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REPORT OF COMMISSION VII 
blue for the separation of hardwood trees from conifers, the delineation of water 
courses, the detection of swampy areas and the mapping of shorelines and other 
features on distant oblique photographs. On the other hand, panchromatic 
minus-blue photography usually is superior to infra-red minus-blue for distin- 
guishing details in shadows, measuring heights of objects, and delineating roads, 
trails and contours in forested areas. Still other film-filter combinations may be 
‘superior for distinguishing certain soil types, for recognizing underwater fea- 
tures, for the detection of insect or disease infestations of crops, etc. 
It is obvious from the preceding that no single film-filter combination is 
superior to all others for making a complete interpretation of the many features 
imaged on a photograph. For example, if the interpreter were given three dif- 
ferent types of black-and-white photography, such as panchromatic minus- 
blue, panchromatic green, and panchromatic red, he usually could make a much 
more complete and reliable interpretation than if limited to a single film-filter 
combination, as he ordinarily is, because of economic considerations. This has 
led to tests by Colwell (1950), O'Neill (1952) and others in which color photog- 
raphy has been used in an attempt to record the entire visible spectrum of each 
object. The single set of color transparencies is then viewed stereoscopically 
over a light table, first with a minus-blue filter; then with a green filter; then 
with a red filter; and, if desired, with any other filters, each of which is optimum 
for the interpretation of certain features. The filter offering best tone differentia- 
tion for any given feature can, in many cases, be predicted from diagrams show- 
ing the reflectance spectra of the various features imaged on the photographs. 
(revised MANUAL OF PHOTOGRAMMETRY—Chapter 12). These reflectance spectra 
are obtainable by laboratory spectrophotometer analyses of samples which 
are representative of the various objects to be interpreted. 
Results of tests on the interpretation of color photography through a variety 
of filters, while very promising, have thus far been conducted on too limited a 
scale to permit final evaluation of color photography for interpretation purposes. 
Certainly, before undue enthusiasm is generated for the use of color photography 
by interpreters, careful consideration should be given to such factors as (1) the 
alteration in color produced by intervening atmosphere; (2) the peculiar type of 
color blindness exhibited by the eye when viewing very small, colored objects 
(Middleton, 1950); (3) the increased cost of film and of processing when color 
photography is used instead of black-and-white; (4) the increased risk to the 
success of a photographic mission imposed by the fact that color film has a 
relatively narrow latitude of acceptable exposure; and (5) the limitations to field 
use of color transparencies imposed by the usual requirement that they be 
placed over a large light table for viewing and examined through a mirror stereo- 
scope. 
As a working compromise on any given photographic project, the added in- 
formation derivable from color photography might be weighed against the 
added cost of procuring it, with the result that sample coverage might be ob- 
tained in color only at selected spots throughout the area. Interpretation of this 
photography and comparison with black-and-white photography of the same 
area might then permit extension of the added information to the remaining 
areas of the project for which only black-and-white photography is available. 
C. PHOTOGRAPHIC FLASH UNITS: 
Because of the difficulty experienced in attempting to interpret features in 
the shaded portions of a photograph, some interpreters recently have expressed 
interest in the possible use of photographic flash units for low-altitude daylight 
photo-reconnaissance. One type of photographic flash unit which was used in