Generazione dell'immagine

Having traced the video processing and illumination paths from their origins, we now come to where they meet: the light engine. This is the heart of the beast. As with the optical portion of the illumination system, we see significant differences here caused by the different panel technologies.

LCD and LCoS panels modulate pixel intensity by controlling the polarization of the light passing through their liquid-crystal material. Since this is not an optics course, I will say only that in combination with other optical elements, this allows the amount of light passing to the screen for each pixel to be decreased from the full intensity possible with the lamp. With an LCD panel, the light is transmitted through the device from one side to another, exiting in the same direction it entered. With an LCoS device, the light reflects off an integral mirror surface after passing through the liquid crystal, causing it to exit in the opposite direction it came in.

In either case, a prism assembly combines the light from the red, green, and blue panels to form a full-Colour image and direct it to the screen. In an LCoS projector, the prism assembly does double-duty, since it also separates the white light from the lamp into its red, green, and blue components. The prism assembly is usually the largest element in a three-panel light engine, with the panels mounted around it. Flexible ribbon cables connect the panels to either the main circuit board or a panel driver board, if the latter exists.

A three-chip DMD projector also uses a prism assembly in its light engine. However, while this performs the same basic functions of separating the Colours from the lamp and combining the modulated images, it works on somewhat different optical principles. DMDs use tiny, hinged mirrors for each pixel; each can flip to one side of the hinge or the other, to display black or white. Light reflected off a mirror is thus either sent to the screen, through a projection lens, or to a light "trap" inside the projector. The geometry of the prism assembly takes the angular difference between these two directions into account.

The optics in the light engine of a single-chip DMD projector is much simpler. With the Colour wheel providing sequentially separated Colours, there is no need for dichroic filters. If the projector doesn't require the ability to shift the image on the screen by moving its projection lens, the engine does not even need a prism assembly. This configuration is typical of the smallest and lightest projector species.