502 BELL SYSTEM TECHNICAL JOURNAL 



1. Considering apertures of a given shape, the more overlap allowed 

 between adjacent scanning lines the weaker will be the extraneous pat- 

 terns but the coarser will be the reproducible detail in the normal 

 image. 



2. Not all shapes of aperture are equally efficient in suppressing 

 extraneous components, and at the same time retaining a given resolu- 

 tion of normal detail. Of the shapes considered, the rectangular 

 aperture is least efficient in this respect, and the full-wave sinusoidal 

 aperture {E in Fig. 12), is the most efficient. 



3. Although not proved, it may be inferred from the figure that the 

 finest resolution in the normal image that can be obtained (assuming 

 a given scanning pitch) without showing a first order extraneous pat- 

 tern on a flat field, is that obtained with the rectangular aperture of 

 width equal to the scanning pitch. 



4. With the most suitable aperture it is possible practically to sup- 

 press the extraneous components, at the expense of coarsening the 

 normal reproducible detail to slightly under twice that given by the 

 rectangular aperture just mentioned. 



The last point in particular enables us to draw a conclusion in regard 

 to the degradation contributed by the extraneous components. For a 

 rectangular aperture of width equal to the scanning pitch it appears 

 that the degradation amounts to a little less than doubling the coarse- 

 ness of resolution to normal detail. This substantially checks the 

 estimate which has already been made above. It may further be sur- 

 mised for all the other shapes of aperture shown with a value of abscissa 

 under 2 that as the degradation contributed by the extraneous com- 

 ponents is reduced, the coarseness of resolution to normal detail is in- 

 creased to just about make up for this, and that in the overall picture 

 the minimum element length which can be recognizably reproduced 

 remains substantially constant at about twice the scanning pitch.^ For 

 aperture arrangements with values of abscissa over 2, either the ineffi- 

 ciency in suppressing extraneous components, or the unnecessarily 

 large overlap, tends to coarsen the overall resolution to a minimum 

 elementary length greater than twice the scanning pitch. In this 

 region the line connecting the points has been dotted. 



* It may very well be that even if all these aperture arrangements transmit an 

 about equal amount of information they do not give the same psychological satis- 

 faction to the viewer at the receiving end. The general effect of a square aperture 

 of scanning pitch width is to give a "snappy" appearance, disturbed, however, by 

 the presence of the extraneous patterns. When these are removed, keeping the over- 

 all resolution about the same, the appearance becomes "woolly" or "fuzzy." 



