F. H. Richardson's bluebook of projection (1942)

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THE PROJECTOR 325 Travel Ghost and Rain (131) As has been explained, the screen image would be hopelessly blurred if there were no shutter to cut off the light while one photograph is removed from the aperture and another replaced therein. The whiter portions of the image, in particular, would be mere vertical streaks on the screen. And if the shutter blade width is reduced too much, permitting more than a minimum of light to reach the screen while the film in the aperture is in motion, a partially streaky effect will be produced. This is called travel ghost. Travel ghost is most easily seen in titles, where white letters against a black background show a grayish "ghost" of part of each letter above or below the letter proper. A similar effect, of course, occurs in each picture, blurring it. Not only incorrect trimming of the shutter blades, but any misadjustment or faulty action of the shutter or the gears that drive it, may produce travel ghost if the misadjustment or faulty action has the effect of admitting more than a minimum of light to the screen during the time film is in motion in the aperture. To eliminate this blurring effect the mechanical cause of the incorrect shutter action must be run down and corrected. (132) Rain is a term applied to thin streaks, like driving rain, appearing in the screen image. These are always the fault of some projectionist, for they represent scratches in the film, which scratches in turn were caused by dirt in someone's projector mechanism. This dirt may be no more than particles of emulsion, or of film coating material, which accumulated in the projector because it was not scrupulously cleaned after every reel. If the projectionist is careless about this matter, a film that arrives in the projection room in the morning in perfect condition may be badly rain-streaked before the end of the last evening show. And the damage is beyond repair; nothing can be done for a rain-streaked print except to scrap it. The duty of the projectionist in this connection is too obvious to need comment.