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PHOTOGRAMMETRIC ENGINEERING 27 
grammetrists and influenced the lens 
manufacturers to devote more attention 
and control to the development of lens 
designs which yielded lenses more nearly 
free from distortion. As an outgrowth of 
this condition, the National Bureau of 
Standards proposed the use of a plane- 
parallel plate between a lens and the 
image-plane to control the third-order 
distortion.? No use of this method of cor- 
rection was made in this country but it was 
applied to the Topogon type of lens pro- 
duced by Zeiss. Russell K. Bean of the 
U. S. Geological Survey brought from 
Germany a lens using the plane-parallel 
plate after the close of World War II and 
subsequently lenses made in this country 
have utilized the same principle. 
The American Society of Photogram- 
metry in the early 1930's appointed a 
committee, under the chairmanship of 
Col. H. H. Blee to formulate specifications 
to govern government mapping contracts 
to be awarded to civilian contractors. At 
that time there were many such contracts 
in connection with the work of the Agri- 
cultural Adjustment Administration and 
other related government activities. The 
National Bureau of Standards was repre- 
sented on this committee and provided 
technical advice. 
Prior to the drafting of these specifica- 
tions it had been customary to give the 
angular limit of resolution in the object 
space or the limit of linear resolution in the 
image space as a measure of resolving 
power; and distortion was measured, 
following the practice of Zeiss publications, 
as a percentage of the distance of the 
respective image point from the center of 
the field of the lens. 
Following the preparation of these 
government specifications, resolution has 
been generally expressed in lines per milli- 
meter in the image plane, and distortion 
is measured as a linear displacement from 
the distortion-free position. Tolerances for 
the performance of airplane cameras were 
formulated,?^ and the government speci- 
fications designated the National Bureau 
of Standards as the agency by which all 
airplane camera lenses should be certified 
before they could be used on mapping 
projects executed for the government by 
civilian contractors. 
After the Air Corps had adopted the 
5-lens mapping camera, lenses were sub- 
mitted to the National Bureau of Stand- 
ards in such number that it became im- 
possible to test the lenses by the optical 
bench method with sufficient promptness. 
Accordingly the precision optical testing 
camera was designed and built.° 
This camera was originally fitted with 
7 collimators spaced 5° to extend over a 
half-field of 30°. Later as wide-angle lenses 
became more generally used additional 
collimators were added to provide for a 
coverage of 45° half-angle. 
In order to better distribute the errors 
arising from distortion over the entire 
negative area with consequent lessening of 
the maximum error, the conception of 
the calibrated focal length was introduced.® 
During the 1930's the emphasis was 
placed on lenses having a focal length of 
1 inches and the performance require- 
ments were constantly being made more 
rigorous. In order to provide the specifica- 
tion writers with performance data on 
typical lenses, tabulations of comparative 
performance were prepared first for 8i 
inch lenses? and next for wide-angle 
lenses.*:? 
The concept of the calibration of the 
metrical qualities of a complete lens cone 
assembly began to assume importance 
about 1938, and a method was developed 
at the Bureau for locating the principal 
point with respect to the fiducial markers 
of the cone.!? 
The volume of camera calibration con- 
tinued to increase and finally in 1949, a 
new camera calibrator was developed and 
placed in operation.H 
This new camera calibrator has proved 
so successful in facilitating the rapid 
accurate determination of the metrical 
properties of precision airplane mapping 
cameras that its principle was at once 
adopted and used in the construction of 
the large Fairchild camera calibrator.? A 
duplicate of this Fairchild equipment in 
in use at the laboratory of the National 
Research Council of Canada. 
During this time, work on the resolution 
of photographic lenses, was not neglected; 
new methods of assessing the image form- 
ing quality of lenses in an objective manner 
are still being studied at this Bureau.!? 
This work on lens performance evalua- 
tion includes the development of a new 
resolving power test chart and the 
microphotometric scanning of the images 
of lines or of edges of opaque diaphragms. 
Recently an apparatus for scanning the