International projectionist (Jan-Dec 1947)

Elements of Projection Optics V. Another article in a series anent the fundamentals of projection optics THE arc larmps generally used for professional motion picture projection are of either the condenser or the reflector type, with the latter, of course, enjoying the widest use because of its particular adaptability for producing an economical high-intensity light. Figure 1 is a diagram of a reflector arc system. The general information conveyed therein is that the rays of light emitted by an electric (carbon) arc within a wide angle are collected by the concave mirror and concentrated in a much narrower angle to form a focus on the film aperture. We all know, of course, that the most brilliant tip is that of the positive carbon which, shortly after the arc is struck, becomes hollow and assumes the shape of a small crater. We see from A and B in Fig. 1. that the image of the crater is projected in greatly magnified form on the plane of the film. To insure accuracy of focusing it is necessary to precisely adjust the distance from the reflector to the film plane and also the distance between the arc and the reflector itself. It is evident from a glance at Fig. 1-B that the condenser system of projection is the more elaborate one, wherein light is reflected from the surfaces of the lenses and is absorbed by the thickness of the glass itself. A common inconvenience encountered with all high-intensity arcs is the socalled pitting of the reflector or of the inner surface of the condenser lens. Small particles of incompletely burned carbon are precipitated from the carbon crater in all directions. Some of these particles strike and adhere to the polished surfaces of the reflector or condenser lens, constituting thereon a thin layer of white scum. Therefore, daily cleaning of these surfaces with the use of a soft, clean cloth is mandatory. When these particles burn on a glass surface for a time they become permanantly imbedded therein and cause cloudy spots. We must never lose sight of the fact that the light seen on the screen is the carbon crater image magnified many thousands of times. If the crater changes its position — improper alignment of the carbons, strong air drafts, etc. — screen illumination will suffer to the same degree as that of the crater change. One of the most valuable aids in maintaining correct arc alignment is the Arc Alignment Bar recommended by the By DR. ANGELO MONTANI Consulting Engineer, N. Y. City Projection Practice Committee of the S.M.P.E. It is difficult to understand why this tool is not used by every theatre ; yet there are many thousands of theatremen who not only do not use it but, in fact, have never even heard of it! The best optics obtainable, being only one link in the projection chain, cannot insure good screen results if other elements of the system are imperfect or improperly used. Reflectors and lenses limited by spherical surfaces introduce distortion in the , path of the light rays. This topic was discussed in detail by the writer in International Projectionist issues for May and July, 1946. Figure 2-A shows a reflector limited by spherical surfaces, and in Fig. 2-B a parabolic reflector. We see that the spherical reflector does not concentrate the parallel rays in a point but rather in a zone of space. This defect (as we have seen in lenses) is known as spherical aberration, which is a consequence of the shape of the mirror. It is possible, of course, to design reflecting surfaces (parabolic) wherein spherical aberration is eliminated and all the rays converge at one point or focus. Automobile headlights, for example, are equipped with parabolic reflectors. Condenser lenses are designed with parabolic surfaces. The strong spherical aberration present in condenser lenses was exploited in the past as a means for FIGURE 1 POSITIVE CARBON feeding more light through the aperture, because the use of a larger light source was permissible thereby. Today the trend is toward smaller and more brilliant sources. Also, the technique of casting large polished lenses has been greatly improved. Condenser vs Projection Lenses It should be emphasized that the theory and technique of the design and manufacture of condenser lenses is basically different from that employed with the projection lens. Persons uninformed about optical techniques might think that projection and condenser lenses are much the same, except that the latter type is larger. The fact is, however, that condenser lenses and reflectors are exposed to such high temperatures that they require special types of glass and special mechanical mounts. It is common knowledge that any solid material expands upon exposure to heat. Glass does too, except that glass presents problems which do not arise with metals. Metals are good conductors of heat and therefore the increase in temperature, and consequently the expansion, is uniform. Glass does not conduct heat, thus the temperature increases and localizes only in the immediate area where heat is applied. Since this results in only a portion of the glass expanding abnormally, the result i9 a tendency of that particular area to crack. Thus the glass used for reflectors and condensers must have a very low coefficient of expansion, which means that it must expand very little when heated. The mechanical mounts for reflectors and condensers must be designed with FILM APERTURE PROJECTION 'LENS ELECTOR POSITIVE CARBON \ CRATER OPTICAL AXIS CRATER IMAGE FOCUSED HERE FILM APERTURE NEGATIVE CARBON CRATER IMAGE FOCUSED HERE 20 INTERNATIONAL PROJECTIONIST • January 194?