Another form of innovative television...
The Eidophor Television System
Note: The information presented here is based on articles
or papers by the following; E. Labin, S. M. P. T. E. Journal,
April 1950; Earl I. Sponable, S.M.P.T.E. Journal, April 1953;
E. Baumann, S. M. P. T. E. Journal, April 1953; Eidophor Training
Manual and brochures, supplied by Bernhard Merk, Switzerland.
From the earliest days of television, large theater size screen
images were a goal for most, if not all of the television pioneers.
Some companies in the movie industry such as Twentieth Century
Fox were also very interested at the time, because this might
provide addition income from their theaters. So they actively
promoted and supported the development of suitable systems that
might accomplish large screen theater television.
The Eidophor system was an example of this and it was in use
extensively from the early 50s, until well into the 80s. EIDOPHOR
is a Greek word combination meaning "Image Bearer".
Invented in 1939, the actual development work began in the early
40s in Zurich, Switzerland, under the direction of Professor Dr.
Fritz Fisher. After considering the many problems, he soon came
to the conclusion that a very powerful arc light source would
be necessary to provide sufficient brightness on a theater size
screen. The next problem was how he could efficiently modulate
such an intense source of light.
Dr. Fisher reviewed all of the light modulators previously
used, particularly the Kerr cell, as was used by Dr. Alexanderson
in his large screen television work. He found the efficiency of
this cell to be much too low for his purposes and so continued
his search. Undoubtedly, Dr. Fisher would also have considered
the Jeffree cell, used in the Scophony theater systems. Unlike
the Kerr cell, which exhibits no memory characteristics whatsoever,
the Jeffree cell was able to store as many as 200 to 300 picture
elements, providing a significant increase in image brightness
on the screen. But even this amount of improvement was not enough
to satisfy Dr. Fisher's goal for brightness.
Dr. Fisher went on to review some work done by Foucault on
the optics of telescopes and also by Toepler who had described
an optical system referred to as the "Toepler Schlieren"
(in German, Schlieren means "streaks" or "striae").
His earliest design based on their work was similar to the
drawing shown here on the right. This is a light control system
based on the phase contrast principle and is a variation of the
Schlieren optical arrangement.
The arc lamp at A, together with the condenser lens B, produces
a uniform illumination of the plane C. A light-modulating or controlling
medium is placed in this plane, between the bar-and-slit systems
at F and G. A field lens is placed so that it images bar system
F upon the opaque bars of system G. The image point at H is located
in the image plane C of the objective lens D. This projection
lens would therefore image the point H at point H' on the projection
screen E.
But this cannot happen because the light beams are being completely
blocked off by the bars of system G. It should be noted that the
incident illumination of every image point at H, is blocked by
the strips of the bar system G. However, if a control medium of
some sort, is located at the image plane C and could be deformed
in a suitable way, diffraction of the light beams would occur.
Those diffracted parts of the beams could pass through the slits
in system G and on to the projection screen as image forming light.
The
next drawing here on the right, shows a control medium, consisting
of a liquid oil film of approximately 0.02mm thickness at the
image plane C. For the sake of this illustration, consider this
oil film as being supported on a thin, flat glass plate.
This layer of liquid is called the Eidophor liquid. It takes
the place of an emulsion on the usual motion picture film in the
film gate, as one would find in the usual projector. If the layer
of Eidophor oil is of uniform thickness and homogeneous, light
passing through the oil film will not be diffracted anywhere in
the image plane C and all of the light passing will be blocked
by the bars of G. No light can reach the screen.
The next step is to create a form of optical inhomogeneity
in the oil film, point by point, that will diffract the light
beam past the bars and through the slits of system G. This is
done with a beam of electrons from an electron gun, scanning an
approximate 3 by 4 inch raster directly on the the oil layer.
The electron gun operating at a 15 kilovolt level, deposits electric
charges point by point, corresponding to the scanned picture.
These charges cause minute wave-shaped corrugations in the surface
of the oil layer. Where the oil surface is corrugated as at H1
on the surface C in the drawing, those light rays passing through
this point are diffracted and no longer blocked at G, instead
passing through the slits and on to the screen. The more the Eidophor
surface is distorted, the more intense is the light reaching the
screen. A brightness range of 1:300 has been obtained.
The drawing to the right shows 
the relationship between the brightness A, along
a line of the image and the amount of the wave-shaped deformation
B, in the surface of the Eidophor liquid. The amount of deformation
on the Eidophor surface is proportional to the desired brightness
level for a corresponding point on the screen.
