Abstract
The low luminance levels of light-emitting diodes (LEDs) compared to
arc lamps make it difficult to design high-brightness LED-based projectors.
Besides, the specificities of LEDs do not always allow using the same design
schemes as with arc lamp-based projection displays. This paper performs a
taxonomy of the techniques that can be used to increase the brightness of
LED-based projection displays. We show that, in étendue-limited systems,
the perceived brightness depends on the system étendue limit, the efficiency
of the light engine, and the source luminance. The ability to improve each
of these parameters depends on the design constraints. The system étendue
limit can be increased at the expense of bulkier, more complex, and more expensive
designs. The light engine efficiency can be increased by using free-form shape
components adapted to the shapes and the emission patterns of the considered
LEDs. The apparent source luminance can be increased at the expense of the
flux by either recycling light or restricting the light collection to a smaller
étendue with higher average luminance. Luminance can also be increased
by using multiple color primaries (spatial multiplexing) or pulsed LEDs (temporal
multiplexing). Finally, we review how light recycling can be implemented to
convert polarization without increasing étendue.
© 2007 IEEE
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