International projectionist (Jan-Dec 1950)

of the frame is seen to be different from the original tone of the unprojected frame lines and film edges. No satisfactory explanation has been offered for this phenomenon: it is one of those occurrences which, though interesting, have no influence on the quality of the projected image. Focus Drift As the projection intensity is increased still further, it is observed that the projectionist must now refocus occasionally during the projection of a single reel of film. This appearance of focus drift begins somewhere in the region 0.35 to 0.40 mean net watt per square mm. Depending upon the circumstances, there may appear to be a rather sudden change in focus with the transition from scene to scene, even though there is no splice in the film at this transition. Or it may appear that there is a gradual loss in sharpness of the projected image obtained at the start of the roll, and this must be corrected by occasional focus resharpening. At intensities where this phenomenon occurs, we believe the only satisfactory solution is more constant attention by the projectionist. The influence of this effect can be minimized, however, by the technique of the initial focusing. Every projectionist has observed that in the setting of the projection lens there is some leeway over which the projected image remains approximately equally sharp. It is apparent that if the lens is set at the mid-point of this tolerance, slight film motion in either direction may not exceed the depth of focus of the projection lens; if, however, the lens should be focused at the limit of its tolerance, film motion in one direction will be obvious immediately as a softening of the picture detail. If the projectionist acquires the habit of setting his lens at the approximate mid-point of this depth of focus, the necessary focus corrections will be minimized. This focus drift is apparently the result of a gradual variation in the amount of negative drift of the film at various points in the roll, induced perhaps by changes in moisture content, degree of exposure to air, and other intangible variables of film handling. When the amount of negative drift is small, variations in the amount pass unnoticed, but with the increase in magnitude of negative drift at higher intensities, some of these variations may be large enough to exceed the 0.002 to 0.004-inch tolerance in lens position for maximum screen sharpness. Image Flutter Beyond the limit of radiant-flux density at which film can be projected without image difficulty, the first warning is a softening of the focus — which we have called "image flutter." The picture is still reasonably sharp upon the screen and it might well be that to the theater audience no lack of quality would be apparent. However, when we project test films which we know to be printed sharply and in which we are familiar with the best quality that can be obtained — so that any departure is obvious — we observe a softening of focus and a loss in detail that cannot be corrected by shifting the projection lens. In other words, at the point of best focus, the image quality is slightly less than optimum. This is a preliminary to in-and-out of focus, which is observed with the next increase in radiation intensity, and may be produced by the same basic phenomena. There is a sharp transition, however, since image flutter is visible only with careful examination, while in-and-out of focus is immediately obvious even to the most uncritical audience. In-and-Out of Focus As the projection intensity is increased, a point is reached where the normal negative drift of each frame in the aperture is modified, and continued projection at higher intensities eventually replaces the negative drift with an actual positive drift or displacement of the film in the opposite direction. The appearance and degree of positive drift are a function of the radiation intensity, together with the previous projection history of the film. We have observed a maximum positive drift of about 0.045 inch. During the time when all the frames show negative drift, or the time when all frames show positive drift, a sharp steady picture can be focused upon the screen. In the transition period, however, it appears that some frames reach the point of positive drift ahead of their adjacent frames, and high-speed motion pictures show that some frames go negative, while others near by go positive. The limits of these two opposite film motions exceed the depth of focus of the projection lens and the phenomenon of in-and-out of focus is observed. Under these conditions, no one setting of the projection lens will focus all frames equally sharp, and the transition from negative to positive is so rapid and unpredictable that the projectionist has no possibility of following it. Film that is going in-and-out of focus is completely unacceptable for theater projection. We have observed that inand-out of focus occasionally may be observed at projection intensities of 0.40 mean net watt per square mm and that, beyond an intensity of 0.50 mean net watt per square mm, in-and-out of focus is almost certain to occur within the first five to ten days of projection in the theater. (In-and-out of focus, as well as many of the other focus effects, can be accelerated by abnormally high moisture content of the film.) Blistering of the Film With the increase in projection intensity to still higher levels, the film is observed to blister during projection. These blisters form in the film base immediately adjacent to the emulsion, where maximum base temperatures are reached. Depending upon the conditions of their formation, these blisters may be so small that the separate blisters are indistinguishable to the naked eye, or they may reach a diameter of approximately 0.005 inch. Once blisters appear, the film is useless for further projection, since the thermal isolation of the emulsion directly over the blister causes it to reach so high a temperature as to burn off and disappear. The unburned blisters show on the screen as dark spots resembling the sudden appearance of severe grain in the image, and the burned-through blisters show as white areas on the screen surrounded by obviously charred emulsion. The exact level at which blistering appears is dependent upon the type of film under consideration, but for release positive, a mean net flux of 0.60 to 0.65 watt per square mm is required. It should be pointed out, however, that in-and-out of focus appears only after repeated projection, while blistering may be observed on the very first run. Accordingly, in an untested setup the in-and-out of focus threshold may be exceeded and even the blistering threshold passed; film projected under such conditions will blister rapidly and nevertheless require a number of projections of the blistered film before in-and-out focus is observed. This represents an extreme case, however, and it is safe to say that tested equipment already shown safe for in-and-out of focus will not blister release positive. It is apparent that the high-intensity effects which depend almost entirely upon film temperature can be modified or prevented only by preventing the film from reaching these damaging temperatures. High-intensity effects that are influenced not only by temperature but by other factors sometimes can be postponed even though the temperature threshold is exceeded. [To be Concluded] Records Light Source Colors A "recording spectro-radiometer" has been developed for comparing and recording the colors of light sources. The instrument breaks up beams into their component colors and makes a graphic record of its measurements of the intensities of these colors. 12 INTERNATIONAL PROJECTIONIST • January 1950