the extraneous light comes from the side. When
one is sitting too close to the screen, lines can be
detected in the image. Silver lenticular surfaces
are used only in tripod or wall mounts that have
a tensioning device to hold the surface flat. It is
important that these screens be positioned in an
exact perpendicular relationship with the projec-
tor and at the eye level of the audience.
Another type of surface is the matte white
which is a nonglass surface that diffuses the
available light evenly across the surface. Because
of this diffusion, image brightness is con-
siderably less than with other screen surfaces.
This type of surface does have some advantages.
The image produced is pleasing, free from grain,
and very sharp. Matte white screens are good for
short projection distances and are, therefore,
very effective in small rooms where the image
brightness needs to be reduced.
VIEWING ANGLE.The relationship of
the screen to the audience is very important. As
a rule the audience should never view the image
from an angle greater than 45° from the projec-
tion axis or outside the viewing angle of 90°.
Beyond this area the image will appear distorted
regardless of the type of screen surface. Objects
and characters appear to be taller and thinner
and printed matter will be difficult to read.
Figure 7-4 illustrates the relative placement of
the audience for a given screen size.
DETERMINING SCREEN SIZE.The
Society of Motion Picture Engineers prescribes
the 2 and 6 rule for determining the screen size
when the size of the audience is known. It is a
simple, easy to apply formula. The rule states
that the distance from the screen to the first row
of seats should equal 2 times the screen width,
and the last row of seats should be at a distance
Figure 7-4.-Seating arrangement in relation to the screen.