A THEORY OF SCANNING 



495 



A circular aperture furnishes a simple example of such reproduction — 

 because its admittance, from its symmetrical shape, is a unique func- 

 tion of the wave length of a component. In other words a circular 

 aperture reproduces normal detail equally well in all directions. We 

 may, therefore, simply plot \^Y(m, n)2^ as a function of the component 

 wave length as in Fig. 16, and this single curve is a measure of how 



0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 



WAVE LENGTH IN TERMS OF DIAMETER OF APERTURE Sh- 



zr 



Fig. 16 — Equivalent transfer admittance for circular apertures at both sending and 

 receiving ends, vs. wavelength. 



well the various normal components are reproduced. The shorter 

 wave components are practically omitted in the reproduction of an 

 image. 



Other apertures do not reproduce normal image detail equally well 

 in all directions because their admittances depend on the slope of a 

 component. To simplify the consideration of such apertures we may 

 resort to a practice commonly used in discussing telephotographic or 

 television systems, and that is, we may take the resolution along the 

 direction of scanning and across the direction of scanning separately as 

 criteria of their performance. 



Neglecting the small slope of scanning lines with respect to the 

 X axis of the image field, the admittance of an aperture for components 

 normal to the direction of scanning is Y(m, 0). Consequently, we 

 may take [F(m, 0)]^ as a measure of the reproduction of normal detail 

 along the direction of scanning. In a similar manner we may take 

 [[F(0, m)J^ as a measure of the reproduction of normal detail across the 

 direction of scanning. 



It thus follows that an aperture gives the same resolution of normal 

 detail along the direction of scanning and across the direction of scan- 



