We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
647 HANDBOOK OF PROJECTION FOR
sented by an ordinary cloth screen and an arc lamp using 25 amperes, the rate of interruptions of the light may be as lowas 36 per second without flicker being objectionably visible. With a more brilliant illumination, however, such as is had with a brilliant screen surface and 60 or more amperes of current, we find it is necessary to increase the interruptions to between 55 and 60 per second in order to render them invisible, or in other words to obtain the effect of even, continuous screen illumination.
MASTER BLADE.— The blade which cuts the light from the screen while the intermittent sprocket is moving is variously called the "master" blade, "working" blade, "main" blade and the "interruptor" blade. We have selected master blade as the most appropriate term. In addition, the shutter has one or more additional blades, commonly termed "flicker" blades, the purpose of which has already been described.
The function of the master blade is to close the lens while the intermittent movement is in action. When the intermittent sprocket starts to move the master blade of the shutter comes in front of the lens and entirely shuts off the light from the screen, passing from in front of the lens and permitting the passage of the light to the screen the instant the intermittent sprocket comes to rest. From this we see that, in theory, the shutter must be so set or "timed" that its master blade will cover the lens, and shut off the light from the screen at the exact instant the intermittent begins to move, and uncover the lens at the exact instant the intermittent sprocket comes to rest. This, however, is to some extent modified. As a matter of fact the lens need be only about three-quarters closed when the intermittent sprocket begins to move, and may still be slightly open when the intermittent movement ceases. This is because of the fact that, with the lens threequarters closed, the light on the screen is very dim, and the impression on the eye therefore diminished to a point where the movement of the picture over the aperture cannot be per*ceived. If, however, the timing of the shutter be such that the lens is open too much, either when the film starts to move or is coming to rest, travel ghost will result.
SHUTTER BLADE WIDTH.— What may be termed the optical balance of the revolving shutter has directly to do with the width of the shutter blades with relation to each other, and with relation to the light openings. It is a well estab