Understanding optical screen technology
Technology Brief
Introduction
Christie’s control room cubes and display enclosures consist of a
DLP® rear projector mounted on a pedestal and surrounded by
an enclosure. The front surface of each enclosure is a projection
screen. The enclosure will also typically contain a first-surface mirror
to direct the light from the projector to the screen (Figure 1).
Figure 1 - A typical rear-projection system
When selecting a rear-projection system the projector usually
gets the most attention, but the screen is of equal, if not
greater, importance. The choice of screen will to a large extent
determine whether the projection system as a whole can live up
to its full potential and meet the needs of the application. Since
rear-projection screens are an integral part of the projection
system’s optics and are typically expected to operate in highambient lighting conditions, choosing the right screen is vitally
important.
Types Of rear-projection screens
Fundamentally, there are only two major types of rear screens:
diffusion and optical. A diffusion screen essentially scatters the
light from the projector that falls on its rear surface, re-directing
it in every direction away from the screen’s front surface in a
hemisphere. The scattered light forms the image you see on the
screen. The scattering process also means, unfortunately, that
a good deal of the projector’s light goes in directions where no
one is looking – off to the extreme sides, for example. This not
only wastes light but means the image that viewers in front of
the screen see is dimmer than it could be.
Almost paradoxically, a diffusion screen tends to hotspot, the
image looking noticeably brighter directly in front of the viewer
than it does elsewhere on the screen. This is due to the fact that
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Figure 1 - A typical rear-projection system
the projector’s light rays hit different parts of the screen at very
different angles, which is something the screen itself cannot
compensate.
Optical screens are designed to avoid the shortcomings of
diffusion screens. An optical screen is usually composed of two
elements: a Fresnel lens and a screen layer (Figure 2). Light
rays from the projector are directed to the Fresnel lens, which
is a sheet typically made of acrylic (or other polymer) a few
millimetres thick, composed of thousands of thin, concentric
rings, each ring diffracting (bending) light in a different, but
exactly calculated way.
Figure 2 - Operation of a two-element optical screen
The rings form a flattened lens that focuses the light from the
projector so that it is directed straight out towards the audience.
In this way all the light rays intercept the second element at right
angles to it, instead of at many different angles. To accomplish
this the Fresnel lens’ focal length must be well matched to the
projector’s throw ratio (the ratio of the projector’s distance
behind the screen to the image’s width).
The second element of the screen is the “screen” element,
which will actually display the projected image. It is also a
polymer sheet, usually containing embossed strips on its
outer (front-facing) surface, each strip having a particular
cross-sectional shape. This type of element, called a lenticular
element, both diffuses the light from the projector and spreads
it over a range of horizontal and vertical viewing angles. Many
different kinds of lenticular sheets are possible, each optimised
for a different range of viewing angles, and hence suited for
different applications.
It is possible to combine the Fresnel and lenticular functions in
a single sheet, with the Fresnel rings on one surface and the
lenticular strips on the other. This is called a single-element
screen to distinguish it from the more common dual-element
variety described above.
Understanding optical screen technology
Technology Brief
Two additional numbers derived from the gain curves are
commonly quoted on screen datasheets. These are the
horizontal and vertical half-gain angles, which are simply the
angles at which the observed brightness is half of the maximum
seen from straight-on.
Ambient light rejection
Most optical screens also have the ability to reject ambient
light that falls on their front surface (the viewing side). This is
often achieved by embedding very thin black stripes within
the lenticular sheet, too small to notice from a normal viewing
distance. These stripes absorb a good portion of the ambient
light falling on the screen from the room, thus preventing that
light from reflecting back to viewers, which would reduce the
inherent contrast within the image. Other, more advanced
methods proprietary to particular screen manufacturers have
also been implemented, some of which cause the screen to
appear black even when the projector is off.
Figure 3
Figure 3 - A cross-lenticular screen
Alternatives to the traditional lenticular element are possible;
most are proprietary to a particular screen manufacturer. One
such alternative uses tiny, glass beads distributed within the
substrate of the sheet’s polymer material. Another uses crossed
lenticulars that together act like prisms to produce a much
wider usable vertical viewing range than a standard lenticular
yet yielding better brightness than available from most beaded
screens (Figure 3).
