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4. Maximising the usable image in-formation density 
The usable image information density is defined as the 
interpretable information quantity stored per unit area on 
a remote sensing data carrier; it is differentiated accor 
ding to the geometric resolution (or spatial frequency, 
radiometric resolution (grey scale, temperature steps 
etc.), and spectral resolution (bandwidth per channel). In 
photographic systems, the maximum is given by the number of 
tonal density levels attainable per film area and by the 
maximum spatial frequency in resolved lines per mi 11imetre; 
it is related to an optimization of exposure, among other 
factors /7/. 
The resolving power of modern aerial films with their 
greatly reduced emulsion thicknesses is only slightly con 
trast-dependent. Consequently, full utilization of the 
potential blackness or density range in the image should be 
aimed at. This means about 1.6 density units in the linear 
part of the characteristic curve of ORWO VF-45 film or 
KODAK PANATOMIC X-Aerographic II contrary to the former 
constant limitation to 0.85. 
This extension of the density range, in combination with 
the details explained in the preceding chapter, improved 
differential exposure metering and the use of microcompu 
ters on the basis of improved computing algorithms, should 
be capable of maximizing the image information density 
recorded with photographic systems. 
The processes of aerial film development must also be 
considered in this context, because the above measures call 
for a film development to a current nominal gradient which 
must be controllable within a film. This requires a modi 
fied continuous developing machine in which the travelling 
speed of the film is controlled automatically by a data 
carrier that is scanned synchronously with the film and may 
be contained on the film itself /2/. 
5. Necessary preparation of scanner data 
Practical tests with push-broom scanners by means of visual 
navigation and without autopilot equipment in small 
aeroplanes have shown that the position and attitude 
accuracy as well as position stability are not sufficient. 
Considerable geometric errors occur. 
The motion-conditioned distortion within one line is 
negligible, but between neighbouring lines it is imperative 
to take it into account because of the occurrence of 
scanninq gaps (Fig. 3), in particular when a small exposure 
time is chosen in relation to the line cycle frequency in 
favour of a small pixel bluring. 
Overlapping or multiple scanning as shown in the example in 
Figure 3, can be corrected afterwards, but missing object 
details with dimensions smaller than or equal to the 
respective scanning gap cannot be regained in the 
subsequent image processing, which means that an on-line 
compensation procedure has to be applied at least for that. 
The overlaps and gaps remain then sufficiently small.