Heat protection filter
A heat protection filter is an infrared blocking filter, usually a pane of glass that absorbs infrared radiation , in the beam path of projectors (including slide projectors and film projectors ) between the projection lamp and the film plane or slide .
Projectors often work with incandescent or halogen lamps. These, in particular, but also other illuminants, have a high proportion of infrared radiation which, without a heat protection filter, would lead to excessive heating of the film or slide. Both the actual slide and the plastic frames are sensitive to heat, the frames can warp, especially glassless slides can deform and fade during projection.
The heat protection filter holds back a large part of the heat radiation emitted by the lamp in the near infrared . Mid infrared is already absorbed by the condenser lenses .
Usually colored glass panes are used as heat protection filters. These also have a certain absorption in the red spectral range and therefore cause a welcome increase in the color temperature . For very high outputs, dichroic infrared cut filters are also used; these do not show any self-heating, but rather reflect the infrared radiation component back into the lamp housing. Older slide and cinema projectors sometimes used duckweed or duckweed cuvettes to absorb the infrared radiation.
There is a concave mirror behind the lamp. This can be designed as a cold light mirror ( dichroic mirror ) for thermal protection , whereby heat radiation passes through it and is not directed forward. This measure is not referred to as a heat protection filter, but can be used alone or as a supplement to a heat protection filter.