Module 31: Projection Displays
31.1. Introduction
Flat panel displays become larger every year, but until large flat panel displays
decrease significantly in cost, projection systems will be marketable. In a sense
CRTs are projections systems, but for large areas, the evacuated glass envelope
becomes too large and heavy. Projection systems using some sort of spatial light
modulation (SLM) have an advantage since the optical path can be tightly folded in a
compact form to save space.
Projection displays generally use a lens to focus the magnified image of a small
internal display, thereby projecting the image onto a screen. Front projection systems
typically use a long throw distance (the room) to project onto a wall whereas rear
projection systems use screens with gain to redirect the light more efficiently.
Another distinction in projection systems revolves around how the image is created.
Light valve projection systems use a fixed light source and an independent
modulation element to form the image. They can have high brightness and contrast
because the light source and modulation can be independently optimized. CRT
projection can have reasonable brightness and efficiency using conventional CRT
technology and optically merging the three monochrome CRT images that generate
red, green and blue.
31.2. Types of Projection Configuration
The primary distinction between projection systems is the configuration: front or rear
projection. The viewer and the projection unit are on the same side of the front
projection screen, while the rear projection system is hidden behind the screen. In
most applications, front projection is not used in homes since it requires a long throw
length, and diffuse reflections from ambient light degrade the contrast significantly.
Rear projection systems consume valuable space behind the screen, even though
the optics can be packaged up compactly, it does use space. A simple configuration
of
a
rear
projection
screen
is
shown
below:
31.3. Rear Projection - Contrast and Gain
Independent of projection technology, the light source, image generator, and screen
are all critical components in optimizing performance. Comparatively speaking,
screen performance is the main advantage of the rear projection over the front
projection screen technology.
Consider the following screen example. The diverging beam of projected light is
directed into the viewing angle of a Fresnel lens as shown below. The image is
formed on a second sheet which has particles dispersed in it to scatter light over a
certain angular range. Focusing and defocusing the light results in passive gain,
which means that the image will be brighter within the intended viewing angle than it
would appear on a Lambertian screen.
The second element also has lenticular elements molded into the lens to expand the
horizontal range. The screen would not be very satisfactory if the horizontal viewing
range were as narrow as the vertical.
These elements also serve to increase light entering the back of the lenticular sheet
becomes concentrated and emerges at the front between black stripes, located here.
These black stripes absorb the light that falls on the screen between lenticular
elements, but the projected light passes through the screen with less than ~10% loss.
A larger fraction of the ambient light falling on the screen from the room is interpreted
and absorbed by the black stripes - this improves contrast.
31.4. Wavelength and Polarization Separation
Many projection displays effectively manage light more efficiently than direct view
displays. As an example, shadow mask CRTs and LCDs that utilize spatial color
synthesis techniques to create full color image are less efficient than monochrome
displays. In a color CRT only about one-fifth of the electrons pass through the
shadow mask and become available for visible light creation. In LCDs the color filter
arrays can reduce transmission by 3-5 times. In projection devices light can be
managed much more effectively. Dichroic mirrors are often used to separate light into
its primary components, as well as to recombine the light after it has been processed.
Polarizers that are used on direct view displays throw away the light through
absorption, but clever optical design can recycle unwanted polarized light and
convert it into a polarization that can be reused. Before we go into the various
projection technology, it is worthwhile to cover some standard tricks of the trade.
31.5. Color Separation and Projection Lenses
In full color, light valve systems, reasonable color separation efficiency is
usually achieved with dichroic mirrors, which reflect a well-defined bandwidth from a
portion of the spectrum and transmit the rest. Dichroic mirrors can perform two
functions in projection display technology: such as splitting out the colors needed
from a broadband light source, or recombining colors from separate spatial light
modulators or individual light sources.
Dichroic mirrors are known to the optics community as interference filters, which
consist of alternating layers of two materials with different indices of refraction. The
reflection at any given boundary is small but when added up in phase, they are very
efficient. The angle of incidence with the dichroic mirror is critical, because of the
change in pathlength associated with higher incidence - therefore the angle at which
the mirror is oriented in the optical system is a design parameter.
The size of the projection is often critical especially in today's market where many
projectors can be carried during travel. One way to create a folded optical design is
the Scholl method, as shown below:
The mirror configuration is the same as of a pentaprism, but the prism has been
eliminated. The nominal angle of incidence for all beams in the system is 22.5°.
Dichroic 1 reflects red and passes the rest of the spectrum, while dichroic 2 reflects
blue. In this particular system dichroic 2 is only required to transmit green light, but
could also be used to transmit red.
Some CRT projections combine three color images in a cube in which two dichroics
intersect one another diagonally, resulting in an angle of incidence of 45°. In this
configuration, the image can be transferred to the screen with only one projection
lens instead of three.
This is advantageous for front projection systems, where the distance from the
projector to the screen and screen size may vary. For rear projection systems, the
distance to the projection screen is fixed, and it is simple to project images from a
CRT to the screen through three separate lenses. In this configuration the three
lenses tend to be large because shrinking the CRT would decrease light output and
resolution.
Projection lenses collect surprisingly little of the light emitted from a CRT. For
example, a typical lens with a 1/f aperture collects light from a cone with a half-angle
of only 27°. Only 20% of the Lambertian distribution passes through this cone. This
inferior collection factor decreases the efficiency advantage of using three
independent monochrome tubes. This is one reason why projection systems are only
typically used only for generating images that are too large for direct view displays.
