Microdisplay-based systems
Microdisplay-based projectors may function in two ways: reflective (such as DMD-based projectors) and transmissive (such as HTPS-based systems). A third hybrid transmissive/reflective technique is provided by LCoS in which a mirror is placed behind the transmissive LC pixel to reflect the light back through the pixel.
A microdisplay-based system consists of two parts: a light source or illumination assembly and a light engine. The illumination assembly con- tains a lamp and appropriate filters, mirrors and lenses to provide light waves to the light engine. The light engine contains the microdisplay and all other necessary splitting, combining and focusing optical arrange- ments to project a full colour image on a screen.
There are two ways of producing the primary colours from pure white light emerging from the illumination assembly: the first is to use R, G and B filters, and the second is to split the light wave into its three primary compo- nents, red, green and blue using dichroic mirrors. The use of filters provide a simple method of obtaining colour, but it does reduce the light intensity and hence brightness of the image. The filterless technique normally requires three microdisplays, making the system bulkier and more expensive.
The illumination assembly
A simplified light source assembly is shown in Figure 19.4. The light source could be an incandescent filament, a tungsten-halogen, fluorescent, mercury, metal halide or arc-lamp devices. Each type offers different white tints, luminance per wattage and lifetime.
The light from the lamp is focused by an elliptical reflector onto a set of ‘relay’ lenses forming a telecentric optical system. A ‘cold’ mirror, one that does not reflect UV and IR waves, oriented at 45º and a UV/IR filter is placed between the first two relay lenses to remove IR and UV radiation from the light beam. A prism plate component may be used between the second and third relay lenses to recover some of the light that would otherwise be rejected by the polariser in the light engine. This component converts about 55% of the ‘wasted’ light into useable light.