The Eidophor principle of modulation is for the cathode beam
to scan the Eidophor surface, controlled by a video signal in
such a way that the resulting deformations are proportional to
the instantaneous values of the controlling signal. The actual
controlling element is the spot size of the electron beam. The
smaller the spot size is, the deeper the deformation of the Eidophor
will be, causing more diffraction of light to take place, in turn
producing a brighter spot on the screen.
The wave-shaped deformations are caused by electrostatic forces
in the oil film, due to the electrical charges placed on the Eidophor
surface by the scanning electron gun. The wavelength of these
deformations is constant, but their height is proportional to
the level of the video signal. As the illumination of the image
points on the screen are always proportional to the height of
the waves at the corresponding point on the Eidophor, the distribution
of light over the projection screen corresponds to the video signal
and thus to the object being reproduced.
The deformation commences at the moment that the electron
beam scans a particular point of the image. By a suitable choice
of the conductivity and viscosity of the Ediphor oil, the deformation
can be preserved for a considerable part of the image scanning
period, so that it disappears shortly before the next scan of
that point. In the ideal case, the deformation of the oil should
remain for the duration of one picture period, but then decay
as quickly as possible. In practice, 70% of the ideal is achieved.
Since the screen illumination is maintained for this part of the
scanning period, a substantial increase in screen brightness occurs
due to this light storage effect.
After considerable testing, the results were encouraging.
A simplified compact prototype model was developed. This is illustrated
in the figure below . Notice that it uses only one bar and slit
assembly, which is reflective and actually does double duty. 
Another change in this prototype was the addition of a color
wheel, developed especially for the Eidophor system by the Columbia
Broadcasting System, using its field sequential color knowledge
and techniques. But before this unit could be completed, Dr. Fisher
had died and his work was carried on by his associates, directed
by Professor Baumann and Dr. Thiemann.
Since there is an electron gun in this system, it might be
well to point out that the electron gun and the Eidophor oil can
only operate in a vacuum. The Ediphor oil characteristics are
subject to change with temperature, so the system includes a means
to stabilize the temperature of the spherical mirror and Eidophor
oil in contact with it. This is accomplished with a small external
refrigeration system.
Another view of the Eidophor Projector is given here. It shows
a side view of the vacuum chamber containing the 
lens systems, electron gun, spherical
mirror and the Eidophor oil surface on the spherical mirror. The
mirror rotates at about one revolution per hour to prevent a gradual
build up of charge that would otherwise change the characteristics
of the Eidophor oil film.This drawing shows an arc lamp, but later
it was found that certain xenon lamps could also be used effectively.
The drawing on the right shows the 
approximate size of the Eidophor projector.
The space requirements are similar to those of a standard 35 mm
movie film projector, as found in most projection booths in theaters
around the world. Not shown in this drawing are the various power
supplies and the vacuum pump that are normally contained in the
same cabinet as the Ediophor projector. Also not shown here are
the cabinets that house the various signal associated electronic
circuits. The over-all dimensions of this machine were approximately
5 feet high; 5.5 feet long and 2.5 feet in width. The weight of
this assembly was 1800 pounds.
This photo to the left shows a complete system, including the
two upright cabinets (6), containing the low level electronic
circuits and their power supplies.
In the main assembly, the projection arc lamp (5) is located
at the top left and the vacuum pump and auxiliary services equipment
(4) are directly below it. The color wheel (3) is located at the
top center. The Eidophor projector (1) is at the lower and center
left. The projection light beam hood (2) is at the top right.
In later models like the one pictured below, the arc light
was abandoned in favor of hi-intensity Xenon lamps rated at either
3000 or 5000 watts. A color dot sequential system was also incorporated,
replacing the CBS field sequential method and the purchaser was
then given the choice of using the NTSC, PAL, SECAM or HDTV color
systems.
The over-all specifications of the more recent models of the
Ediophor systems were most impressive. They included these:
Screen sizes up to approximately 40 by 50 feet; 80 times brighter
than than the best three tube CRT systems; up to 1250 lines horizontal,
120 Hz vertical; Video bandwidth, 50 Mhz; all digital control;
white field brightness levels of over 10,000 lumens; projection
throws of over 650 feet.
What a fantastic system! An engineering marvel, if there ever
was one!! Fabulous!!! (Editor's comment)
In spite of it though, the Eidophor is becoming obsolete.
It looks as if it will undoubtedly be replaced by the LCD and/or
the DLP device, manufactured by Texas Instruments, basically an
integrated circuit with teeny,tiny little movable mirrors,
(pardon the scientific terms).
Peter F. Yanczer
2/18/07