Data- and Video Projectors
Autor: Mag. Erhard
current research and development
director of Science-Vision, the research division of In-Vision.
Published in the annual report of the
HTBLuVA M÷dling, 1999.
About 90% of the information from the environment are perceived by the human
eye. Of course this has to be regarded in everyday teaching in classes, and so
many teachers labor with blackboards or overhead projectors to visually
provide study content for their students. Occasionaly, unfortunately much too
rarely, there is a video that suits the study content. Here trouble starts: TV
sets are broadly available, but what is the viewing angle for the student from
the last row? At least he won't be able to recognize details. Let's therefore go
into the cinema and turn on the video projector. This takes time and is not
always possible, but no student is disadvantaged.
But how does such a video projector work, and why can not every classroom have
its own projection device? The second quesiton is easy to be answered: The
projectors are (still) too expensive and/or (still) provide poor intensity of
light for a classroom that is not shaded. The question about the functionality
is not as easy to be answered. In recent time various companies brought
projectors to the market that are based on quite different functionalities. Up
to a few years ago, video projectors were almost entirely CRT projectors,
that are projectors where the TV picture is separated into the three basic
colors, which are displayed on cathode ray tubes each and projected by three
lenses on the screen. This technic has a couple of disadvantages. Starting with
the difficulties to bring the three single images on top of each other when
changing the position, up to the physically caused connection of the picture
quality with the intensity of light: Because of the mutual repulsion of
electrons, an increase in intensity of light, that is only to be achieved by a
more intense electron beam, inevitably leads to a bigger pixel and
therefore to a lower resolution, that is the sharpness of the image. This system
is neither suited for day light projection nor for high definition TV (HDTV).
Modern video projectors are so-called light-valve projectors. In general,
light valves consist of a controllable substance that imposes the image
information to the luminous flux from a light intense projection lamp according
to electronically applied signals. One possibilty to influence the luminous flux
by the image is the liquid crystal display (LCD), and various companies brought
projectors based on this technology for special applications as well as for the
consumer sector to the market. Most of these projectors are based on slide
projection, that is the light of the condenser is directed through the LCD and
influenced for the image information. The principle is the same as for a slide
or film projector where the slide or the film strip is replaced by a LCD. Since
LCDs only can operate gray tones, for the color projection usually a system of 3
LCDs is used. Here the white light from the condenser is first separated into
the three basic colors (red, green and blue), and after passing the LCD is
mirrored together by a prism and projected by a common lens. The most light
intense of such projectors can reach up to 1000 ANSI-lumen on the screen and
would be quite suited for the classroom. Further increase of the luminous flux
is limited though: The functionality of the LCD is based on tilting the
polarization plane - useless light (black parts of the image) is absorbed in the
polarization foil. Since light is a form of energy, it causes heating of the
polarization foil that generally is glued to the LCD. And LCDs only work up to a
temperature of about 65░C.
LCD projectors that use color LCDs are older and therefore cheaper, but bigger
and less intense in light.
Apart from these systems which are based on the principle of slide projectors,
there are also methods based on epiprojection. The oldest projector of
this kind, capable of large-screen projection, is the so-called Eidophor
projector. It contains a shperical reflector with an oil film that is
deformed according to a charge pattern written by an electron beam. The
light intensity is not reduced by reflection, but its direction is changed
slightly (phase-modulated) on those spots with a deformed oil film. The
conversion from phase modulation to intensity modulation (projection image) is
accomplished by an optical streak dark field system. Simply spoken, those light
beams that were not changed in direction are reflected back to the lamp, and
those that changed direction due to the deformed oil film go through the
projection lens and cause an according pixel on the screen. Since the light
intensity only depends on the bulb used, large images with high resolution are
possible. And since the light that is not for the image is reflected and not
absorbed, there is no heating. Such projectors were used in the past few years
for example during the "Wiener Festwochen" in front of the city hall in Vienna
for the projection of operas and concerts.
Recently, there has been intense research in the field of light modulation that
should eventually lead to a projection system similar to Eidophor projectors.
One of these methods is downright the ideal combination of mechatronic
and optical principles. Since this system is on the verge of bringing on
to the market, I want to describe it here in a little bit more detail. The heart
of this projection system is an arrangement of mirrors that consists of a large
number of tiny single mirrors. This arrangement is called DMD (Digital
Micromirror Device). Each single quadratic mirror has a length of 16Ám (even
more recently 13Ám), and is supported by torsion springs. In figure 1 you can
see the arrangement of two such mirrors. In the idle state, that is without
activation, the mirror planes of the single mirrors (in figure 1 drawn
transparently) are arranged parallel to the base. If they are controlled by
electronics (CMOS matrix), they can be tilted into two depicted directions: In
one end position they are tilted by -10░ (recently -12░) (off state), and in the
other end position they are tilted by +10░(+12░) (on state). The tilting angle
is limited by the stop position. The mirror tiltings are caused by electrostatic
attraction, caused by the voltage difference between mirror resp. moving
electrode and the corresponding memory component on the CMOS matrix.
