8.1 Pixel Size 
When digitizing photographic images, the appropriate pixel size for 
the lens/film combination can be found. The pixel size is determined 
by the spatial frequency content of the image according to ( 3.1 ). 
The cut-off frequency can be estimated with the methods described in 
Section 3.2 and 3.3. TM or AIM curves normally integrates the effect 
of having a human observer in the system. Working with a digital 
system, the threshold modulation used for estimating the cut-off 
frequency should be related to the properties of the emulsion and the 
digitizer only. 
When pixel size decreases, the signal to noise ratio decreases. A 
pixel size corresponding to the estimated cut-off can give an unfa- 
vourable ratio. In this case the smallest detail of interest in the 
image has to be considered. If it is considerably greater than the 
resolution limit of the photographic system the sampling interval can 
be adjusted accordingly and the signal to noise ratio improved. 
8.2 Image Quality and System Performance Measures 
Image quality and system performance can be evaluated in terms of 
signal to noise ratios. The methods presented in Section 6 are useful 
and allow for a thorough analysis of the digitizing system in relation 
to the photographic system. These methods can also be used in the 
analysis of systems where the digital image is recorded directly. 
Another possibility is to approach image quality and system perfor- 
mance in terms of information theory. In this case image quality can 
be expressed as information content. This quantity is related to the 
ability to retrieve object structure and to the ultimate resolution 
inherent in the system. We can also make use of information density 
expressed as bits/ unit area. It is possible to apply such methods 
both to digital and emulsion based imaging systems . Thus it becomes 
possible to rate images from different imaging systems. 
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