494 BELL SYSTEM TECHNICAL JOURNAL 



duce a relative loss in the reproduction of the shorter wave components. 

 Consequently there is a loss of definition in the finer grained details of 

 the normal image. This type of distortion due to aperture loss may be 

 termed simple omission of detail. 



In addition to the simple omission of detail, the normal image is 

 masked by the presence of the extraneous field. The more pronounced 

 features of this field are the line structure and serrated edges that it 

 superposes on the normal image. Its presence is not only displeasing, 

 but it also masks the normal image components and thus results in a 

 further loss of useful detail. This type of loss may be termed a 

 masking of detail or a masking loss. It is true that the extraneous com- 

 ponents may sometimes give rise to an illusory increase in resolution 

 across the direction of scanning in special cases where they add on to 

 the diminished normal components in just the right phase and magni- 

 tude to bring the latter back to their phases and intensities in the 

 original image, giving no resultant distortion whatever. (In all such 

 cases, however, to obtain this benefit it is necessary to effect a quite 

 accurate register between the original image and the scanning lines or 

 the distortion is very large. Such accurate registering is generally 

 impractical and may be definitely impossible if the registry required for 

 one portion of the image conflicts with that required in another portion. 

 Such cases may, therefore, in general be disregarded.) 



The Reproduction of Normal Detail 



The preceding theory permits a numerical calculation of the repro- 

 duction of detail in the normal image. This is given directly by equa- 

 tion (38) above. 



In order to make some of the discussion in the following pages more 

 concrete and specific the sending and receiving apertures will be taken 

 alike; this condition, therefore, gives [F(m, w)]^ as a measure of how 

 well the various components are reproduced. If a picture be assumed 

 in which all the original components have the same amplitude then 

 \_Y{m, n)'J' is the amplitude of the reproduced normal components. 



The relative admittance for any given pair of apertures may be 

 calculated from equations (20') or (38'). Such calculations have been 

 made for various apertures and the results summarized in Appendix II. 



The admittance of an aperture is not in general uniquely determined 

 by the wave length of a component, but also depends on the orientation 

 of the component with respect to the aperture. The admittances of 

 reasonably shaped apertures do, however, decrease in general with 

 increasing numerical values of the indices m and n ; and the shorter wave 

 components are, therefore, in general, less faithfully reproduced than 

 the longer wave ones. 



