Radar System Engineering

SEC.7.3] AIDS TO PLOTTING AND CONTROL 221
The display is formed as in any other cathode-ray tube, except that it
is necessary to minimize the diameter of the focusing and deflection coils
to prevent them from cutting off too much light. Between 10 and 12 kv
is applied to the final anode of the tube.
Signals appear as magenta-colored patches against a white background,
and they can be viewed in the presence of a reasonably high level
of ambient light. This property, together with the large, flat, parallaxfree
image, makes the display extremely useful for measurement and
plotting. However, it has serious inadequacies which arise from shortcomings
of the skiatron tube. Contrast is always low, particularly at
low duty ratios. Repeated signals have a very objectionable tendency
to “burn in,” becoming stronger than transient signals and remaining on
the tube long ,after the radar echo that created them has moved away or
disappeared. This difficulty can be considerably alleviated by occasionally
raising the tube temperature and scanning the screen with a weak
electron beam, both of which measures tend to bleach the screen. A
fairly intense burned-in pattern can thus be removed in one or two
minutes.
In its present technical state, the skiatron is most useful for observing
and plotting the courses of ships, from either a shipboard or a coastal
station. It is less satisfactory when used to display aircraft signals,
because of the poor contrast in their more rapidly moving echoes.
Rapid Photographic Pro.iection.-The inadequacies of other methods
of providing a large projection display of a radar indlcat or led to the
development, by the Eastman Kodak Company, of means for the
photography of one full 360° sweep of a PPI, rapid photographic processing
of the exposed film, and immediate projection of the developed picture.
The equipment is shown in Fig. 77.
The camera uses 16-mm film, of Which 350 ft are required for continuous
24-hr operation at two scans per minute. An instantaneous blue
phosphor serves best for photographing. After exposure to a full scan,
each frame is processed with metered quantities of developer and fixer,
the total processing time being 13”5 sec. A total quantity of less than a
gallon of the two solutions is required for 24-hr operation. The projector
unit consists of a 300-watt lamp and a simple optical system. The film
is cooled by an air blast during projection. The maximum diameter of
the projected image is 8 ft, and images of a given point can be repeated on
successive frames within a circle of ~-in. diameter at this magnification.
Plotting means employing this device were just being worked out at
the end of the war. ~ It appeared that this was a highly effective and
satisfactory method of large-scale direct presentation of a radar display.
1L. L. Blackmer, “ P41 (Photographic Projection PPI),” RL Report No. 725,
April26, 1945.