NEC WT600 White Paper

NEC
White Paper
WT600 Used In Rear Projection Systems
The WT600 has a very short throw distance that makes it an attractive product to use in a rear projection (RP) system.
By using the WT600, the depth of the rear projection system can be reduced by half of a system using a traditional
projector. It is possible to design a system with a 100" diagonal screen that has a total depth of approximately 24"
(single mirror system).
The key to using the WT600 in a rear projection system is 'screen selection'.
The WT600 has a very steep projection angle (55°).
When the WT600 is installed on the opposite side of the screen from the viewer in an RP system, the image path
comes through the screen at a very steep angle. When the light path passes through the screen, the screen diffuses
much of the light so that the image can be seen from many angles. However some of the light keeps traveling at that
steep angle after it passes through the screen and keeps traveling up so that some light may pass over the viewers'
head when the projector is installed in desktop orientation. This portion of light is lost to the viewer (see diagram below).
'Hot spotting' can also occur in areas on the screen, usually noticed at the very bottom of the screen in the middle,
when the projector is installed in desktop orientation. This is also dependant on the height of the viewer's seating. In
testing the WT600 in the environments described, we have found the location of hot spotting and the intensity is not
very noticeable compared to a traditional projector with a rear lens where hot spotting tends to happen more around the
center of the screen and usually at more intensity.
Designers of RP Systems are also careful to use a screen and system designed to avoid the possibility of 'secondary
reflection'. Secondary reflection can occur when an RP system with mirrors is used. Often diffusion screens have a
shiny side which when facing the projector can reflect some light back to the mirror which in turn will reflect back onto
the screen a second time (see diagram). This can cause a double image to occur. Secondary reflections can be
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SOME LOST LIGHT
DIFFUSED
LIGHT
NEC Solutions (America), Inc.
Visual Systems
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