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.
For daylight projection the translucent screen should be used, but in this case the brightness of the picture will depend on the amount of daylight you can keep away from the screen. This screen is only used for rear projection, and with the aid of a mirror is ideal for use in schools or anywhere where space is limited. Its use also allows the teacher to keep his place at the front of the class and yet control the projector at the same time. The correct layout for front and rear projection is described in the chapter on Exhibiting and Layout.
A white wall may also be used as a screen, and even the ceiling when the show is for hospital patients lying in bed.
Testing Screen Brightness
There are two kinds of light-meters used by projectionists — ■ those which measure the light from the lamp and those which measure light reflected from the screen.
Light can, of course, be measured in many ways, but the basic unit for all measurements is candle-power. The power of a lamp may be described in candle-power, or in wattage, which is an electrical unit. But at the moment we are concerned only with light measurements. And the first thing to get straight is the difference between lumens and lamberts.
The efficiency of a lamp is described as being so-many lumens per watt, and by using an appropriate meter we can measure the number of lumens emitted from the lamp.
Measuring lumens, however, is not very practical for the projectionist, and for various other reasons he uses a meter marked in foot-candles. Known as a photometer, it can be held in front of different portions of the screen to measure the amount of light received from the lamp.
The photometer may contain a photo-electric cell, or perhaps a plate on which is coated a light-sensitive substance which, when exposed to light, will produce a small current capable of actuating a needle against a graduated scale.
Holding the meter in front of the centre of the screen will give you the highest reading, but as this may bear no relation to the amount of light reaching the edges of the screen, the meter should be moved about over the screen-surface and a number of readings taken. The average reading obtained is then that of the
F 81