September
2006
Black
Levels and Solid-State Projectors
Solid-state
projectors (LCD, DLP and their derivatives) are
being used as a replacement for CRT projectors in
visual display systems for simulation and training
because of their brightness, fidelity and lower
life cycle costs. The solid-state projector is clearly
an improvement over CRT projectors in all categories
except one
..black level.
What
is Black Level?
The
Black Level of a display device is the light output
produced with no signal input. In CRT projectors,
the light source (electron beam) is turned completely
off. In solid-state projectors, the light source
(mercury or xenon lamp) is always on.
The LCD panels in LCD projectors modulate the light
using cross polarizing panels. The panels are not
capable of completely extinguishing the light transmission
in the off condition. At best, approximately 0.1%
of the light is passed thru the panels and projected
on to the screen (1000:1 contrast ratio).
DLP projectors use oscillating mirrors to control
the light instead of LCD panels. It is a little
known fact that the mirrors always have a partial
"ON" duty cycle, even in no signal (black)
scenes, resulting again in some light being transmitted
to the screen.
Both technologies use beam-splitting devices in
the optical block to separate and then recombine
the red, green and blue components of the white
light source. With thousands of lumens of light
being reflected around in such a small area, there
is no way to completely eliminate "stray"
light from reaching the lens and on to the viewed
image.
Why
is Black Level Important?
Typical daytime images are composed of illuminated
and non-illuminated areas represented by the palette
of colors that can be provided by Computer Generated
Graphics today. In these scenes, your eye will perceive
the non-illumination of black objects as a function
of the brightness of the illuminated or white objects.
The human eyes response to this contrast ratio will
"trick" the eye into perceiving the black
areas as being true black. The greater the difference
in illumination of an image, the higher the contrast,
the blacker the dark levels appear.
In typical dusk or night scenes, the illuminated
areas are dim and represent a smaller percentage
of the overall image content. Under these conditions,
limited Black Level performance of the display system
results in a visible gray background and illuminate
your training area.
Many
visual display systems are comprised of multiple
projectors or channels configured to produce a single,
large continuous image. In these applications, the
individual images from each channel have an area
of overlap to insure a smooth transition of objects
as they cross these boundaries. CRT projectors with
their typical 400-800 lumen output used electronic
edge-blending to create these overlap areas. This
technique is also being used by solid-state projectors
with 1500 to 5000 lumen output. However, the electronic
edge-blending processors cannot compensate for the
"non-black" background produced by solid-state
projectors. The result is that in low light level
scenes, a visible vertical band will appear in the
overlapped regions of the composite image.
Are
there solutions to improve Black Level on Solid
State Projectors?
The
response to that is unfortunately yes and no. There
is no current solid state technology that meets
the absolute black level that can reproduced by
a CRT projector today
.nothing is better than
off and solid state light valves are always on.
The improved contrast ratios touted by some projector
technologies is a combination of reduced "stray
light" in the optical block and increased brightness
in the highlight areas. The inability to completely
"turn off" the light valve still inhibits
reproduction of "true black". However,
there are ways to approach approximately 90% of
the black level capability of CRT projectors and
eliminate the "banding" of the image overlap
regions.
Display
Solutions currently uses multiple techniques depending
on the solid state projector type, the lens throw
ratio and light output. These devices are a combination
of optical grade neutral density filters, solid
edge baffles, graduated edge baffles and bulb correction
technology. These techniques, can be added to all
brands of solid state projector technology and the
resultant image is many steps closer to the black
levels currently provided by CRT's.
Conclusion
Optimizing
the "black level" performance of solid
state light valve projectors in simulation theater
applications requires understanding the limitations
of the technologies and the capabilities of the
compensation devices available and designing the
visual system to match those capabilities. Display
Solutions can best help you achieve the maximum
performance from your theater design by working
with you in the early stages of your projects. We
offer services ranging from visual system design
consulting to display system products including
projectors, baffles and filters and electronic edge-blend
processors. We can also provide complete "turn-key"
visual systems including installation and alignment.
Projection
Design SVGA DLP Projector with Wide Angle Lens and
Edge-Blend Bat Wing Baffles (Black Level Enhancement
filter not shown)
For
more information, contact Ken Boyd at 978-897-9680
or email at ken@displaysolution.com.
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