P3D Heat Study part 7 of 7 (Schott IR Glass)

[Tom Hubin <thubin@xxxxxxxxx>]

Hello,

And finally, part 7 of 7. There is more math here than in other parts of
this report. But it would not make sense to send only this part to T3d.

Please comment and discuss this report freely. I am even interested in
spelling errors.

As far as the percentage of projector lamp power that becomes UV, IR, or
visible light, I made an engineering estimate applying Plank's Law for
blackbody radiation. If you have better numbers please provide me with
them and some kind of reference or justification.

I am still looking for off-the-shelf replacements for the IR filters. I
found 63mm diameter and 6mm thick KG1 glass for the Carosel projector
for $30 each. But these would require machining of the mounting hardware
to accomodate the larger diameter and thicker plate. So it is not the
drop-in replacement that I want.

Tom Hubin
thubin@xxxxxxxxx
AO Systems Design  

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        Schott Infrared Filter Glasses

        A typical projector lamp filament with a 3200K color temperature 
        converts about 9 percent of the electrical energy to visible 
        light, 1 percent to ultraviolet light and 90 percent to infrared 
        light. In other words, the invisible light energy radiated by 
        the filament is about 10 times the visible light energy 
        radiated. To avoid undesirable heating of the slide film, the 
        ideal light source would radiate 100 percent visible energy and 
        no invisible energy. As a practical matter, however, something 
        close to 90 percent visible energy and 10 percent invisible 
        energy would be acceptable.   

        A 3mm thick plate of Schott KG3 glass transmits more than 82 
        percent over most of the ultraviolet and visible spectrum and 
        less than 0.035 percent over most of the infrared spectrum. The 
        remainder of the light is absorbed by the glass and converted to 
        heat. Combining the lamp spectral characteristics with the KG3 
        glass spectral characteristics we can expect the light 
        illuminating the film to be composed of less than 10 percent 
        ultraviolet, more than 89 percent visible, and less than 1 
        percent infrared.      

        A 3mm thick plate of Schott KG1 glass transmits more than 86 
        percent over most of the ultraviolet and visible spectrum and 
        less than 1.1 percent over most of the infrared spectrum. 
        Combining the lamp spectral characteristics with the KG1 glass 
        spectral characteristics we can expect the light illuminating 
        the film to be composed of less than 9 percent ultraviolet, more 
        than 80 percent visible, and less than 11 percent infrared. 

        Either of these glasses would be acceptable for a 3mm thick 
        filter. For either glass type, the invisible light energy 
        reaching the film is much less than the visible light. However, 
        the KG1 glass might be preferable since it transmits about 5 
        percent more visible light than the KG3 glass, thereby producing 
        an image that is 5 percent brighter. 
        
        The light energy absorbed by infrared absorbing filters is 
        converted to heat. Even with forced air cooling, these glass 
        plates can reach temperatures in excess of 200C (392F) in a 
        condenser system for a 500 watt lamp. These glasses are rated to 
        withstand a temperature of 250C (482F) if not tempered and 300C 
        (572F) if tempered. For this reason it is customary to use 
        tempered glass in high powered projectors.


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