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520 T. M. C. LANCE November
output measured under peak white conditions dropped by about 10% when the temperature of the outside face was raised from 20 C to 100 C ; the temperature of the screen material itself under steadystate thermal conditions was then about 90 ° higher than the outside temperature.
The main difficulty in the problem of cooling the phosphor is that the thermal conductivity of the glass is low. Unfortunately in order that the tube shall withstand the atmospheric pressure with safety, particularly considering the bending moment on the glass seal, we have to use a plate 5 mm thick. From the point of view of thermal conductivity the phosphor layer corresponds to an extra 1 mm of glass. If, however, the face seal could be made in a manner whereby the stresses across the joint were normal to the radius at that point then the glass could be reduced to at most a third of this thickness, thus reducing the thermal conductivity so that the cooling of the phosphor would not be a serious problem, and an increase in luminous efficiency could be expected.
Experiments with an air blower have shown that there is a good possibility of obtaining adequate cooling by this means, and we are experimenting in the design of a silent nozzle to give an air jet to maintain the tube face at room temperature.
Effect of Heat on Phosphors
Experiments made 18 months [early 1948] ago on the effect of temperature rise, show that the efficiency of the yellow silicate dropped by 50% at 120 C, which meant that for satisfactory operation the outside face of the projection tube would have to be maintained at a temperature of less than 0 C, and plans were seriously discussed for cooling the face by a liquid cell; the projector would then have to include a small refrigerator.
The improvement in the temperature effect brought about by 18 months' work in phosphor development is clearly shown in Fig. 5.
(a) Is for a blue sulfate which we were forced to use at that time in the absence of any other blue phosphor;
(6) Is the temperature characteristic of an early projection silicate; while
(c) Is the best projection silicate at present available.
It has become obvious to us that the design of the projection cabinet must be very carefully worked out to restrict the entry of dust and moisture. It may even become necessary that heaters will have to be included to prevent condensation within the cabinet when the apparatus is not in use.