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  • doddel commented on claudiopolis's instructable LED Projector Lamp v.2.09 months ago
    LED Projector Lamp v.2.0

    The multiple LED lightsources with a single reflector system do create light of course but most of the light will not make it through the projection system. This is due to the so called 'Etendue' of any optical system, the product of the area through which the light traverses or from where it originates and the spatial angle it occupies. A loss-less system can only exist if the source's etendue is equal to or smaller than that of the rest of the optical system. If the etendue of the source is much larger than that of the optical system, most of the light will not be guidable through the system. The projection system has been designed for a high pressure arc lamp that is characterized by an enormous luminance, a light flux emanating from nearly a point, i.e. a very small surface. These l...see more »The multiple LED lightsources with a single reflector system do create light of course but most of the light will not make it through the projection system. This is due to the so called 'Etendue' of any optical system, the product of the area through which the light traverses or from where it originates and the spatial angle it occupies. A loss-less system can only exist if the source's etendue is equal to or smaller than that of the rest of the optical system. If the etendue of the source is much larger than that of the optical system, most of the light will not be guidable through the system. The projection system has been designed for a high pressure arc lamp that is characterized by an enormous luminance, a light flux emanating from nearly a point, i.e. a very small surface. These lamps have many negatives (short lived, UV, explosion, modest energetic efficiency) but are hard to beat when it comes to their optical geometry, all the light comes from one small point and facilitates an optical system to capture, guide, and image it. The Luminance of a high pressure Mercury arc lamp is in the order of 4 Giga Candela per m2. That of a LED about 8 Mega Candele per m2. A factor of 500 less!So when it comes to converting electrical energy into light energy without many contraints where that light goes, the LED wins (lumens per Watt) but when it comes to creating light that is useful in a projection system the LED is far inferior. Conclusion:Most projection systems are designed such that they operate, even with a high pressure arc lamp, in an etendue situation that is already not lossless.So the total light emitting surface of the LED should not be bigger than that of the arc in the arc lamp. Assuming for simplicity that the arc is a little sphere with radius 0.5 mm, its surface is 4 * pi * 0.5² = appr. 3 mm².Unfortunately from such a small surface a LED can only produce a few lumens. Had this not been the case of course all projectors would have been using LEDs to start with. Latest solid-state trend is the use of laser diodes. They achieve much higher luminances than LEDs can but are costly at the power levels needed.

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