International projectionist (Jan-Dec 1946)

Elements of Projection Optics IV. Another article in a series anent the fundamentals of projection optics By DR. ANGELO MONTANI Consulting Engineer, New York City IN I. P. for July we discussed the luminosity of projection lenses, which topic leads naturally to consideration of the optical system which feeds the light into the lens. A modern reflector light system is always composed of a curved mirror which collects the light radiated backward and reflects it forward toward the condenser lens. There are several types of these systems in use which attempt to exploit the maximum of light from the source and employ differently shaped mirrors and condensers. Generally the surfaces of the mirror and lenses are not spherical. The common scope of all the systems consists in producing a surface of the maximum and most evenly distributed luminosity (for a given source) said surface being located in the second principal plane of the projecting lens. This plane must be imagined perpendicular to the optical axis and passing through point P of the projecting system. (See point P in Fig. 3 in our article in I. P. for July.) It is evident, although very often overlooked, that the projecting lens cannot transmit more light than that concentrated by the mirror-condenser system. Disregarding the absorption and reflection losses, if a lens transmits all the light it is receiving, then a lens of larger . aperture will not increase the brilliance of the image upon the screen since it will not get any additional light from the source. A lens of smaller aperture, on the other hand, will reduce the intensity of the projected light since it will be unable to utilize the entire amount of light produced by the illuminating system. Practically the optimum condition of -MOUNT F20NT ^ I2EAB (film side) a FEONT EEAE. FIGURE 1 utilization of available light is achieved when a light spot of the diameter of the lens of even brilliancy can be collected on a white surface in the position where the back element' of the projection lens is generally located. Due to the high concentration of heat which would carbonize any paper or cardboard, some other non-combustible and non-glossy surface must be employed. The observation should be made, of course, through a deep neutral glass. In this way it is possible to determine the correct distances yielding even illumination and correct size of the spot. The smaller the spot of light, the more brilliant it will be; on the other hand, the spot must fill the entire surface of the back element of the lens. Uneven distribution of the light in the spot may cause objectionable "hot spots." When this is the case, some scattered zones of the screen appear to receive more light than the neighboring zones, particularly at the sides. Hot spots can be readily detected by throwing the beam on the screen without any film in the gate and positioning an even deep neutral screen in front of the projecting lens. Due to the responsitivity of the eye to the illumination stimulus, urevenness on the screen is better detected when the light is rather dim. 'Flatness"* of the Field Elementary optical laws teach us that it is impossible to obtain even illumination of the screen with relatively simple means. The center of the field will always be more brilliant than the sides. Practically, however, it is possible to make the eye ignore such a difference for small field extensions because, luckily, the eye does not record the absolute ratios of illumination but only their logarythmic values. The center of the screen receives rays perpendicular or almost perpendicular to it. The portions around the edges, however, receive oblique rays which for an equal angle of spray fall on a larger area and thus occasion diminishing brilliancy. In projection in general the angle of the field is kept rather limited and the difference in luminosity between the central zone and the sides is almost below the intensity discrimination of even a trained eye. The most popular of the projection lenses in use today is the direct derivation of the very first objective which was designed for photographic work. Since this type of projection lens is most widely used, few descriptive words are necessary about it. A schematic of the lens is presented in Fig. la. This lens is corrected for color, spherical FIGURE 2 aberration and astigmatism. The prototype of this lens was calculated in 1840 by a Viennese mathematician named Petzval. and at that time it was not color-corrected because the proper glass was not then available. Sometimes the second doublet is cemented (the elements having a different shape) and this is found on projectors which do not develop too much heat, such as the 8 and 16-mm types. Other manufacturers invert the position of the elements of the second doublet, positioning the convergent lens inside and the divergent lens outside, as in Fig. lb (Dallmayer type) . Lenses of the aforementioned geneial construction (Figs, la and lb) can be easily manufactured in large apertures up to //1.9 for moderate fields of from 12 to 14 degrees. Since the field is limited, the image projected is satisfactorily sharp and flat. A second type of lens sometimes used in projection is schematically sketched in Fig. 2. Such a lens derives from the Cooke lens designed for photographic cameras in 1895 by H. Dennis Taylor. This lens possesses a satisfactory flat field which is much more extended than the one shown in Fig. 1. It is made with apertures up to //3 and yields sharper images having increased contrast effect.1 Other types of lenses have such limited applications as to not warrant a description herein. In general, motion picture projection lenses cover a narrow angle of the field since of necessity the screen must 1 For a popular exposition of the various types of louses refer to "The Evolution of the Photographic Objective," by A. Montani, American Annual of Photography for 1946, p. 149. INTERNATIONAL PROJECTIONIST October, 1946