These disadvantages, however, have been eliminated with the 
development of the Stereo Image Alternator. It is based on the relatively 
simple principle of the flicker system. In practice, the two stereo 
images are alternately projected to the platten at a rate of 60 cycles per 
second and are viewed through a shutter operating at the same rate. The 
result is a steady presentation of a stereo model at a brightness twice 
as great as that previously available utilizing anaglyphic techniques, 
complete image separation, a less critical light balance, improved model 
definition, and a stereo projection system compatible with color photography. 
In reviewing this paper at this point, several general statements 
concerning the progress of automation in photo interpretation are apparent. 
They are: 
Considerable effort and work is being expended toward the 
development of fully automatic interpretation systems and 
component parts of such systems. 
It has been demonstrated that pattern recognition techniques 
and computer manipulation of images is feasible so long as 
imagery is of high contrast and the sampled population is 
relatively small. 
Little progress is being made toward the development of auto- 
matic interpretive systems that will function with the almost 
infinite image variables that are present in even the simplest 
of conventional aerial photographs.