British Kinematography (1950)

54 BRITISH KINEMATOGRAPHY Vol. 16, No. 2 There are now two differential equations: and 82 0, 8*2 82 e2 8*2 pi q-i *2 8^ 8f 862 8/ /fe a y in the emulsion = 0 in the celluloid and the boundary conditions are: (i) (ii) 0X = 02 = 0 for 80, 8* all 0 or x at t = 0 S02 a, -g— = 0 on x = b, for all /. 8* (iii) 02 = 02 and 60, 802 ** 8* ~kl 87 on x = 0 for all /. These equations have been solved using the Inverse Laplace Theorem and Cauchy's Theorem to give the temperature rise in the base. The numerical result of this fuller treatment is within 2 per cent, of the result of the simpler method, thus justifying the assumption of a very thin emulsion layer. The full solution has also been obtained for the condition in a theatre projector of the emulsion towards the arc. The numerical result is the same as before showing that the temperature rise within the base is the same to the accuracy to which the computation has been carried. DISCUSSION Mr. H. D. Waley : Have you worked on any of the new projector illuminants — high pressure gas arc, the zirconium arc or the high-pressure mercury arc ? The Author : I have not made any measurements on these. High-pressure mercury discharge lamps will give a relatively high luminous efficiency, but are somewhat coloured and difficult to incorporate in an optical system. Mr. W. Buckstone : Have you made measurements on the electronic flash lamp ? The Author : No. Although these will give a brilliant flash for single exposures, the light output is limited for repeated flashing in a projector. Mr. R. H. Cricks : Years ago experiments were made in cooling a process projector with solid C02. The Author : The extremely low temperature of solid CO 2 might be of value in certain applications if it could be used. Mr. S. A. Stevens : In a slide projector trouble may arise from condensation on the cover glass. Sometimes the slide breaks instead of the gelatin turning brown. The Author : Gelatin, being a spongelike material, can easily hold a considerable amount of water, particularly if inadequately dried after washing or exposed for long to humid air. This is driven off when the slide warms up, and may condense on the cooler cover glass. Whether a slide fails by breakage, or charring of the gelatin, depends on several factors, including the density and shape of the picture, the type of slide holder and stresses already in the glass. Mr. R. A. Tomes : It is important not to de-nature the gelatin by excessive pre-drying. Mr. C. G. Heys-Hallett : We have tried liquid cooling of slides by immersing them in oil. After a few seconds bubbles of gas rise from pin-holes in the slide, and then dark streams appeared on the screen, caused by refraction by the heated oil. Mr. R. Robertson : How about filtering light for ordinary kinema projectors? A piece of ON20 glass without some form of cooling would stand up to an ordinary kinema arc. The Author : It is generally desirable to place a glass filter as far from the gate and as near the arc as possible so that the light will be least concentrated. In a kinema projector it will almost certainly be necessary to cool the glass by air blasts. Mr. B. Honri : Some years ago I saw a device aimed at removing scratches from film, in which a liquid was applied to both sides of the film and evaporated in the gate. The Author : This method is extremely effective in making old scratched prints appear and sound better. The liquid on the emulsion side will contribute to cooling the film in the gate and this idea might be developed to be a significant help, if required. It also cleans oil off the film. EXHIBITION OF FRENCH An exhibition of French scientific instruments is on view until February 26 at the Science Museum, South Kensington. Among the official French organisations providing material are the National Centre of Scientific SCIENTIFIC INSTRUMENTS Research, the National Office for Aeronautical Studies, and the Atomic Energy Commission. Lectures are being given in connection with the exhibition and there is a bilingual catalogue of over 100 pps., price 2s.