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of the visual acuity of the image analyst which are almost 
identical with these three image quality characteristics. 
Specifically, it is highly relevant for three corresponding 
questions to be asked: (1) to what extent can any given image 
analyst (or applicant for work involving image analysis) per- 
ceive differences in tone or color contrast between an image 
and its background (e.g., does he have some kind of color 
blindness?); (2) to what extent can he resolve fine detail 
and thus exploit the sharpness of an image?; and (3) to what 
extent can he perceive stereoscopic parallax, if at all?; 
unfortunately some image analysts have only one good eye. 
b) It follows from the above that a competent image analyst, on 
examining a given type of imagery, can determine to what 
extent that imagery is of suitable quality in terms of each 
of these three attributes. If, on making such a determination, 
he finds deficiencies in one or more of these attributes and 
wishes to make improvements in that attribute when acquiring 
additional imagery in the future, he needs only to turn to 
the list of factors described above that govern the quality 
of that attribute. In so doing he can promptly learn what 
approach needs to be used in rectifying the deficiency, what- 
ever it may be, when more photography is being taken. 
Two measures of image quality that were being developed at about 
the same time as the above have become known, respectively, as 
"acutance" and the "modulation transfer function". Both of them 
probably are more precisely measurable and consequently more 
appealing to the physicist and mathematician than the "tone- 
sharpness-parallax" concept that has just been described. But 
experience has shown that both of them are much more difficult 
for the average image analyst to comprehend, and to identify 
with. Furthermore, neither acutance nor the modulation transfer 
function gives due consideration to stereoscopic parallax. In- 
stead, each concerns itself primarily with the "sinusoidal res- 
ponse curve" that is obtained when a plot is made of the change 
in tone or brightness per unit of distance along the edge or 
perimeter of a feature, as imaged on photography. Acutance deals 
with only one such edge (e.g., the classic photograph of a "knife 
edge") while the modulation transfer function deals with a whole 
sequence of such edges, repetitively spaced, but at progressively 
closer distances, as in a resolution target. 
By about 1960, a systematic approach to the factors governing the 
amount of information derivable from image analysis tended to 
deal in one way or another with four components, only the first 
of which has been alluded to in the preceding paragraphs. These 
components, at least as set forth in one representative treatment 
(American Society of Photogrammetry, 1960) were described in the 
following statement: "If aerial photographic interpretation in 
any of its applied forms it to be employed successfully, four 
conditions must be satisfied: (a) The aerial photography must 
provide images of suitable quality for extracting the type of 
information that is to be obtained through photo interpretation; 
(b) the men performing the photo interpretation work must have 
been properly selected and trained; (c) the equipment used in 
viewing, measuring, and interpreting the photographic images 
must be of suitable quality; and (d) the methods and techniques