580 BELL SYSTEM TECHNICAL JOURNAL 



square apertures plus the filter. According to the scanning theory of 

 Mertz and Gray,^ the pair of apertures in tandem was equivalent, in 

 frequency limitation, to a single aperture 1.35 times as long as either 

 one of the pair. Taking the width of this single aperture (1.35 times 

 the length of one half cycle) equal to the width of the figure of confu- 

 sion, the height of the figure of confusion was calculated from its area 

 to be 1.9 times the height of one scanning line or one scanning pitch. 

 This was the measure by subjective comparison of the resolution across 

 the scanning lines. 



Under the condition of equal resolution along and across the scanning 

 lines, the figure of confusion would have to be square and its width 

 would then also be 1.9 times the scanning pitch. The width of each 

 one of the pair of equivalent tandem scanning apertures would be 

 1.9/1.35 or 1.4 times the scanning pitch. That is, two rectangular 

 scanning apertures, each 1 line high and 1.4 lines wide, used in tandem 

 without electrical band limitation, would yield an image having equal 

 resolution along and across the scanning lines. 



The nominal frequency band associated with such scanning apertures 

 is 1/1.4 times that associated with square apertures. That is, the 

 nominal video frequency band, in cycles per frame period, required for 

 equal horizontal and vertical resolution is 0.70 times one half the 

 number of square scanning elements per frame area, reckoning a 

 square scanning element as an area of height and width equal to the 

 scanning pitch, or spacing between scanning lines. 



For comparison with the value 0.70 which we have just found, the 

 following values of nominal band width coefficient have been lifted 

 from their contexts in the references: 



(a) Kell, Bedford and Trainer (1934) 0.64 



{b) Mertz and Gray (1934) 0.53 



(c) Wheeler and Loughren (1938) 0.71 



{d) Wilson (1938) 0.82 



{d) Kell, Bedford and Fredendall (1940) 0.85 



Acknowledgment 

 This work has been done under the direction of Dr. P. Mertz and 

 with the extensive assistance of Mr. T. R. D. Collins. To them, and 

 to my other colleagues who have given of their time and counsel, I 

 wish to extend my appreciation of their help. 



APPENDIX 



1. Determination of the Size of the Figures of Confusion 

 The image was put out of focus by moving the projection lens nearer 

 to the film gate, throwing the plane of sharp focus beyond the viewing 



