International projectionist (Jan-Dec 1949)

skilled craftsmen. With large lenses and prisms, it is very difficult to produce the glass perfectly homogeneous. If inhomogeniety of the glass is indicated, then one surface of the lens or prism is rubbed by a small polisher to deform the surface so as to neutralize the effect: only a few wave-lengths of glass would be so removed. The removal of the lenses from the pitch is facilitated by putting the blocks of lenses into a refrigerator. The pitch contracts at a different rate to the lenses and the lenses can easily be removed quite clean. Edge-Grinding and Assembly Lenses* after being polished on both surfaces, have to be edge-ground so that the optical axis is true with the edge. To do this the lens is stuck with wax onto a chuck. The lens is warmed and moved on the chuck until reflected images seen in the lens surfaces are perfectly steady. The spindle, together with the lens, is then transferred to a mechanical edge-grinding machine and the lens is edged to the correct diameter. For work of the highest accuracy, the lenses are edge-ground by hand, the operator checking the concentricity of the lens continually. Chamfers are produced at the same time as edging. Lenses which have to be cemented are now stuck together with Canada balsam, and set up concentric on a chuck as if setting up for edging. The lenses are then ready for mounting and are bevelled into their cells. The cells are assembled into the lens bodies, and the completed lenses are ready for test. Anti-Reflection Coating of Lenses When visible light passes through an optical system, about 2% is absorbed per centimeter of glass. At each air-glass or glass-air transmitting surface between 4 and 8% is lost by reflection, dependent on the refractive index of the glass. The reflected light which is lost suffers multiple reflection between the lens surfaces and finally makes its presence felt as a general background of illumination in the image, whether photographic or projected, causing a reduction in contrast. In bad cases, flare spots are present also. By means of surface coating, very nearly all the reflected light is eliminated, passing instead through the system to increase the brightness of the image. The prime advantages of surface coating are, therefore, increased transmission and improved contrast. The film is only a few millionths of an inch in thickness, being produced in high-vacuum plants. The surface treatment of lenses has opened the way to more complicated lens systems, since the necessity of keeping the number of elements to a minimum has been largely removed. The largeaperture projection lenses now employed resemble photographic anastigmat lenses more and more in their construction. The separation of the various elements is critical, and if dismantled, the lens has to be very carefully re-assembled. Such lenses should therefore be sealed. Transmission Calibration Scale To quote an example of the increase in transmission caused by blooming: one class of F:3.5 lenses has a transmission of 70% prior to blooming and 91% after blooming. The transmission difference between various makes of lens, bloomed and unbloomed, when set to the same aperture value, has become serious in the case of taking lenses for color films, and it has been proposed to mark photographic lenses with a transmission scale. The scale will take into consideration the light-transmitting power of the lens, so that all lenses of whatever make, bloomed or unbloomed, would transmit exactly the same amount of light when set to the same T-number.* There are mechanical and optical tests to be applied to the finished lenses, and from an optical point of view the focal length and resolution are the important criteria. Formulating Resolution Data Photographic lenses were formerly tested by photographing a test chart to ascertain that the resolution was satisfactory over the area to be covered by the lens. The testing of long focus lenses in this way is costly, as the plates are very big; also much storage room is required to keep the records of lenses which have been manufactured. A method of test was devised, therefore, which enables the equivalent focal length of the lens to be ascertained, and at the same time test visual and photographic resolution. The instrument used is called a "nodal slide". The lens under test is arranged * See "T-Number vs. F-Number Lens Markings," IP for March, 1949, p. 17. Coarse emery particles at 12x magnification. These sharp particles are used in grinding lenses. by trial so that a pivoting point on the lower carriage passes through the back nodal point. In front of the lens is a collimator with a resolution graticule. The image of this resolution graticule, called a Cobb-type test object, is focused by the microscope and the visual resolution as the lens is swung through any angle can be examined. The photographic resolution on any type of emulsion can be determined by removing the microscope and putting a 2% x 3%-inch plate into the repeating back of the instrument. A series of resolution photographs is made with the lens rotated through various angles, then further sets of photographs are made in and out of the visual focus position. In this way, the resolution data for any focal length lens can be obtained on a very small plate. The equivalent focal length of the lens is shown on a scale and is the distance between the pivoting point and the focal plane. Projection lenses are tested usually for resolution and focal length on a small nodal slide, and then finally in an ordinary projector, using a piece of transparent quartz bearing a multitude of fine detail as the test object. A foco-collimator is used for the precise determination of the equivalent focal length of short focus lenses. Special test benches are used for testing enlarging and process lenses, also for testing such lenses as wide-angle survey and gauge projection lenses, for which freedom from image distortion is the important criterion. The standardization of a transmission scale is one of a number of optical matters at present receiving consideration by the British Standards Institution. Another example is a recent draft specification for 35-mm projection lenses which makes provision for a barrel diameter of 80 mm, permitting a range of lenses working at F:1A. This indicates the trend of new design. The popular projection lens barrel diameter size in the past was 52.4 mm, but more recently with the introduction of F:1.9 collecting systems and lenses to match, the 70.65 mm 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 surfacetreated and hermetically sealed, the inner optical surfaces are thus always clean, and the projectionist need only wipe over the external surfaces. Discussion : Question. To what light does the quarter wave-length refer? Is the transmission selection according to the wave-length of light? Answer. We generally coat the surfaces (Continued on page 26) 22 INTERNATIONAL PROJECTIONIST • April 1949