0 
successive Fourier transform of this product signal 
and displays it in its front focal plane. The correlation 
signal appears directly on the optical axis in the 
second frequency plane. This term is separated from 
the other light by a pinhole field stop. A phototube, 
placed directly in line with the pinhole, measures the 
intensity of the transmitted light. Since the correla- 
tion function is in terms of the light amplitude, a 
measure of its intensity implies that the squared 
absolute value of the correlation function is mea- 
sured. The difference in the characteristics of the 
correlation and its squared value are not too signifi- 
cant. However, since an intensity measurement is the 
only method of removing information from the opti- 
cal system, it does imply that negative signals can 
never be obtained directly from the correlator 
output. 
MULTI-APERTURE OPTICAL CORRELATOR 
A block diagram of the stereo-image measurement 
system is illustrated in Figure 7. The basic apparatus 
of the system is a multi-aperture optical correlator, an 
image scanner, and a signal processor. The configura- 
tion of the optical correlator departs from the single- 
aperture image/image correlator in that it contains 
the necessary optical elements and sensors to provide 
a multi-aperture parallel processing capability. The 
image scanner is included as part of the optical train 
of the correlator. 
Stereo Diapositive 1 
    
    
Collimated 
Laser Beam 
1 
Fourier- a 
Plane N 
i 
I-Lens v 
Image Scanner 
Scanner 
Synch Signals 
Stereo 
Diapositive 2 
Figure 7 Coherent Multi-Aperture Optical Correlator 
76 
Signal 
Processor 
The multi-aperture capability of the optical corre- 
lator implies that the whole image is not processed as 
a single entity but is segmented into areas that are all 
processed separately but simultaneously. This type of 
parallel processing is what is required to measure 
stereo-image alignment and first-order distortion er- 
rors. The system illustrated has five apertures. The 
alignment error signals are derived from a correlation 
measurement of the whole image, as defined by the 
imaging lens and the phototube behind the pinhole 
filter. The first-order distortion measurements are 
obtained from the four correlator outputs defined by 
the quadrant lens and detector assembly. Here, four 
different areas of the original aperture are correlated 
simultaneously and independently. 
The image scanner is a rotating canted optical flat. 
Its function in the optical correlator is to rapidly scan 
the first stereo image over the second so that direc- 
tional derivatives of the five correlator outputs can be 
measured. The directional derivatives are generated in 
the signal processor from the correlator outputs. 
The signal processor is an electronic computer 
system that generates the stereo-image alignment and 
first-order distortion error signals from the outputs of 
the multi-aperture correlator and the image scanner. 
The processor correlates the output of the five light 
sensors with a sine and cosine signal generated by the 
image scanner to obtain the directional derivative 
measurements. The alignment and first-order distor- 
tion error signals are then obtained by summing the 
       
  
   
Distortion 
   
Quadrant Lens and 
Detector Assembly 
Alignment 
Beam NN Detector 
CS 
Splitter O- Lens 
Low- Pass 
Filter (Pinhole) 
KOWALSKI