integration and, according to the authors, appears to be as accurate 
as the scanning interval allows. It is necessary to orient the 
interferograms orthogonally so the resulting data from the scanner 
is registered in a common coordinate system, 
    
  
  
    
  
   
   
   
    
   
    
   
   
  
  
  
   
     
  
    
   
  
  
  
   
  
The auxiliary lens that photographs the fringes is located 
such that its aberrations do not affect the measurement of the 
fringe locations. 
Comparison tests were made using a 50mm SIRA standard lens. 
Within a spatial frequency of 100 lines/mm, there was good agreement 
on MTF values with those certified by the British Calibration Service. 
The WSI system has been demonstrated at NBS to be a versatile, ¢ 
inexpensive, and simple alternative for interferometric testing 
of lenses. The authors note that further improvements are possible, 
particularly to the computing techniques. 
5.9.2.3 Pupil Function: The National Bureau of Standards, according 
to Swing53 has chosen to establish the pupil function as the basis 
for measurement of optical performance. From this function, the 
transfer function, impulse responses, and lens aberrations may be 
calculated. Swing reviews the reasoning that led to this decision: 
that for optimum confidence, any technique developed by NBS should 
be both coherence-independent and object independent. Swing notes 
that "Because the OTF results from a correlation of the pupil function 
with the complex conjugate, and because such a function cannot be 
unfolded (since the phase cannot be usefully retrieved), we are 
limited in the extent of application when OTF is measured directly. 
A great deal of additional information can also be derived from 
the pupil function: the lens aberrations. These are obviously 4 
useful to the manufacturer and the designer. Thus, from users' 
and designers' point of view, the pupil function is more useful, 
whereas for the manufacturer or quality-controller, the OTF alone 
may be sufficient." 
The Wave-front shearing interferometer52 developed at NBS 
met the requirements for measuring pupil function. The data is 
obtained in a pair of interferograms taken 90? apart. Analysis 
depends on accurate measurement of the location of fringe peaks 
and a computer program. 
5.9.3 University of Arizona 
  
5.9.3.1 The Significance of Phase Transfer Functions: