International projectionist (Oct 1931-Sept 1933)

March 1932 INTERNATIONAL PROJECTIONIST 19 NO. 6 NO. 7 r ^nT^ NO. 8 NO, 10 could get maximum brilliance; but we can't, so let us see what is the next best thing. Consider first a central pencil of light (Fig. 6). We said that the ideal would be to concentrate the light on one point. Here we have it. The direct line on which this wave-front travels (shown by the middle line), does not change course. The front diverges from a brilliant grain of incandescent carbon in the crater to the condenser; then converges to appoint on the film near the center; then diverges to the projection lens, and then converges to a point on the screen. What happens when we get away from the central pencil? To get the proper viewpoint we must start at the screen and trace backward to find where the illumination needed must originate in order to give the necessary light upon the screen. The lines in Figure . 7 represent the boundary of a wave-front coming from one corner of the screen; it comes to a point at the corner of a frame of film. Now, to get the same amount of illumination at this point as at the center this wave-front should continue to advance along the same line. See where it lands. In order to furnish that illumination a 12 or 14-inch condenser would be required. A condensing lens of that diameter of the proper focal length to keep the arc within 4 inches of the lens (which must be done to get proper efiiciency), would be so thick that spherical abberation would make impossible its use. Reflector Arcs Now, perhaps, you get a clearer idea as to why the reflector arc is so much more efficient than the straight arc. In handling wave-fronts the mirror acts just like a lens, except that the light passes through but a very thin thickness of glass. There is no difficulty experienced in making a mirror of 6, 8, 12, or 14 inches in diameter. The trend toward reflector arcs is thus inevitable. First we had the 20-ampere lamps, then the 30-, 60-, and 70-ampere lamps. The next development will be 90 and 100ampere reflector arcs, which I expect to see on the market this year. The question of whether to use a 6V2 or 7^ focus or any other focal length piano convex 41/2-inch diameter condenser has become only of academic interest, due to the introduction of the reflector arc, and is no longer of interest to us. Previously it was almost wholly a matter of "cut and try." It could have been reduced to a certainty, but it never was. All that I have ever seen written on the subject approached the topic from the wrong angle, the theories upon which these expositions were built were fundamentally wrong, and the writers thereof endeavored to find an explanation for what was found in practice to be correct, instead of trying to determine theoretically what would be best and then attempting to reduce that determinaion to practice. Bausch and Lomb undoubtedly dug into the subject from a scientific angle and, instead of trying to determine which of the popular combinations was best, found that none of them was even fair. They then began developing their Cinephor series of condensers, the latest of which is the cylindrical Series II now in use in the recent models of high intensity lamps. Let us get back to the projection lens for a moment. The Petzval Type In 1840 Petzval brought out a portrait lens which worked at the remarkable diaphragm (for that period), of f/3.5, when at that time f/60 was considered good and when, in fact, portraits were very difficult to take because the subject could not sit still long enough to allow for sufficient exposure. I can't go too •deeply into apertures just now, but suffice it to say that for a lens of 6-inch E.F. the free opening of the Petzval would be 1.7 inches, and of the f/60 it would be .1, so that the Petzval would have 17 times the diameter and would pass about 290 times as much light. Owing to the large relative aperture, the Petzval was immediately adapted for lantern work. Figure 8 is a sketch of a Petzval lens taken from a book published about 40 years ago. This lens consisted of a cemented front combination of flint and crown glass and a back combination of flint and crown separated by an air space, the combinations being separated by a distance depending upon the focal length. Let us look back to the state of science and industry in 1840. The arc lamp had not been invented; automobiles, telephones, and phonographs were missing. Railroads were just being developed, and the electrical industry as a whole was not even dreamed of. In thinking of the great development in other lines one might expect the projection lens to develop space. Let us see. The lower half of Figure 8 was taken from a circular received recently from an optical company* describing their newest development in projection lenses^ Ninety years have not changed the fundamental design of projection lenses one iota. But, while the principle of the lens has not been changed within this time, there has been great development in the execution of that design. Hartung^, after describing the Petzval as an extremely good projection lens says: "The Petzval objective must be very carefully constructed if it is actually to show its whole efficiency. Unfortunately, this cannot be said of most of the so-called projection lenses made on the Petzval formula." So that while both sections of Figure 8 look identical, the newer lens, and all other lenses built on this formula, are good or bad not according to the design but according to the care and skill exercised in the execution of the design. In this particular lens' it seems to me that the great advantage for projectionists lies in the fact that it is so constructed that if all the glasses and all the rings are removed and scattered about, they cannot be put back together in any way but the right way — certainly a great help to a hard-working service department. The passing of the straight arc and the accompanying introduction of the Cine Kollmorgen Optical Co. ' '*Solex". ' "Optics for Photographers."