NOTE: Both systematic and random differences exist between laboratory distortion measurements 
and the departures from ideal central projection found for a camera in flight. Some causes are 
curvature of the earth, atmospheric refraction, camera temperature, air pressure, temperature 
and pressure gradients near the camera and aircraft, lack of flatness of the emulsion, and 
dimensional changes in the emulsion. For the most accurate work account must be taken of such 
differences. This can be done from photographs of a test area taken with the camera, the 
aircraft, and the conditions all the same as in the survey photography or data on the 
individual effects for the conditions of use can be obtained and combined. 
Discussion 
The NRC camera calibrator (Carman and Brown 1978), USGS calibration facility (Tayman 1978), and US i 
Air Force, Ogden, Utah, camera calibration laboratory use multicollimator test instruments for testing 
mapping cameras photographically. Laboratory calibration procedures offer a high degree of control 
which relates to the precision with which the camera can be mounted and held during calibration, the 
uniformity of light, the temperature, the pressure, and a standard target for comparison of image 
quality. The field test range fly-over method also meets the requirement for photographic type 
calibration, but does not offer the same degree of control. The aerial mapping camera is a precision 
measuring instrument. Although fundamentally an optical instrument, it is dependent, for proper 
functioning, not only upon proper design but also upon proper performance of a large number of | 
elaborate and complicated mechanical parts, among which are the film magazine by which the film is 
advanced, constrained to lie flat, and brought in contact with the camera's focal plane. It is for 
this reason, that the U.S. Geological Survey performs also operational type photographic calibrations. 
The complete camera system is operated in the laboratory to make film test exposures. From these 
exposures, contact glass (micro-flat) diapositives are printed. Using these diapositives, measurements 
are made for calibration and performance evaluation of the lens, camera and magazine system. The 
fundamental requirements for any type camera calibration should be that the negative, when exposed in a 
magazine, has the same metrical characteristics and accuracy as results from flash plates or goniometer 
measurements. This method of calibration was brought about by discovery or knowledge of 
camera/magazine malfunctions that can only be detected by an operational type test. The following are 
camera/magazine malfunctions that affect the true calibration of a camera system: 
1. Platen not seating properly on camera focal plane. The platen pressure may be too great, thus 
causing platen deformation, or too low, with the platen not seating firmly on the focal 
plane. 
2. The height of the focal plane with reference to the camera body is not equal on all four 
sides. This condition can cause the platen to be out of contact with the focal plane on one 
or two sides during the film exposure. 
3. Vacuum-induced platen deformation. 
4. Malfunction of fiducial mark illumination or data chamber registry. 
We recommend that laboratory calibration should include a test enabling the -control under operational I 
conditions of the magazine. i 
2.2 Definition | 
2.2.1. Fiducial Centre: Point of intersection of fiducial axes. 
2.2.2. Principal Point of Autocollimation: The centre of the image formed in the emulsion plane by Il 
the camera lens from an incident-beam of parallel light which in the object space is | 
perpendicular to the emulsion plane. 
NOTE: The principal point of autocollimation is the type of "principal point" which can be | | 
determined most directly and accurately. Other definitions of principal point have been 
suggested as partial remedies for inaccurate lens centring. If it is desired to use 
one of them for some particular application the test data are sufficient to permit is 
mathematical determination. 
2.2.3. Measured distortion: Measured distortion is a vector quantity, being the displacement from the 
theoretical image point of an ideal camera of the chosen calibrated principal distance to the 
actual image point for the camera under test. It has as components radial measured 
distortion and tangential-measured distortion. Radial measured distortion is positive when it 
is outward from the principal point of autocollimation. Tangential measured distortion is 
positive when it appears as counter clockwise to an observer who is in the image space and is 
looking toward the lens. The origin for distortion measurement is the principal point of auto- 
collimation. Measured distortion is zero at that point. 
2.2.4. Theoretical Distortion: Theoretical distortion is an aberration affecting the position of 
images off-axis, caused by the fact that objects at different angular distances from the axis 
undergo different magnifications.  Numerically, theoretical distortion is the displacement from 
the theoretical image point of an ideal lens of the same principal distance to the theoretical 
image point for the actual lens design under study. It is positive when it is outward from the 
centre of the field. Theoretical distortion is purely radial and is completely symmetrical. | 
For a perfectly made and perfectly centred lens, measured distortion is equal to theoretical | 
distortion when the position of the image plane and the principal distance are the same in both 
cases. 
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