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within the triangle. This "triangle of equipment' ' may be moved to any position in the room, providing, of course, that the relative positions of each part are not moved out of line with the others. The mirror naturally halves the length of the throw and, unless the mirror is a good one, the picture may be thrown slightly out of focus or perhaps distorted.
You must not use a mirror which distorts the reflection, and before using it you must see that it is thoroughly clean and polished.
In positioning the equipment, the height of the projector lens should be exactly the same as the height of the centre of the mirror and the centre of the screen. And the mirror must be set on a line at right angles to the screen's centre.
The figures given in the illustration are for the average layout, but should a larger screen be used the distance between the projector, mirror and screen will be correspondingly greater.
There is also a right and a wrong side to the translucent screen, but in most cases the rough side should be towards the audience.
<^\
/ M PROJECTOR
/ ^
^-A
\%
>
/ ^
TRANSLUCENT / SCREEN \
/ \
/ \
/ \
/ v
/ \
/ \
/ \
/ \
/ \
/ v
/ \ / VERY BRIGHT \
\ / BRIGHT \
/
/ NOT BRIGHT ENOUGH \
/ PICTURE AREA * /«-0F 0000 VISIBILITY-^ — i .1
/ \ A» GOOD VISIBILITY \
^GOOD VISIBILITY
Fig. 25
REAR PROJECTION WITH MIRROR Showing how the brightness of the picture and the width of the area of good visibility can be controlled by the length of the throw.
Tests carried out with a number of different types of translucent screens show that the area of good visibility corresponds to an area enclosed by lines drawn from the mirror to and beyond the edges of the screen. This is explained by the diagrams in Fig. 25. Note that the width of the area of good visibility ateo corresponds to the actual width of the projection-beam after
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