Projection display throughput: Efficiency of optical transmission and light-source collection

The optical system for a projection display based on three miniature reflective spatial light modulators (SLMs) is described. The total projection display light throughput is a function not only of the optical system efficiency but also of the light-collection and light-coupling efficiency referred to here as the lamp-SLM coupling. The optical system efficiency is the transmission of the optical components in the projection display. These are examined in detail through measurements and estimates of the components in the system. The various optical components include UV-IR filtering, illumination optics, polarization optics, color separation and recombination optics, SLM efficiency, and projection optics. The lamp-SLM coupling, which is the amount of usable light that can be collected from a particular lamp coupled to the projection optical system, is determined by the light-source luminance, the efficiency of the light-collection optics, and the optical system étendue. For small SLMs, less than 50 mm diagonal, for example, the lamp-SLM coupling efficiency falls off rapidly with SLM size and optical system f-number. The dependence of this coupling efficiency on SLM size is determined from measurements of the light-collection efficiency as a function of aperture size, where the apertures are used to simulate SLMs of the same dimensions. A variety of arc lamps were investigated for use in the projection display based on IBM reflective SLM devices. The lamp-SLM coupling dependence on arc gap was determined. The measurements are used to compare various lamps and to estimate directly the throughput for the complete projection system. The SLMs used in the projection display are liquid crystal devices which utilize only one polarization of light while discarding the second. Converting the discarded polarization into useful light can in principle double the throughput of the projector. However, polarization conversion results in doubling of the size of the light source and thus produces less efficient lamp-SLM coupling, particularly for long-arc-gap lamps. Measurements and analysis of throughput enhancement by polarization conversion are presented, and the dependence on arc gap and optical system étendue is discussed.