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Lets say I have a strong halogen light source which has a lot of IR radiation (e.g. a construction lamp).

If I change the transparent front glass from the enclosure with a IR filter that blocks visible light, what will it do with the heat inside the enclosure?

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If your filter absorbs the IR it will heat up the filter. If you're using some form of etalon to reflect the IR back then it will heat up the whole lamp.

In the days of my youth I used a 1kW xenon-mercury for UV irradiating materials, and we had to use a water cooled chamber containing copper sulphate solution to remove the IR before the light reached the filter. Without the water cooled chamber the lamp would destroy the filters in a few minutes.

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Meanwhile, the filter and lamp will lose energy to the surrounding air. Eventually the components will reach a temperature at which the energy being added by the lamp is balanced by the energy being lost to the air. At that point, the temperature of the components will stop rising and remain constant. – garyp Jun 19 '14 at 15:02

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