PHOTOGRAMMETRIC ENGINEERING 
7 
on the aerial negative whenever a pressure difference of one inch or more of 
mercury exists at the platen. 1 his in turn, provides to the film checker in the 
1 hoto Squadron and the photogrammetrist, a simple but accurate check on the 
vacuum system of the camera. If the vacuum recording appears, then the film 
flattening system functioned properly. If no vacuum recording appears, then the 
film checker knows something was wrong with the system and the mission can 
be reflown. 1 his will eliminate film containing distortions due to insufficient 
vacuum from being forwarded to the map compilation organization. 
Other recordings furnished in the camera are: fiducial markers and film 
shrinkage markers, the calibrated focal length of the lens to the nearest 1/100 
millimeter, the lens and camera serial numbers, altitude above sea level, time of 
day, negative number and pencil notations as desired. The recordings are placed 
in the area between the negatives. The fiducial marker adjacent to the recordings 
has been made larger than the others. This is to indicate which set of recordings 
applies to which photograph and to indicate the direction of film transport. The 
numerical recordings are imaged on the film by means of a simple lens system 
and incandescent light. The fiducial markers, however, utilize both natural and 
artificial illumination for exposure. An examination of the design of these mark 
ers will illustrate how this is accomplished. It will be noted from Figure 1 that 
the marker consists of two units—a round dot and a “V” shaped notch. The dot 
is illuminated by artificial light, and the notch receives its illuminations from 
the light of the terrain image. This combination was selected as a safety factor. 
The dot is believed to provide the photogrammetrist with the most precise indi 
cation of the fiducial marks position. However, having some doubts as to the 
reliability of incandescent lights, the natural lighted markers were added to give 
maximum assurance that aerial photography would have fiducial marker record 
ings. The chamber containing the recorded instruments, counters, etc., is con 
tained within the camera body. 
The camera body comprises the inner lens cone, the shutter and the case 
drive assemblies, the recording chamber, the camera mounting ring and an outer 
shell. Probably the most important thing that can be brought out at this point 
is that the outer shell of the camera body provides the necessary structure on 
which to mount the various sub-assemblies. This eliminates the need of attach 
ing anything, other than the shutter assembly, to the inner lens cone. In this 
way the inner cone is furnished maximum protection from any force that might 
introduce distortion, thereby assuring the using organization long camera life 
between camera calibrations. 
The Rapidyne shutter, as before stated, is a drawer type, high speed, high 
efficiency shutter. Its sequence of operation is somewhat different from most 
conventional shutters, in that it has two complete sets of shutter blades and 
mechanism. One set of blades and mechanism is called the A side and the 
other is called the “B” side. Each side has only one function to accomplish at 
the time of exposure. The “A” side is normally closed before exposure while the 
“B” side is normally open. At the instant of exposure the “A” blades open, 
allowing light to pass through the optical system and after a predetermined 
length of time, corresponding to the shutter speed, the B blades close. This 
means that the shutter designer has a much simpler design task, as it is not 
necessary to control the shutter blade acceleration and speed characteristics 
through a continuous open-and-close cycle. This greatly reduces the inertia of 
the various parts which in turn permits a reduction in the size of the parts, etc., 
until we have the shutter reduced to its present size. Anothei by-product of this 
type of design is long shutter life. This results from the fact that the shutter