However it is done, the ambient light rejection function of most
optical screens, combined with an optical screen’s inherent gain
allows images with very good contrast to be achieved in normal
levels of ambient lighting, such as would be found in a control
room or the higher levels typical of a meeting room or office.
It should be noted that most rear-projection screens aren’t
designed to reject ambient light incident on the rear (projection)
side of the screen. That’s quite understandable, as this is the
Screen gain
An optical screen is a passive device and is therefore not able to
amplify light in the sense that a transistor amplifies an electrical
signal. Nevertheless, it is able to produce much brighter images
than a diffusion screen. This is due to the combination of the
Fresnel and optical screen elements, which allows light from the
projector to be concentrated in a range of viewing angles much
narrower than a hemisphere.
Instead of wasting light off to the sides where no one is viewing,
an optical screen preferentially directs that light toward the
audience, resulting in a substantial increase in brightness. The
increase in brightness directly in front of the center of the screen
compared to that which would be seen on a theoretically perfect
diffusion screen is called the screen gain.
Since it is only a single number, screen gain can’t completely
describe the gain performance of an optical screen. That
requires a pair of curves, one showing how the gain varies with
horizontal viewing angle and the other similarly showing how
gain varies with vertical viewing angle (Figure 4). Since an optical
screen directs light away from the sides towards the center, the
gain necessarily decreases with increasing viewing angle. How
fast and in exactly what manner it decreases depends on the
specific design of the screen.
Figure 4 - Vertical and horizontal half-angle curves
side the light from the projector is hitting: It generally wouldn’t
be a good idea to do anything that might decrease that light.
Hence, most rear-projection systems must be housed in an
enclosure, either a self-contained one – often called a “cube”
– or as part of a separate projection room adjacent to the main
room. In either case, it is common to use one or more firstsurface mirrors to fold the optical path between the projector
and the screen, thereby reducing the depth required behind the
screen.
Understanding optical screen technology
Technology Brief
Choosing an optical screen
Many different types of optical screen are available today, one
to fit almost every rear-projection application. The choice of
screen will depend upon a number of factors:
• Budget – Different types of screen use different materials
and manufacturing processes, both of which affect cost. In
general, lenticular screens will cost less than designs that
use embedded glass beads or crossed-lenticular elements.
• Viewing positions – The location of viewers in the room and
the angles at which they see the screen should be strongly
considered. For installations where viewers can be positioned
off to the sides, a screen with a wide horizontal viewing angle
will be required. When the screen may be viewed from a
different height, a large vertical viewing angle will be required.
A screen that is part of a videowall in a typical control room,
for example, will require wide viewing angles both horizontally
and vertically because of the various locations of the viewing
stations and the higher elevation of the upper rows of the wall.
The benefits of optical screens
By combining a Fresnel lens with a lenticular, beaded or crosslenticular structure, whether implemented in a single sheet or
multiple layers, optical rear-projection screens enable bright
images with excellent uniformity that are visible over a wide
range of viewing angles. In addition, they almost universally
employ technology to reject ambient light from the viewing
side, resulting in excellent contrast and enhanced readability.
When properly matched to a projector’s throw ratio, an optical
screen will ensure the best possible image quality for any given
rear-projection application.
• Size and shape of screens – Sizes range from 40 inches
diagonal to as large as 200 inches diagonal. The most
common aspect ratio (the ratio of width to height) is 4:3.
Other common aspect ratios are 5:4 and 16:9. However, not
all sizes are available for a given screen type. For example,
cross-lenticular screens are currently available only up to
80 inches diagonal when sized for a 4:3 aspect ratio.
• Ambient light - Since screens usually have to contend with
a certain level of ambient light, some degree of ambient
light rejection in the screen will be required in order to
ensure sufficient contrast in the image. A higher screen
gain can also combat ambient light but with most screen
types there is a trade-off between gain and viewing angle.
Wider viewing angles usually mean lower gain. However,
better screen designs, such as cross-lenticular, can provide
good performance for both parameters simultaneously.
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For the most current specification information, please visit controlrooms.christiedigital.com
Copyright 2009 Christie Digital Systems, Inc. All rights reserved. All brand names and product names are
trademarks, registered trademarks or tradenames of their respective holders. Canadian manufacturing facility
is ISO 9001 and 14001 certified. Performance specifications are typical. Due to constant research, specifications
are subject to change without notice. Understanding optical screen technology tech guide July 09.