MTF curves for lenses have been published (Welander, 1968) and manu- 
facturers supply MTF curves for their films. If TM curves are not 
zed available they can be approximated by a straight line between the 
resolution values for object contrast 1000:1 and 1.6:1 (Welch, 1974). 
This corresponds to modulation of approximately 1.0 and 0.23 
) respectively. The resolution values of the quoted contrast ratios are 
generally supplied by the film manufacturer. 
  
nal 
of The second method is based on the use of a system MTF measured and 
as computed from operational imagery ( Welch, 1974 ). The threshold  mod- 
fer ulation of the emulsion is computed as: 
MTF 
mo - M. * AD / Y (£3.47) 
T 
Here Y is the slope of the characteristic curve of the emulsion and 
) AD the minimum density difference required to resolve a target. AD can 
  
  
    
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
be measured or calculated from MTF and TM curves. A horisontal line 
ect is drawn through the value of M in the MTF plot and the intersection 
is the predicted resolution. M normally varies between 0.05 and 0.2. 
ion 100 100 
ss. 80} 80 Zn Lens + image motion 20 um 
ted F es E 
be 60F 60 | 
« 40H T oL 
«v 
tem . lL € L 
. c 
ing 2 z 
TM : = 
i * 20r Image motion 20 pm 20 [- 
e 
ame 
is : 
for 10 L—L 1.1 L4 1 1 1 IL dL 10 1114 Lid 1 i1 1 
der 5 6 10 15 20 30 40 50 60 100 5 6 10 15 20 30 40 50 60 100 
of Spatial frequency (cycles/mm) Spatial frequency (cycles/mm) 
à Fig. 3.2. 
MTF and TM curves for different contrast ratios used for prediction of 
system resolution. Left : Without image motion. Right : With image mo- 
tion. Shaded area represents uncertainty in determination of lens MTF 
( After Manual of Photograeaetry J). 
3.3 Application Dig] 
  
The techniques described in section 3.2 are used for evaluation of sy- 
stems for aerial photography. They are used to analyze the influence 
T of the system components and to optimize the system. When predicting 
resolution with these methods a human observer is integrated in the 
evaluation through the TM curves. For a digital system it seems more 
11] appropriate to neglect the human observer and focus on the properties 
100 of the digitizing system. The second technique described above could 
be modified in this direction. It can be changed so that instead of 
using threshold modulations related to the human eye a threshold for 
the digitizer is used. To do this the influence of the film 
granularity ( section 4.1 ) must be taken into account. 
  
  
The number of available density levels of an emulsion can be computed 
from the formula ( Manual of Remote Sensing, 1983 ) 
  
  
143