Light output and light distribution over a screen was already a very important issue for slide and film projectors when it became even more critical with the emergence of digital projectors and rear-projection TVs over the past 15 years. Such projectors use liquid crystal displays (LCDs) or Texas Instruments digital micromirror devices (DMDs) to modulate image information onto the light beam, which is then finally projected to a screen. Especially during the early times, transmissivity of the displays was very low. Also, the filaments of halogen lamps or the arcs of metal-halide discharge lamps were rather bulky and did not match the dimensions and accepted apertures of the displays. In addition, spectral uniformity over the field became an issue, especially in industrial applications like business presentations and process visualization. Special measures were implemented to efficiently collect the emitted radiation from the light source and to homogenize spectral and brightness uniformity over the screen. For color images, complex optical relay systems were established to efficiently split and recombine the light into three color channels: red, green, and blue (Fig. 11.1).
New light sources with significantly smaller arc gaps and higher luminous efficacy in conjunction with the ongoing introduction of smaller imager displays enabled the development of more compact projectors (Fig. 11.2) with continuously higher light output and lower power consumption. While maintaining brightness and spectral performance, miniaturization and cost optimization became a driving force for optical system design. This allows the introduction of front and rear projection engines for large-screen consumer projection TVs, but again it reinforces the need for more compact and cost-efficient optical systems. Recent advances in light emitting diode (LED) development enable battery-powered, ultra-compact pocket projectors, which soon will be applied in consumer rear-projection TVs.
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