What exactly is a "heat filter" in the lamphouse, and what function does it serve?

The manual for my 4000 watt ORC lamphouse says it should have one in the lamphouse, but I can't find it. I've looked and explored inside the lamphouse and all I've found is the bulb, the reflector, the douser, and some dead bugs.

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Smaller projectors (e.g., slide projectors and 16mm projectors) often used a "heat glass" like a Pittsburgh 2043 filter to absorb unwanted infrared. But all the excess energy is absorbed by the filter, so high power levels could crack or shatter the filter. The spectral "cut" of these filters was not precise, so they often removed some visible red light, making the light on the screen appear more cyan.

Theatre projector lamphouses usually use dichroic heat filters that are made by depositing very thin layers of metal (often nickel) and dielectric on a substrate, such that light passing through the coatings is separated into various wavelengths by constructive and destructive interference. For example, a metal reflector can have a dichroic coating that reflects visible light, and absorbs infrared energy. Or a piece of quartz or heat resistant glass can have a dichroic coating that transmits visible light, while reflecting IR to a heat sink.

Sometimes, multiple "heat filters" are used --- e.g., a dichroic coating on the main reflector that reflects visible light, along with a separate filter in the light beam that transmits visible, and reflects any remaining IR to a heat sink. Whatever design is used, much heat energy is removed from the light beam, so it needs to be properly managed with "heat sinks" and ventilation.

Dichroic coatings are fragile, and can be damaged by improper cleaning, reducing their efficiency. Excessive heating (i.e., poor ventilation) can also damage the coatings.
http://automatedhd.com/dichroic/coatings.htm
http://www.dichroic.com.tw/02.htm
http://www.dichroic.com.tw/03.htm

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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com

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It's always best to run a test. Using a loop of a dark-scene trailer, you should be able to run several hundred passes without visible heat damage or film deformation. Of course, a black-and-white (silver) image is most sensitive to heat damage, as it absorbs both visible and infrared energy.

I highly recommend reading the Kodak paper "Projection Performance of Theatrical Motion-Picture Films Using Xenon Short-Arc Lamps" by Paul Preo, published in the September 1983 SMPTE Journal.

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