International projectionist (Jan-Dec 1948)

the crater. In all arc lamps the positive crater must face the collector (mirror or condenser) squarely if optimum results are to be obtained. The angle at which the positive crater burns should be checked frequently during projection, and the position of the negative adjusted to keep the crater formation perfect. The last two factors listed concern external influences on the arc stream. These may be harmful or beneficial. In the non-rotating type of high-intensity arc an auxiliary magnetic field is introduced to reinforce the natural field produced by the arc current. The effect of magnetic flux may be demonstrated by bringing the tip of a magnetized screwdriver close to a burning arc and observing the repelling effect on the tail-flame. So sensitive is the arc stream to magnetic influence that instructions regarding the supplementary magnets must be rigorously observed to insure proper formation of the ionized gas ball in the tositive crater of the arc. Arc-disturbing drafts in the lamphouse give rise to serious operating difficulties, with both low and high-intensity arcs producing a flickering light. The ventpipe dampers may be closed off, of course, but this procedure permits poisonous arc fumes to endanger the projection crew1. A better plan is to have the vent system remodelled for satisfactory operation. If the theatre owner refuses to comply, the powers of the local Board of Health or of an industry-labor board may be invoked. Heat-resistant glass discs should be placed over the lamphouse cones when air-circulating rear shutters are used; otherwise the use of protective glasses should be avoided. Even when clean, these glasses absorb from 5 to 10 percent of the light. The Condensing System If all electrical and mechanical conditions are such that a perfect positive crater of maximum brilliance can be maintained, the next step is to examine the functioning of the light-collecting and -converging system — the reflector or condensing lenses. Because a mirror can reflect only the light that it intercepts the importance of maintaining a perfect crater is obvious. It is possible, however, that the mirror is not reflecting light efficiently. These factors govern the performance of a mirror: 1. Reflective power. 2. Transmission of the glass. 3. Diameter of the mirror. 4. Focal length. 5. Parabolization. The reflective surface of an arc-lamp 1 See "Toxicity of Carbon Arc Gases," Leroy W. Latuwsky, M.D., IP for February, 1948, p. 10. mirror is a thin film of silver or aluminum on the rear surface of the glass. This film reflects from 85 to 95% of the visible light falling upon it. The twoway transmission of light by the glass is seldom greater than 96%, and it is greatly decreased when the surface is soiled by carbon dust, core dust, etc. A dirty mirror may reflect less than half the light it intercepts. In addition to decreasing the efficiency of the reflector, a coating of core dust discolors the light. The pitting of mirrors by particles of carbon thrown out from the crater seldom occurs in low-intensity lamps. In high-intensity lamps, however, pitting is unavoidable, and the emission of carbon particles from the arc continues all the time the arc burns. At the time of striking the arc droplets of molten copper may be thrown onto the reflector. If the arc is struck inexpertly, violently-ejected carbon vapor condenses on the mirror in the form of black soot; but this is easily wiped off and does not injure the surface of the glass. On the whole, even moderately severe pitting does not affect the performance of the reflector nearly so seriously as does core dust. Optical Surfaces Cleanliness Even though the projectionist is unable to prevent pitting, it is his duty to keep the mirror clean at all times. Particles of copper and carbon can be removed from the surface of the glass by means of a razor blade or the edge of a copper coin. The white core dust should be wiped off with a soft, dry cloth after every hour of projection. Because core dust contains basic oxides of rare-earth metals, it exhibits a pronounced tendency to "eat" into the glass, especially when hot, thus making it difficult to remove when it has been left on the mirror for a long period of time. When the dust does not wipe off easily, the reflector must be removed and scoured with Bon Ami or lens-cleaning paste on a moistened cloth. Care must be exercised not to wet the back of the mirror. Ammonia solutions should never be used for cleaning mirrors, as the fumes may work through the backing and adversely affect the silvered surface. A mirror, like a lens, has a focal length which governs the "working distance" (mirror-aperture distance) and the "arc focus" (mirror-crater distance). The elliptical or parabolic "figure" of the reflector is intended to be such that a reasonably sharp image of the positive crater is thrown on the aperture plate when the correct working and arc distances are employed. If the degree of parabolization is insufficient, spherical aberration results with a consequent "hot spot" and fadeaway. The projectionist will find it worth while to adjust by trial and error the mirror-aperture distance until that point is located where screen illumination is brightest and has no objectionable fadeaway2. In cases where fadeaway persists at every practicable setting, the size (geometric speed) of the projection lens may be inadeqate. Condensing lenses usually consist of two elements, the collector and the converger. Such lenses may be cleaned by methods previously given for reflectors. Aperture and Film Plane Next to be considered is the aperture, or, rather, the film plane over the aperture. We can profitably examine the light shield which is interposed between the cooling plate and the lamphouse light cone in front-shutter mechanisms. In many instances the older type light shields are too narrow to accommodate the full width of the light beam from a reflector arc, hence produce a fadeaway at the vertical edges of the screen image. In such cases light shields of adequate size must be substituted. Flatness of the film over the aperture is absolutely essential. Any movement of the film to and from the projection lens is tantamount to moving the lens itself in and out of focus. The effect of badly buckled film cannot be "cured," but it may be minimized by having a perfectly straight film course (this involves the lateral positions of all sprockets in the head and of the gate itself), and by replacing gate film tracks and gate-door tension pads whenever they evidence wear. An even or similar degree of tension must be exerted on both edges of the film, but, strangely, the precise degree of tension of the pads, so important in many ways, has little influence on the inand-out of focus effect of buckled film. Worn film tracks in the gate are exceedingly bad because they distort the film plane sufficiently to preclude the possibility of obtaining a sharp focus over all the screen. The tracks should be checked occasionally with a short steel straight-edge. The rotating shutter, no matter what its construction or position in the projector, has, but one function, that of providing a cutoff of light while the intermittent sprocket moves, and a balancing cutoff during the middle of the intermittent's rest period. The utilization of the shutter as a fan or air-circulator to cool the gate is of questionable value. If the shutter is mis-timed, the streaky effect known as "travel ghost" will appear. If the shutter be "early" (opens the lens before the film has stopped) the ghost appears to extend downward from bright objects in the picture; if it be "late" (the (Continued on page 26) 3 See "Complete Projection Data Charts, II," IP for May, 1947. p. 13. INTERNATIONAL PROJECTIONIST • May 1948