Increased lumens per etendue by combining pulsed light-emitting diodes

LED-based projectors have numerous advantages compared to traditional projectors, such as compactness, larger color gamut, longer lifetime, and lower supply voltage. As LEDs can switch rapidly, there is the possibility to pulse. However, there is also an important disadvantage. The optical power per unit of etendue of an LED is significantly lower than, e. g., an ultra-high-performance (UHP) lamp. This problem can be remedied partly by pulsing the LEDs. If one drives an LED with a pulsed current source, the peak luminance can be higher, albeit the average luminance will not increase. By pulsing two LEDs alternately (50% duty cycle), their increased flux can be added up in time and will generate a higher average flux within the same etendue. We combine the LEDs with a polarizing beam splitter (PBS) and change the polarization of one LED with a switchable retarder. The achieved substantial net gain after all losses is 36%. (c) 2006 Society of Photo-Optical Instrumentation Engineers.