The problem of collecting light in a more effective manner has been addressed by
incorporating an interference filter in the CRT face plate. In this application the
angular dependence of the mirror can increase the light output from a CRT. The
figure below shows the light emerging from the phosphor layer and encountering an
interference filter before entering the face plate.
The filter cuts over from transmitting to reflecting in the 30°-40° range thus narrowing
the emission angle light that would accur at layer angles and not be collected by the
projection lens.
Light that is emitted at larger angles will be ejected by the filter and not collected by
the projection lens. Since the phosphor layer is diffusely reflecting, a portion of the
reflected light can be scattered back into the collection optics where the filter
transmits. Therefore, more light is collected by the projection lens, and unwanted
wavelengths emitted by the phosphor layer can be rejected by the filter. A gain in
luminance can be achieved, however, interference filters are costly.
31.6. Polarization Separation
Most liquid crystal technology requires polarized light input. Practically speaking,
more than 50% of the light is absorbed by the first polarizer, and the second polarizer
can also absorb some of the selected components. Because of this limitation, various
projection designs that utilize LCD technology try to manage polarized light.
The figure below shows a compact projection design in which the three color images
are combined by crossed dichroic mirrors.
Crossed dichroics have been used transmissively to combine color channels in both
CRT and LCD based systems, but in this configuration also work in reflection. As a
result the same dichroic beam splitter can be used to both separate and combine the
colors. Half of the system aperture is filled with the polarization that passes directly
through the polarizing beam splitter. The other polarization is reflected internally and
becomes rotated by 90° after twice passing through a quarter wave plate. The
reflected light emerges below the original beam and fills the remainder of the system
aperture.
The electrooptic spatial light modulators in the system act as switchable quarter wave
plates. Light that has its polarization rotated by one of the modulators subsequently
reflects from the internal diagonal in the polarizing beam splitter and passes out
through the projection lens. The rest of the light passes through the beam splitter and
returns to the source. Light that is returned to the source may be reflected and added
to the output. A major challenge is to conceal the bonding between the two values of
the aperture.
A modifiedsystem shown below does not double the aspect ratio of the source but
alters the capability to recycle the unwanted polarization.
In this configuration the mirror reflects the S-wavelength, which is polarized
perpendicular to the plane containing the incident and reflected propagation
directions, back into a polarizing beam splitter. P wave is defined as polarized in the
plane of incidence, which passes through the beam splitter and can be used by the
system. The S- wave is then reflected by the beam splitter back to the source. A
quarter wave plate is placed between the lamp and the polarizing beam splitter. The
effect on the polarization of the reflected light is the same as in a circular polarizer
used as a contrast enhancement filter.
The principle axes of the quarter wave plate are at 45° with the respect to the
polarization axes of the S and P waves. Assuming that the light undergoes specula
reflection inside the lamp, two passes through the quarter-wave plate rotate the plane
of polarization by 90°. The recycled light in this case becomes P-wave. A problem
with this system is that the intensity and the
reflections.
spectrum can be influenced by multiple
Another possible configuration is shown below.
In this case the unpolarized light will traverse through a polarizing beam splitter. The
S and P polarizations emerge from the beam splitters and are reflected by mirrors
towards the liquid crystal spatial light modulation from opposite directions. After
passing through the modulator, the two beams retrace one another's paths back to
the beam splitter. At individual pixel positions the modulator rotates the polarization of
the two beams by 90°. In this case the S changes into the P wave and the P changes
into the S wave, and the two beams will retrace each other's paths into the light
source. In the pixel position where the two beams pass through the modulator un
unaffected, however both will emerge from the forth face of the beam splitter and
enter the projection lens. Both ferroelectrics and twisted nematics can be used to
simultaneous modulation of oppositely -propagating, orthogonal polarized light
beams. For this particular configuration, the system treats the two polarizations
symmetrically, but it is difficult to devise a color version that is as compact as the
system described above.
31.7. CRT Based Projectors
Typically projectors based on CRT technology are comprised of three individual
CRTs with dedicated projection. Most CRT projection systems are designed for rear
projection applications.
The figure below shows an example of a CRT projection that is designed for high
definition television (HDTV).
Typically, the faceplate of the CRT is concave. Negative curvature on the inside of
the faceplate improves the image quality in two ways:


The curvature tends to flatten the field of the projection image (which is more
difficult to accomplish with extra lens elements as the projection distance
increases).
The curvature also makes the projected image brighter at the edges. The
angular distribution of light emitted by the phosphor face plate at any position
is centered about the direction normal to the face plate. The normal is tilted
into the lens at the edges, so light emitted there is collected if the screen were
flat.
The coolant between the first projection lens and the face plate improves the
longevity of the phosphor and tube in three ways:



It conducts heat away from the face plate.
It reduces temperature gradients, minimizing tube cracking.
It is index matched with the faceplate to suppress unwanted reflections,
thereby improving contrast and brightness
To optimize brightness, large optical elements in CRT based projectors are utilized.
The faceplates are usually 5-7 inches (diagonal) to maximize efficiency. The
efficiency of phosphor depends on the excitation energy pulse phosphors saturate at
high input power because all the activation centers become excited. The light output
depends on the density and decay time of activation centers, and after saturation,
increasing the input power does not increase the light output. Making smaller CRTs
with higher resolution would require less phosphor at each pixel and less light output
at saturation. The figure below shows the efficacy of some common phosphor
materials.