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102 PHOTO INTERPRETATION PICTURE, COLWELL 
c. Dr. Ellis Rabben (1955, 1960) has written two excellent articles on the visual 
factors in photo interpreters. Such articles could only have been written by one having 
a thorough grounding in both photo interpretation and human optics. 
  
Fig. 8. Conventional panchromatic minus-blue aerial photography (left 
photo) is unsuitable for detecting early evidence of diseases in this orange 
grove. Infrared photography (right photo) readily permits the distinction 
to be made, however. On the right photo all dark-toned trees are diseased, 
and all light-toned trees are healthy. (Photos courtesy U.S. Bureau of 
Reclamation.) 
The valuable contributions which these and other workers recently have made in 
the *human factors" field indicate that still greater gains are yet to be made through 
this type of research. In fact, I believe that at present the photo interpretation research 
dollar, when invested in human factors research will buy more to further the state of 
the art than when invested in any other way. 
2. Photo interpreters should better appreciate the value of special purpose photography. 
Regardless of the kind of information he may be asked to extract from aerial photo- 
graphy, the civil photo interpreter of today still has a strong tendency to make do with, 
or even to express a preference for, conventional panchromatic minus blue vertical 
aerial photography, of his study area, scale 1 : 15,000 to 1 : 20,000, taken near noon on 
  
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