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40
BRITISH {CINEMATOGRAPHY
Vol. 16, No. 2
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.
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 contributing 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 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 wavelengths which might be absorbed by the glass, the lens
TO WATER
MAINS
Fig. 3. Combined heat-absorbing filter and condenser lens for a projector.
does not get unduly hot and it is therefore 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 circulate round 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 water-cooled 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 sets into the cell.
Efficiency of Heat-Absorbing Cell
The total amount of heat taken up by this cell is 3,600 watts. About onethird of this is unwanted radiation removed from the light-beam, and the remainder is radiated on to the metal-work of the cell which is in the hot lamphouse.