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380 XII. PEOCESSING AND EELEASE PEINTING
raw stock upon which the copy is to be made, and (6) a frame (usually a casting) — for mounting all the equipment and parts needed.
Picture Printers: Continuous Contact Printer. The continuous contact printer is one of the most common in commercial use. It moves the two films in intimate physical contact at uniform velocity past the printing gate, where the raw stock is exposed to the light from the printer lamp that is "modulated" by the image on the film to be copied. (The sequence of parts is : light source, printer sprocket supporting the image-bearing film with its emulsion facing away from the lamp, raw stock with its emulsion facing the emulsion of the image-bearing film; gate closure which supports the gate shoes that press the raw stock against the imagebearing film by means of springs.)
Printing usually takes place at the periphery of a large printing sprocket that propels the films together past the printing gate ; it is this printer type that is capable of operating at high speed. A common speed for a 16-mm printer of this type (such as the Depue, Fig. 93) is about 60 ft. per minute ; 35-mm printers are in use that run at speeds well over 200 ft. per minute. As the speed is increased, it becomes increasingly difficult to maintain good contact between the films being printed. Continuous contact printers are used for release printing where some sacrifice in resolving power can be made for the sake of high running speed and correspondingly lower print price. The prime disadvantage of the continuous contact printer is that a loss of at least 30% in resolving power occurs with even the best construction possible; poorer constructions result in still higher losses.
As the speed of printing increases, it becomes increasingly difficult to effect light changes. These are changes in illumination that are required to correct for the undesired density variations in an image-bearing film being copied. There are two general methods of light-changing used with such printers : (1) a change in lamp brightness — usually effected by a rheostat in the lamp circuit of an incandescent lamp, and (2) a change in the aperture or diaphragm in the illumination system, the change in illumination being proportional to the change in area of the diaphragm or aperture. There is appreciable thermal lag in an incandescent lamp filament; for example, a printer operating at 20 ft. per minute that uses a 500-w. lamp as a light source will require about 4 or 5 printed frames to complete a change of brightness of 50% when the change is made with a rheostat. Mechanical systems of light change that