PEE ES ara 4 
  
  
  
  
  
20. 
and also as a general measure of image quality, since the FL would 
normally occur in the range of significance quoted by him. Hufnagel 
also successfully applied the Wiener Spectrum in his studies, for 
modifying visual performance due to the effects of granularity. As 
stated earlier the Wiener Spectrum is being studied in a current 
research programme for combining granularity effects and MTF's. 
Scott (1968) in an extension of Hufnagel's work found that a 5% 
difference in the width of the spread functions of two photographs 
can be detected when the photographs are of the same scene, while 
this factor increases to 10% for photographs of different scenes. 
These figures approach the measuring precision of MTF's of photo- 
grammetric systems, and thus demonstrate the importance of accurate 6G 
determinations of MTF's for future image quality assessment. i 
With Scott's data, it was also possible (Trinder, 1973) to prove 
that the FL for grainless and grainy photographs are approximately 
equal (see sec 2.4). This adds weight to the earlier findings of 
Hufnagel, regarding the significant section of the MTF curve for 
purposes of interpreting a photograph. Though attempts have been 
made by some researchers to include the full shape of the MTF curve 
in image quality assessment, significant agreement has been obtained 
in several works through the FL. 
4.4 Conclusions 
(i) An understanding of factors affecting interpretability 
involves many parameters» few of which have been investigated to date. 
Some knowledge has been gained of the photographic object sizes for 
detection and recognition, (ii) For ranking of photographs in terms 
of quality, it appears that the high frequencies are most significant 
in determining which photographs are considered of higher quality than 
others, (iii) In addition, observers are sensitive to variations in 
MTF's which approach the measuring precisions of MIFs. 
  
5. Final Remarks 
Current research experiments on the significant parameters of 
image quality reveal that there are as yet many unknown factors to be 
investigated. Generally attempts have been made to use a single 
parameter e.g. Frequency limit or spread function width, to express 
performance in terms of image quality. A more thorough parameter may 
be more suitable, which describes the shape of the MTF curve and the 
effects of granularity. Finally this parameter must be related to 
various visual tasks in a general manner so that the effects of using 
different components can be estimated.