P3D Heat Study part 2 of 7 (IR vs Heat)

[Tom Hubin <thubin@xxxxxxxxx>]

Hello,

As promised, here is part 2 of 7.

Tom Hubin
thubin@xxxxxxxxx
AO Systems Design

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        Infrared Energy and Heat

        Infrared energy is not heat. While traveling through space it is 
        just invisible light energy. However, when it is absorbed by 
        something it usually gets converted to heat. That is when the 
        trouble begins.   

        I calculate that a typical projector lamp with a filament color 
        temperature of 3200K will convert about 9 percent of the 
        electrical energy consumed to visible light, 90 percent to 
        infrared light, and 1 percent to ultraviolet light. All energy 
        absorbed by the film will be converted to heat. It is 
        unavoidable that the visible energy absorbed is converted to 
        heat. But this heat is of little consequence when compared to 
        the heat that can be caused by absorption of infrared energy. 
        Infrared energy in a slide projector is nothing but trouble. 
        
        When film and mounts overheat it is likely due to excessive 
        infrared energy absorption. In the future we may use light 
        sources that do not waste power on unwanted and destructive 
        infrared energy. For now it is customary to filter the infrared 
        out of the light path somewhere in the condenser optics. An 
        infrared absorbing filter can be used to absorb most of the 
        infrared energy while passing most of the visible light. In so 
        doing, the filter gets very hot. The temperature of the first 
        surface can easily exceed 200C (392F), even with forced air 
        cooling. If the filter does its job, very little infrared energy 
        will reach the film and as a result only the visible light 
        absorbed by the film will cause any heating on the film. 

        The infrared absorbing filter is often referred to as heat 
        absorbing glass. This is a misnomer. The glass absorbs infrared 
        energy and radiates heat. The notion is that it took the heat 
        away from the film.


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