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Welcome to the coarse: Projection Screen Science Made Simple. This projection
screen course is brought to you by Draper. A world leader in the design and
manufacture of audio-visual products. Draper has been manufacturing quality
products since 1902.
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The screen tint should be considered when selecting a screen surface. The surface
tint will affect how well it performs in a room with ambient light and viewing cone.
White surfaces are very diffusive and wash out easily in rooms with ambient light.
These surfaces are best for darker rooms with very wide seating configurations.
White surfaces are typically available in higher gains so for applications that don’t
have enough system brightness, using a higher gain white material can help bring
the system brightness up, but may not give good system contrast.
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Grey surfaces help to reject ambient light and improve black on the screen. These
surfaces absorb more stray light with their darker tint, and help to improve the
blacks on the screen. Darker grey materials usually have more reflective
components to increase gain and reflect ambient light away from the viewers. Grey
surfaces typically are not available in higher gains. Also when calculating system
brightness, factor more for grey or dark grey surfaces since these darker surfaces
absorb more whites and bright colors to improve blacks in the image. Calculating for
a little more brightness will compensate for this and still get the benefits of
improved blacks. A combination of grey surfaces not being available in higher gains
and needing more projection brightness to properly light up the screen can make it
difficult for applications with tight budgets.
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Matt White is a very popular material. It is a highly diffusive surface, with very little
reflectivity and the white color helps to spread the projector light very evenly across
the screen giving a very uniform image and is good for very wide seating
configurations like theaters. These materials are so diffusive that if ambient light
hits them, it will also be diffused, compete with the projector light and the image
will be washed out. These surfaces are best for dark applications and very wide
seating configurations.
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High Performance screen surfaces are typically dark grey in tint to help absorb stray
ambient light and deepen black levels even more than light grey surfaces. They are
also typically very reflective so that they reflect the projector light back to the
viewers, but reflect off axis ambient light away from the viewers. They may also
have a lens structure that can block off axis ambient light from reflecting toward the
viewers. This is most important for light fixtures in the ceiling that are mounted too
close to the screen surface. It is always recommended to install light blocking
shades at windows.
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In a projection system, we look at ambient lighting in two places, at the screen and
at the viewers. Light hitting the screen washes out blacks and decreases the image
contrast. Light in the viewers eyes competes with projection light getting to the
viewers eyes and determines how bright a system is needed.
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The more light hitting the screen surface, the more the black in the image gets
washed out decreasing image contrast. Projectors cannot project black, only white
and color. Black is the absence of light. The only way to get black on any screen is to
have a dark room. Grey and high performance screens can only help improve black
in the image. A good target for a commercial application with projection is to have
no more than 5 ft candles of ambient light hitting the screen. If you have over 5 ft
candles of ambient light hitting the screen, a white surface will have difficulty
showing good black and a grey or higher gain white surface should be considered .
If you have over 8 ft candles of ambient light hitting the screen, a grey surface will
have difficulty showing good black and a high performance surface should be
considered.
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In most commercial projection applications a good target is to have 30 foot candles
of ambient light on the viewers for task lighting (note taking/reading). Having too
much ambient light on the viewer is not good for projection. When there is too
much ambient light the iris in the human eye begins to shut letting less light in. The
projected image will not look as bright. It is important that the amount of foot
lamberts (covered later) of projected light coming from the screen is always much
higher than the amount of foot candles of ambient light on the viewers. You never
want the ambient light on the viewer to compete with the projected light.
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Image contrast affects how well you can read which means it affects detail and
resolution. Low contrast reduces image detail and resolution, high contrast
increases detail and resolution.
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Image contrast is even more critical when the application involves high resolutions
graphics, medical imaging, control rooms and CAD applications.
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System contrast is an average of measurements of black at different locations on
the screen compared to white at different locations on the screen measured in the
existing lighting conditions of the application. Ambient light will affect the
measurement of black. When measuring system contrast an ANSI checkerboard
pattern is used.
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We can calculate what the brightness of a system could be. Brightness is measured
in foot lamberts. The basic formula for ft lamberts is ft lamberts = screen gain x
lumens / sq ft of image. This only helps to calculate how bright the image could be,
not how good it will look. You can have two systems in the same lighting producing
the same amount of brightness, but the contrast could be very different between
the two if they have different screen surfaces. We can also calculate what the
system contrast ratio is which will give us an idea of how good the image will look in
the application. There are a few calculators available that can do this. But you need
to know the following:
-Content/Use of the system
-Image Size
-Projector Brightness
-Throw Distance (or Lens Ratio)
-Light level at the screen
-Light Level at the viewer
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There are now standards created by Infocomm that list the minimum recommended
system contrast ratio standards for four different viewing categories. The four
viewing categories are:
Passive viewing, which is viewing content that you don’t engage in or are reading
information (i.e. moving graphics for retail..).
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Basic Decision Making which is primarily presentations.
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Analytical Decision Making, which is more critical viewing of detailed content (ei.
Spreadsheets, control room, CAD..).
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Full Motion Video, which is high definition content (ei. Theaters, screening rooms..).
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Draper’s Surface Selector Pro helps designers calculate system brightness and
system contrast by factoring in ambient light. The latest version adds some ‘real
world’ choices to the viewing categories on top of the existing Infocomm PISCR
standards.
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