Fig. 1: Two DMD pixel. (Mirrors are
Starting point for the production of a DMD is the creation of a CMOS circuit on
an according substrate. A relatively thick oxid layer is applied to the last
metal layer of the CMOS and using the special chemical mechanical polishing
method (CMP) it is exaclty planar polished. This is necessary in order for the
single mirrors to have exactly the same conditions for the procuction. The
creation of the micro mirror structure is achieved using six photo masks with
witch the electrodes, the torsion springs, the mirror as well as distant pieces
are formed out of aluminum using thin film technical methods. To produce the
necessary distances between the aluminum layers, additional layers made from
organic material are also applied. The structure of the aluminum layers is
formed by plasma etching using a SiO2
- etching mask. After
completion of the last layer (mirror layers) the organic layers are dissolved
again by plasma etching. At last, a thin oil film is applied so that the stop
positions of the moving electrodes will not stick to the support by adhesive
forces during switching.
Figure 2 showes the structure of a micromirror retainer yet without the mirror
as an image of a SEM (Scanning Electron Microscope). You can see clearly the
torsion springs and the stop positions at the vertices of the moving electrode.
Fig. 2: Photo of torsion spring and moving
electrode made by an electron microscope.
The DMD can be understood as a "light switch" (SLM, Spatial Light Modulator)
which either reflects the light bundled from the condensing lens through the
lens system or or aside from it (DLP, Digital Light Processing system). In fig.
3 this principle is shown for a single mirror. Since the change of light
direction is twice as large as the rotation angle of the mirror, the condensing
lens can be pivoted by about 20░ from the projection axis. It is important that
the so-called "0░-light" does not enter the projection lens, since a pixel error
would cause a bright, disturbing image pixel. In the practical
construction of course this design is not possible, because the DMD area has to
be illuminated uniformly and the light has to be concentrated in the entrance
pupil of the projection lens to obtain a bright projection image. Therefore,
further optical elements - prisms and/or lenses - have to be arranged.
Fig. 3: DMD
as an optical switch.
Different gray tones can be optained by varying pivot durations of the micro
mirrors, since the eye integrates repeating light pulses within short times
(1/50 second). Since the minimal optical switch time is about 2 Ás, there are
enough time intervals at a refresh rate of 50 images per second to display 256
or even 1024 different gray tones.
But how can we add color to the system? One possibilty results from the short
switch times of the DMD and the fact that the human eye integrates consecutively
displayed colors and lets us recognize them as a mixed color. The image
information provided by an RGB signal is switched consecutively by the
electronics of the DMD and therefore consecutively projected. Between the
condenser lens and the DMD there is a so-called color wheel, a tripartite filter
wheel, that lets red, green and blue parts of the white light pass through its
color segments. Of course an exact synchronisation between rotational speed and
position of angle of the color wheel and the controlling of the DMD is necessary
so that the DMD will for example exactly switch to the gray tone fraction of the
blue image information when the blue segment of the color wheel enters the
optical path of the condenser.
Another possibility to build a color projector is using three DMDs. Here the
light first has to be split into its three basic colors by a prism system and
after reflection by the respective DMD reflected back together. One possible
design of such a projector is shown in fig. 4 in a simplified way: The light of
a metal vapor or xenon lamp is first aligned parallel by a paraboloidal
reflector and directed by a condensing lens to the DMD. In order to keep the
distance between the projection lens and the DMD small, the reflection is
accomplished by a so-called TIR-prism. "TIR" means "total internal reflection".
The angels of the prism are devised in such a way that the light coming from the
condensing lens is totally reflected, but the light coming from the DMD can pass
unobstractedly. The two single prisms of the TIR prism are not glued to each
other, but have a distance of about 10Ám. The color separating prisms have to be
designed so that the distance from each DMD to the projection lens is exactly
the same. Before gluing the prisms of the color splitting system, dichroic
coating is vapor deposited. These coatings reflect only a certain spectrum range
(blue or red), and let light of different wavelength pass almost completely. For
example, in fig. 4 the first color separating prism reflects blue and lets green
and red pass through. The blue light reaches the DMD3, is reflected there
according to the image information and comes after repeated reflection at the
dichroic layer of the first color splitting prism into the projection lens.
Fig. 4: Optical system of a 3-chip DMD
The resolution of such a projector depends on the size of the DMD. Since the
central distance of two micro mirrors is 17Ám, a DMD with a diagonal of 0.7 in.
(17.8mm) can achieve a resolution of 800x600 pixels (SVGA). For XGA resolution
(1024x768), a diagonal of 0.9 in. (22.9mm), for SXGA resolution (1280x1024
pixels) a diagonal of 1.1 in. (27.9mm) is necessary. Recent DMDs with 13.68Ám
mirror distance can have SXGA resolution at a diagonal of 0.9 in. Also
projectors with HDTV capable resolution (1920x1080 pixels, 16:9 aspect ratio)
have been introduced at international fairs. The light intensity is by all means
comparable to the abovementioned Eidophor projectors.