British Kinematography (1949)

42 The popular projection lens barrel diameter size in the past was 52.4mm. but more recently with the introduction of //1.9 collecting systems and lenses to match, the 70.65mm. diameter jacket has become almost universal. There are, however, certain mechanical restrictions in some projectors which do not permit the use of this larger diameter jacket in all focal lengths. Modern projection lenses are surface treated and hermetically sealed, the inner optical surfaces are thus always clean, and the projectionist need only wipe over the external surfaces. Development woik on optical matters is constantly in progress by British manufacturers. During the war a number of new types of optical glass were envolvtd and this has enabled further improvements to be made in taking and projection lenses. REFERENCES 1. /. Brit. Kine; Soc, Vol. 4, No. 3, July, 1941. 2. Brit. Kine., March, 1948, p. 91. 3. Fifth Newman Memorial Lecture, " Transm'ssion Calibration of Lenses," to be publ.shed in a future issue of Brit. Kine. DISCUSSION Mr. W. Braby : To what light does the quarter wave-length refer ? Is the transmission selection according to the wavelength of light ? The Author : We generally coat the surfaces to have a maximum transmission and minimum reflectivity in the app]egreen part of the spectrum at about 5,500A. Away from that "minimum position, you do get a certain amount of light reflected. Mr. W. Braby : If you coated for a particular wave-length, what percentage ga n wou!d you expect for a particular monochromatic light ? The Author : There is a condition that the refractive index of the film should be the square root of the refractive index of the glass. It is not possible to obtain that condition precisely, because there are not many minerals of a suitable type for evaporating on to the glass surface. We have to use magnesium fluoride. Mr. W. Buckstone : How does the reflectivity curve continue into the infrared ? At 12,000A would there be any improvement with coated lenses ? The Author : Yes, there would still be a considerable improvement. It is possible by evaporating successive layers of different minerals to attain an achromatic effect. Mr. A. Duerdoth : Is it likely that faster aperture lenses than are now available will come into use soon ? If that is so, it will mean re-designing the collecting system of the arc lamp. The Author : The case is really that the light collecting end of the system is lagging behind the projection lens. The wider the angle of pick-up the greater the trouble with heating. Mr. R. H. Cricks : Regarding the de-. cU-nhilitv of matching apertures from th2 mirror to the lens, recent papers in Ameri can journals have suggested that the illumination at the edges of the picture is improved if the projection lens is of wider aperture than the mirror. What are Mr. Anstis's views on this subject ? The Author : I think that one American writer was endeavouring rather to point out that certain American projection lenses gave some cut-off of light due to their improper design. Mr. Oram : You have that case in some American projectors using //1.9 lenses giving only //1.9 at the centre. If you are free to increase the diameter, vou will get// 1.9 at the margin. Mr. F. G. Gunn : What do you consider is a natural basis for the calibration of lenses in transmission values ? The Author : The old basis was the relation of the diameter of the lens to the focal length. This gave the /value. That does not take into consideration the transmission of the lens. If one imagines a lens of 100% efficiency, that is the theoretical basis of the t system. Mr. J. Hill : Am I not right in thinking that the coating is not permanent? The / number of a lens is permanent, but the t scale would vary in time.' The Author : Modern coatings are hard and substantial. Over a number of years the transmission would not vary appreciably. Mr. C . F. Knott : How is an aspherical surface produced ? The Author : They cannot be produced in the manner as for ordinary lens surfaces. The grinding and polishing tools when producing spherical surfaces oscillate in a free motion over the top of the lens. In making aspherical surfaces, one has usually to depend on some kind of a formula or link mechanism for producing the curvature mathematically.