International projectionist (Jan-Dec 1950)

heat from the margins of the film by the face of the intermittent sprocket, it will be found that the edges of the film are considerably hotter than the middle. Moreover, as a European correspondent has pointed out, "the first meters in a reel always feel cold, while the end can be very hot." Since both the first few feet and the last portions of a reel receive the same amount of light (and radiant heat) at the aperture, it is logical to conclude that the heat has come from the gate runners which increased in temperature (up to a certain limit) as the projection of the reel progressed. But this does imply that the heat received by the film from direct irradiation is unimportant, however. Two entirely distinct phenomena are involved. The beam of light at the aperture communicates very little heat to the film, raising the temperature of the middle portion of the film only a few degrees above room temperature. When nitrate film attains a temperature of about 180° C. (356° F.) it ignites; so it is obvious that even a "1,000-degree light beam" is incapable of raising the temperature of film exposed to it for 1/32 second or 1/48 second to even the ignition temperature of 180° C. ! What is the explanation of this apparently paradoxical situation? Use of Glass Light Filters In passing through the intense beam of light the surface of the emulsion in the darker portions of the picture image may be heated to several hundred degrees above the ignition temperature of nitrate film. But the layer so heated is so thin that very little heat (calories) is communicated to the film base; and nothing other than a move or less severe warping, or embossing, of the film results. The lighter portions of the emulsion absorb very little heat by direct irradiation, for much of the infrared radiation passes through the film with the visible light. FIG. 2. Various types of film tears, with suggested measures for correcting each: A — Oneperforation tear, corrected by trimming to smooth curves. B — Two-perforation tear before and after trimming. C — Three-perforation tear, best corrected by cutting and splicing. D — Roughened film edges trimmed smooth. The phenomenon is analogous to passing one's finger through a candle flame (temp. 1,100° C.) without getting burned. When extremely high arc amperages are used, the superficial heating of the emulsion in the dark portions of the picture is so intense that a slight blistering of the gelatine occurs. This is similar to the blistering and charring of the emulsion of negatives in cameras photographing, without filters, the explosion of oldtype atomic bombs at a distance of about six miles.2 The use of special heat-ab?orbing glass filters with all arc lamps using more than 70 amperes is recommended. However, one thick filter should be used in place of two or more thin glasses to avoid surface-reflection losses. Film Damage During Projection There are three important types of film damage for which the projectionist is usually responsible, namely, (1) the mutilation of leaders by painting crosses, numbers, etc., on them (2) the placement of picture-defacing changeover and other cues at the end of the film, and (3) the shortening of leaders, runouts, opening titles, and The End titles by the practice of "doubling up" short reels (splicing two short subjects together and winding them on one large reel). Conscientious projectionists deplore these practices. "In the matter of conserving leaders, projectionists can help by using great care in threading and by refraining from marking reel numbers, titles, etc., on the leaders with indelible ink. Notations should be confined to the first few feet of the plain 'protection leader,' and then written only with easily-erased, cellophane-marking crayon. "The painting of squares and crosses on threading-up footage-number frames has high nuisance value, as has also the cementing of opaque strips of film across leaders . . . "Inasmuch as the standard release print depends for its effectiveness on the exact positioning of the cues and leader footage numbers, projection efficiency is reduced when these have been mutilated. It is the responsibility of the distributor to replace leaders which have become unduly shortened through repeated use, and it also behooves him not to overdo the replacement of film to the extent of interposing several yards of black film between footage number 3 and the start of the picture. Such excess footage must be removed by the projectionist. "The motor start and changeover cues are frequently rendered unsightly by projectionists who fear that they may not catch the printed cues when they flash on the screen. Much worse is scratching curtain cues into the emulsion, for their presence is likely to be confusing to projectionists who subsequently use the film. Curtain and lighting cues should be marked on the film with crayon, and then wiped off when the film has completed its run. "Motor start and changeover cues which are really too faint to be seen on the screen may be rendered visible by lightly scoring them with a regular cuemarking device, several of which are on the market. Holes should never be punched in the film for any reason, and those distributors who punch identification symbols on titles which are to be projected should refrain from the practice."3 Repairing Prints, Splicing All film (even new prints) should be carefully inspected by the projectionist prior to showing, and all physical defects corrected. The inspecting operation is performed by rewinding the film slowly by hand and passing the film between thumb and finger in order to d" tect splices and torn edges. Every splice should be examined for strength by twisting or bending it very slightly, special attention being directed to the ends of the splice at the edges of the film. Splices that lift at the ends, but are otherwise strong, can often be "doctored" by dabbing a little film cement between the two stubs and pressing with the fingers. Torn edges and sprocket holes may be corrected by judicious trimming, as shown in the accompanying illustrations, if the defect is not too great. The secret of making smooth and perfectly strong film splices is found in the operator's technique and in the film cement used. A satisfactory film splice has a greater tensile strength than the {Continued on page 31) 3 "Projection-Room Handling of Prints" by Robert A. Mitchell, IP for March 1948, p. 5. The peculiar black spots in pictures of atomicbomb explosions are due partly to photographic reversal caused by intense overexposure and partly to a burning away of emulsion which, in such cases, may attain superficial temperatures of thousands of degrees for a very small fraction of a second. The film base does not catch fire. FIG. 3. When unshrunken film is spliced to shrunken film, the resultant rough edge should be trimmed smooth. 16 INTERNATIONAL PROJECTIONIST • MARCH 1950