International projectionist (Jan-Dec 1950)

Heating of Film by High-Intensity Arcs By HUGH McG. ROSS SUPPLEMENTING THE DATA ON MEANS FOR REDUCING THE HEAT PRODUCED BY HIGH-INTENSITY ARCS THAT HAS APPEARED IN THESE PAGES ARE THE APPENDED EXCERPTS FROM A PAPER PRESENTED RECENTLY BEFORE THE BRITISH KINEMATOGRAPH SOCIETY AND PUBLISHED IN THE JOURNAL OF THE SOCIETY. IN ORDER to reduce the heating effect of a high-intensity arc on film, it is clearly desirable to remove the infrared radiation and, perhaps, also the ultra-violet radiation. Water, even in a very thin layer, is virtually opaque to radiation above 2.35 microns (1 micron 1 one-thousandth of a mm.). This is of particular value in keeping cool the lenses and other parts of the optical system, because crown glass begins to absorb quite heavily at longer wave-lengths than this. Perhaps the most convenient infra-red filter is provided by type ON20 heat-absorbing glass made by Chance Bros. This is almost colorless and it absorbs well in the near infra-red. Its transmission increases slightly as the wave-length increases and only becomes negligible above 3.4 microns. For many applications this type of filter is suitable, and the heat which it absorbs may be dissipated by natural convection to the air, or the glass may be cooled by blowing air on it. Combined Heat-Absorbing Cell, Lens It is probable, however, that the most suitable infra-red filter for practical use in high-power projectors is a combination of a thin layer of water with a sheet of ON20 glass. This absorbs well in the near infra-red, provides full protection to the lenses, and is reliable and stable in use. Figure 1 shows diagramatically such a filter combined with the first condenser lens of a process projector. The arc runs at 300 amps, with a 16-mm positive carbon, and 2% inches away from this is the front window of the cell S1/^ inches diameter. This window is made of quartz in order that it may readily withstand the heat of the arc and its flame. On the other side of the quartz there is a narrow water channel, the water being constantly circulated by a pump and motor. The efficiency of cooling the quartz, and the whole front metal plate of the cell facing the arc, is so good that after running the arc at 300 amps for 30 minutes it is possible to switch off, open the arc door and immediately hold one's hand against the quartz. A further consequence is that the cool quartz window is relatively little damaged by spatter from the arc, and being quite easily replaced, it acts instead of a spatter glass. Second Dual-Purpose Channel The light, after passing through the quartz and first water channel, passes through a sheet of ON20 glass and into a second water channel. The water channels serve the double purpose of cooling the ON20 glass and also contribute to the filtering of the light. In particular, the first water channel absorbs much of the infra-red radiation, thereby reducing the amount which the ON20 glass has to absorb. If only a heat-absorbing cell were required, the farther window could be made of optical glass. But in this present example the light next passes into the first condenser lens of the optical system. Because the filter has removed the radiation of wave-lengths which might be absorbed by the glass, the lens does not get unduly hot, therefore it is made of crown glass for best optical performance. Similarly, the other lenses of the system are not heated seriously. Only about 3 pints of distilled water to w*re» ♦ 1 MAINS A A circulate around the cell, and of course the heat which passes into the water has to be continuously removed. At each end of the cell are cylindrical tanks which ensure that the stream of water is evenly distributed across the aperture, and in one of those there is a coil of copper tube. Cold tap-water passes through this (and is used also to cool the watercooled jaws of the arc) and the heat from the circulating water is transferred to the tap-water which runs to waste. Of course, none of the tap-water gets into the cell. Efficiency of Heat-Absorbing Cell The total amount of heat taken up by this cell is 3600 watts. About one-third of this is unwanted radiation removed from the light beam, and the remainder is radiated on to the metalwork of the cell which is in the hot lamphouse. It is estimated that such a cell and lens absorbs only 10% more of the visible light, compared with an ordinary lens. This is, in effect, recovered by avoiding the use of a separate spatter glass. It may be added that the measured absorption of visible light from an arc by a simple cell comprising two glass windows and two inches of water is about 15%. The addition of 2-mm thickness of ON20 glass to such a cell absorbs only a further 6% of visible light from an arc, due to low reflection losses. An ON20 glass filter in air absorbs about 12% of the visible light from a tungsten bulb. Luminous Efficiency of Radiation When a beam of radiation is absorbed in any way, the energy of the radiation is converted into heat: Unless there is sufficient cooling, the temperature of the object absorbing the radiation will therefore rise. Contrary to what is often implied, it makes no difference to the heating effect whether this radiation is in the infra-red, the visible, or the ultraviolet regions of the spectrum. One way of looking at this is to realize that just because the human eye can see radiation of certain wave-lengths, this is no reason for these wave-lengths to lose their heating effect. Consequently, in FIG. 1. Combined heatabsorbing filter and conEmotorC denser lens for a projector. Note set up of electric motor, circulating pump and coil for the water-cooling. INTERNATIONAL PROJECTIONIST April 1950 17