travels through the optical path in the reverse direction to the 
viewing light. The spot on the face of the cathode-ray tube is 
sharply focused on the emulsion of the photographic plate. 
Blue light from the cathode-ray tube spot transmitted through 
the photographic emulsion is reflected by the first dichroic mir- 
ror into the photomultiplier tube. Sensitivity of the photomul- 
tiplier tube to room light and to light emitted from the illumi- 
nating lamp and scattered off surfaces in the optical path is 
minimized by inserting a blue filter in front of the photomul- 
tiplier and a yellow filter in front of the illuminating lamp. 
The problems associated with a flying spot scanner in this 
application are the stability of.the scanning pattern and the 
light output of the phosphor. 
Scanning Pattern Stability 
Any error in the position of the scanning patterns on the 
left and right cathode-ray tubes will be interpreted by the cor- 
relator as a parallax error between the photographic plates. 
Errors in the y direction are corrected electrically and are unim- 
portant. However, error in the x direction will cause the system 
to adjust the z carriage of the Planimat to restore the relation- 
ship between the left and right raster, thus producing a heighting 
error. The sensitivity of the system to this source of error is 
considerably reduced by using a demagnification ratio of 10:1 
between the cathode-ray tube and the film. 
Several factors cause scanning pattern position errors. 
External Magnetic Fields. In many environments strong 
magnetic fields are common. The electron beam in a cathode-ray 
tube is deflected by passing through any magnetic field. Even 
the variations of the earth’s field would be noticeable in the 
present application. To avoid this source of error, the cathode- 
ray tubes have two layers of magnetic shielding. The inner layer 
surrounds only the neck of the tube where the electron beam 
velocity is slowest and hence most sensitive to magnetic fields. 
The outer layer covers the entire tube. Measurements show that 
this system eliminates the effect of all magnetic fields likely to 
be encountered in practice. 
Internal Magnetic Fields. No amount of shielding will elim- 
inate the effect of fields originating within the shielded space. 
In fact, the shield could easily worsen the situation. To eliminate 
this problem, it is necessary to exclude all magnetic materials 
from within the shield. The deflection yoke cannot be excluded, 
and to keep a field from building up, a degaussing circuit is in- 
corporated in the design. This circuit operates automatically 
each time the machine is turned on, and, if desired, can also be 
operated by pushbutton. 
Electrical Deflection System Variations. The electrical 
circuits controlling positioning of the raster have been designed 
to give a stability of better than 0.1 percent to prevent unwanted 
drift. 
Light Output Variations 
There are two main causes of variations in the light output 
of a phosphor: granularity and fatigue. Granularity produces 
relatively high frequency variations in light output. If these are 
not corrected at the source, there is no way of distinguishing 
them from variations produced by the photographic images. 
When the scanning pattern is varying in size and shape as in the 
present system, phosphor fatigue can produce variations in light 
intensity within the scanned area, which can cause false correla- 
tion. To reduce these effects to acceptable levels, an antiscreen 
noise circuit is used, consisting of a photomultiplier tube and 
high frequency amplifier (see Fig. 4). This circuit monitors the 
intensity of the cathode-ray tube spot and provides a bias to the 
grid of the cathode-ray tube to maintain the intensity constant. 
In addition to these short term intensity variations, the 
cathode-ray tube gradually loses emission over a period of a few 
thousand hours. To stabilize against this variation, which would 
affect the efficiency of correlation, a stable photocell is used to 
control the grid voltage of the cathode-ray tube. This circuit 
enables the cathode-ray tubes to be replaced when necessary 
without any further adjustment to the circuit.