754 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1952 



averaged over all time, for various values of the time shift t. In the case 

 of a picture transparency, the optical transmission is a function of two- 

 dimensional space, expressible in polar coordinates as T(s/^), and the 

 autocorrelation can be expressed in analogous fashion. The time variable 

 t is replaced by the space coordinate s/^, and the correlation time shift t 

 is replaced by a space shift As/d_, so that the new expression is 



<l>{As/l) = Tis/l) T{s[l+ As/i), 



(2) 



averaged over as much area as practicable. This space-domain auto- 

 correlation is much easier to measure than the time-domain autocorrela- 

 tion. We need merely measure the relative optical transmission of two 

 identical cascaded transparencies, shifted from register by a variable 



PARALLEL — 

 LIGHT 



-OPAL GLASS 



' CONDENSER 

 COMPOSED OF 

 TWO 5-INCH 

 ASPHERIC LENSES 



LIGHT SOURCE 

 GE-1493 



Fig. 2 — Basic arrangement of picture autocorrelator. 



