel, you can block all light from escaping if the light waves are traveling on a plane that the filter blocks. See Figure 1.

Inside an LCoS Panel
Figure 1

Now we can talk about how the panels are used in an optical engine to produce and image.

An LCoS optical light engine incorporates three small LCoS panels (AKA SXRD or D-ILA panels; see Figure 2.), a lamp, filters and prisms to create an image.

LCoS Panel
Figure 2

White light is provided by way of a lamp. The lamp light is first passed through a dichroic mirror (a mirror that reflects only certain wavelengths of light while letting all others pass through) that splits the white light into yellow and blue light.

The yellow light is then passed on to an additional dichroic mirror that splits the yellow light into red and green light.

The red, green and blue light is then passed through polarizing filters before being sent to directional prisms that reflect light coming from a certain direction, but allow the same light to pass through if coming from a different direction. These prisms are used to bounce the light off of the three LCoS panels.

After the red, green and blue light is bounced off the LCoS panels, it is sent to another dichroic prism, which recombines the three primary colors. After the three colors are recombined, the light is passed through the main lens and out of the projector. See Figure 3.

Inside an LCoS Projector
Figure 3

Each LCoS panel controls one of the three primary colors. For example, if we were to show an image of an open green field with a blue sky, the LCoS panel that controls red would block all light from passing on to the dichroic prism, while the LCoS panel that controls blue would only allow blue light through for the areas of sky. Meanwhile, the LCoS panel for green would do the same for the areas of the green field.

Each LCoS panel has individual cells that are each controlled by a separate transparent transistor that can apply current to the liquid crystal within the cell. The resolution of the display determines the number of cells, or pixels, that the LCoS panel has.

In this way, each panel can control what color each pixel is going to be at any given moment. If a pixel needs to be white, all three panels let through light at their corresponding pixel, and the three primary colors are recombined by the dichroic prism to create white light.

All pixels working together in the projector can then paint a picture in much the same way a printer uses individual dots to make a color image on paper.