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
The next drawing here on the right,
shows a control medium, consisting of
a liquid oil film of approximately 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 Eidophor system was an example of this and it was in use extensively from 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 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
The Eidophor principal 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
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
After considerable testing, the results were encouraging. A simplified compact prototype model was developed.
This is illustrated in the figure above . 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 two upright cabinets (6), containing the
low level electronic circuits and their power
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 40 by 50 feet; 80
times brighter that the best 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
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
Peter F. Yanczer