British Kinematography (1947)

84 stage ; the second stage is fully efficient. A pre-exposure at high intensity can therefore eliminate low-intensity failure. The ratio of sub-image to full image increases with the intensity of light. Low-intensity postexposure is a method of developing this sub-image. High-intensity reciprocity failure can be eliminated by a post-exposure at low intensity and is therefore confined to the second stage of latent-image formation. REFERENCES 1. Mott, /. Brit. Kine. Soc, 7, 1, 5 (194+). 2. GURNEY & Mott, Proc. Roy. Soc— A. 164, 151 (1938). 3. TRIVELLI, /. Frank Inst.. 241, 1, (1946). 4. Hall & ScHOEN, /. Opt. Soc. Amer., 31, 281 (1941). 5. Webb & Evans, Phot. /., 83, 188 (1940). 6. Luppo-Cramer, Phot. Cm., 72, 1, (1936). 7. See e.g., Berg, Mvrriage, Stevens, Phot. J.. 81, 413 (1941). S. In preparation, Phot. J. 9. DEBOT, Bull. Soc. Roy. Sri., Liege, p. 675 (1941). 10. Berg, Phot. J., 86B, 154 (1946). 11. Burt :>n & Berg, Phot. J., 83B, 1 (1946). DISCUSSION Mr. G. S. Moore : There may be a possibility of using latensification technique in sound recording, where exposures of the order of l/20,000th second are usual. From the work that has been done on the internal and external latent image, have there emerged any possibilities of using better developers than our present developers for improving speed of kine. negatives or sound recording films ? The Author : With variable area sound film, at any rate, the main requirement is to get sufficient contrast and resolution ; one would not have thought that speed was a major consideration. A post-exposure is liable to decrease the contrast rather than increase it. Regarding the question of using internal development for increasing speed, tests have been made at various times, and have always been disappointing. It looks as though by far the best hope is to find a developer which works well with small latent image specks. In the developer known as RAF pyro-metol, we have one that does just that, and often gives considerably higher effective speeds. Mr. B. C. Sewell : Would you get a particularly fine-grain image with pyro-metol developer ? The Author : Pyro-metol tends to give rather a grainy image. It is a very energetic developer, which seems to have an almost explosive action on the grains, thus making them larger than would an ordinary developer. Mr. L. Isaacs : What type of developer would be a surface developer ? The Author : All commercial developers are a fairly close approximation to a surface developer, but to get a perfect one sulphite must not be used as a preservative because of its solvent action, but carbonate is satisfactory. Glycin carbonate, and also a catechol formula, are satisfactory surface developers. Mr. L. Isaacs : What developer would you recommend for use with long exposures and low intensity ? The Author : One would give a preexposure at a fairly high intensity of light. If really long camera-exposure times are in question, one would then get quite an appreciable increase in speed. The preflash exposure time should be of the order of l/25th second, but not much shorter. This causes a sub-image to be formed. If the exposure time is much shorter, one also gets an internal image which counteracts the effect of the sub-image on the surface. Using an exposure time of l/25th second, then, the intensity should be so adjusted as to obtain a fog level of not more than 0.2 or 0.3 on development in Kodak formu'a D76 or D16. Doing this, one can reckon to get a speed increase of two or three times if the camera-exposure time is of the order of two or three minutes. It does not matter what colour of light is used for the preflaslrng as long as it produces a sufficient density in the time stated. Mr. G. S. Moore : A simple rule to remember this fogging technique for increasing speed is : if your camera-exposure is going to be of long duration, you give a short pre-exposure : if your camera exposure is going to be short, of the order of l/25th second or less, you give a long fogging