500 BELL SYSTEM TECHNICAL JOURNAL 



transmitted by telephotography and television. It can be removed 

 only by insuring that Yirn' , n') shall vanish whenever n' = N, so that 



F(0, 0) • Y(fx, fxN) = 0. 



The requirement can be met for the elementary shapes of apertures 

 A, E and Foi Fig. 12, but cannot be met in the others. In these other 

 cases the overlap between adjacent scanning lines is usually adjusted 

 so that the requirement is met for fx = ± 1, to remove the most serious 

 pattern. Thus, for example, for the circular aperture B this requires 

 an overlap of around 25 per cent. 



The Reproduction of Detail 



In optical instruments the reproduction of detail is usually measured 

 by what is called the "resolving power" which in turn is defined from 

 the smallest separation between two mathematical point (or parallel 

 line) sources of light in the original which can be distinguished as 

 double in the reproduced image. 



For the present it is perhaps simpler to consider another criterion of 

 the resolving power, namely, the shortest element length in an image 

 resembling a telegraph signal, used as an original, which can be recog- 

 nizably reproduced with certainty in the received picture. For reasons 

 that have already been mentioned above it is necessary to insist that 

 the received picture be recognizable with certainty without any registry 

 requirement between the original image and the scanning lines. 



Using this criterion for the resolution along the direction of scanning 

 and assuming the apertures at the sending and receiving ends to be 

 rectangular and of the same length with respect to the picture size, 

 the minimum signal element required for a recognizable picture (as set 

 by the apertures as distinguished from the electrical transmission 

 circuits) will be of about the length of either aperture. For other 

 shapes of aperture the minimum element length will be very nearly the 

 length of the equivalent rectangular aperture using the term "equival- 

 ent" in the same sense that it was used in the discussion regarding 

 Fig. 12. 



According to the same criterion, for the resolution across the direc- 

 tion of scanning the minimum element length required for recognizable 

 transmission, in the case of a rectangular aperture of width equal to the 

 scanning pitch, will be twice the scanning pitch. It will be noted that 

 this is twice the length which would be required if only the normal im- 

 age components were reproduced, and this difference may be considered 

 as a measure of the degradation caused by the masking effect of the 

 extraneous components for this arrangement of apertures. 



