International projectionist (Jan-Dec 1946)

Incandescent Lamps for Film Projection WHEN in addition to the normal 35-mm film, the narrower film (originally 9x/2 mm, later 8 and 16-mm also) began to be used, it seemed as first as if not only the field of application but also the projection apparatus for the two types would differ in principle. The main problem in projection technique is that of passing the large light flux, which must be thrown on the screen, through the small film aperture, whose area in the case of 16-mm film is about one-fifth and in that of 8-mm film about one-twentieth of that for normal film. Narrow film was therefore predestined for use in the home or in small auditoria with a small projection screen, where less light was needed. And while normal film was projected with the help of arc lamps and mirror condenser, the narrow film projector for the small audience worked exclusively with incandescent lamps and lens condensers. Only in that way could they be made sufficiently small and cheap, safe in use, easily transportable and fairly foolproof. Wide Gains by 16-mm Film At the present time, however, the former sharp boundary between normal and narrow film projection is becoming vaguer. This is due mainly to the fact that the advantages of the narow film (cheaper installation, lower film costs, possibility of the use of non-inflammable film material) have gained for it more territory which belonged originally only to normal film: the halls and auditoria in which 8 and 16-mm film is shown are growing larger and larger and approaching the size of ordinary motion picture theaters, and non-portable apparatus is now being made for that size film. Another reason for this development was that the noisless and quietly burning incandescent lamp, which was being manufactured for the growing subnormal film projectors in types of steadily increasing power, was displacing other light sources, even in normal film projectors. This was possible to a certain extent in smaller theatres, but not in larger ones where, because of its greater brightness, the arc lamp continues in use. During the last few years before the war the super high-pressure mercury lamp appeared as a new serious rival of motion picture light sources, and the intrusive narrow film has in the meantime also taken possession of this aid in order to win new territory. In discussing incandescent lamps for film projection we shall not, according By J. J. A. MANDERS Research Laboratory, Philips Incandescent Lamp Company, Eindhoven, Holland to the foregoing, be able to draw any sharp boundary between lamps for normal film — called theatre lamps — and those for narrower film. There is indeed a difference in lifetime: according to universally accepted standards, the lifetime of theatre lamps must be 100 hours; while in the case of narrow film lamps 50, 25 or even only 10 hours is considered sufficient. This difference is, however, based more on tradition than upon logi FIGURE 1. The optical system used for film projection. The light from the filament of the / lamp L is collected over / a wide angle by thel^jw condens3r lens C and, \ focussed on or near the \ film window F. The film picture passing F is projected by the objective O on the screen S. cal grounds. In principle, the requirements for theatre and for narrow-film lamps are the same, and the fact that in practice each type of projection lamp is stated to belong to one or the other category is mainly because generally each type of lamp can be used only in a certain type of projector which will naturally be intended for either normal or narrower film. Enormous Brilliance Required In projection the film picture occupying the aperture is thrown on the screen as an enlarged image by the objective [see the right-hand part of Fig. 1]. Now, even if all the light falling on the film reaches the screen, the intensity of illumination of the screen will be more than 60,000 times as small as that on the film aperture. If, for example, it is desired to have 50 lux1 on the screen (75 to 100 lux is usually demanded), the intensity of illumination on the film, taking into ac count the losses by reflection and absorption in the objective (about 50%), must amount to about 6 million lux! This enormously intense illumination is obtained by projecting the image of a very bright light source — in our case the filament of the projection lamp — on or near the film aperture by means of a condenser system (see the left-hand part of Fig. 1 ) . As may be seen, the condenser has the function of collecting the light emitted by the source over a very wide angle and concentrating it on the film within a smaller angle. Naturally, if the objective lens is not entirely filled, much unused light passes the objective. ! In order to obtain maximum illumination on the screen it is desirable to raise, the brightness of the light source^as high as possible. In addition, the loss factor must be kept as small as possible. In our case the losses at the aperture constitute a large percentage of the whole. Positioning of Filament Because the filament of an incandescent lamp is not a uniformly luminous plane, it may not be focused exactly on the aperture. If it were, the structure of that filament image would also be porjected on the screen. The filament is therefore focused farther away, either in the objective or at a spot between the objective and the aperture. In other words, the aperture is not situated at the narrowest part of the beam of light produced by the condenser, and a larger or smaller part (70%, for example) of this beam is cut off by the edges of the aperture. The more uniform the brightness of the filament, the Characteristics of the optical systems in which film projection lamps are used demand that the following requirements be met: the filament must have great brightness and must fill a given area as completely as possible, the diameter of the bulb must be small, and the lamp must be adjustable automatically. Herein is explained how these and other requirements are satisfied, with particular emphasis on filament construction and the part played by the gas filling. This is the first of a series of articles which cover comprehensively the topic of film projection lamps and which merit intensive study by professional projectionists. 12 INTERNATIONAL PROJECTIONIST November 1946