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FIG. 1 — ■ An interference-type reflector reduces heating of the film and mechanism by reflecting only the useful light rays to the projector aperture. Invisible heat-producing infrared rays pass right through the mirror and into the rear of the lamphouse, where they are dissipated harmlessly.
FIG. 2 — When a ray of light falls upon a thin transparent film equal in thickness to the length of the light waves, or V2 this length, the waves are reinforced and totally reflected. On the other hand, if the thickness of the film is equal to % or !/4 the wavelength, "cancellation" of the reflected waves results in total transmission. These phenomena are known as "interference."
\ t \ \ t \
Thickness of thin nlm = 1 wavelength (A)
Light reflected Transmitted Reflected Transmitted
of which has a 21-inch reflector! These values of radiant flux are actually in the same heat range as those obtained with small and medium-size Suprex lamps having silvered mirrors.
The second aspect of the heat problem concerns the conduction of large amounts of heat from the metal rails and tension shoes of the film gate to the perforation margins cf the film. Thermal conduction is very deleterious to the film, causing it to warp, shrink, and become brittle. It is prevented by using water-cooled gates in the projectors — an absolute necessity in the operation of many drive-ins and other large theatres. Use of Strong TufCold reflectors in the arc lamps eliminates the need for water-cooled gates in many instances, however, by drastically reducing the heat content of the radiation focused upon the aperture plate of the projector mechanism.
Heat of Arc Radiations
An ordinary silvered mirror reflects all of the radiations of the high-intensity carbon arc — ultraviolet, visible, and infrared — about equally well. The ultraviolet wavelengths in highintensity arc radiation extend from 3250 Angstroms down to 4000 Angstroms and account for 4% of the radiant energy emitted by the arc. The region of visible radiation — light — extends from 4000 Angstroms (violet) through all the colors of the spectrum down to 7500 Angstroms (deep red), accounting for 40% of the energy. The Infrared wavelengths, which account for the remaining 56%, extend from 7500 Angstroms down to about 40000 Angstroms (but are actually rather weak beyond 20000 Angstroms).
It is obvious that complete removal of the invisible ultraviolet and infrared wavelengths from the arc radiation will reduce the heat to only 40% of its original intensity. That is, the visible light, alone, accounts for 40% of the total heating effect of high-intensity arc radiation.
Cold Mirror Best Heat Reducer
No actual heat filter or cold mirror is quite as perfect a remover of ultraviolet and infrared as this theoretical ideal; but of all the means known for reducing the heat content of arc radiation, the interference-type cold reflector is by far the most effective, as tests amply prove.
Suppose a silvered-glass reflector be used in an arc lamp, and the relative intensities of screen light and aperture heat obtained each be asigned a numerical value cf 100%. On this basis, the insertion of an absorption-type
International Projectionist
April 1961