6 
The image correlation system consists of three new components, 
plus the AP-Z computer. The new components are: 
(1) Flying-spot scanners 
(2) Scan generator 
(3) Video correlator 
The flying-spot scanners consist of two cathode-ray tubes (CRT's) and 
two photo-multiplier tubes (PMT's). The CRT's scan conjugate areas 
on the two photographs, while the PMT's sense the amount of light 
which passes through the photographs. These components and the 
associated optical elements are incorporated in the viewing unit with 
% 
modifications as required to the standard AP-2 unit. 
The scan generator and video correlator are housed in a separate 
cabinet, Figure 4, which is the major physical addition to the basic 
AP-2 system. The scan generator produces the X and Y deflection 
signals for the CRT's, representing the required image scan patterns 
in photo coordinates. 
As a first step in this process, a basic X and Y scan pattern of 
the desired type is produced in model coordinates. The pattern in 
model coordinates is then transformed to separate sets of scan signals 
for each photograph. In this process, information from the computer 
is used to control the size, shape, and rotation of the two scan patterns 
in accordance with camera orientation, camera geometry, and terrain 
slope, so that conjugate areas are simultaneously scanned on the two 
photographs. This approach to scan shaping eliminates the need for 
optical image-distortion devices on the viewing unit. 
The scan pattern generated on the face of each CRT is optically 
transferred to the moving carriage, duplexed with the illumination 
provided for visual observation, and imaged on the photograph. The 
CRT light transmitted through each photograph is separated from the 
visual illumination by color filters and detected by the PMT. The re 
sult is a video signal representing the optical density of each elemental 
area as scanned. 
^Optics and viewer modifications were designed and produced by 
OMI (Rome, Italy).