WORKING GROUP 1 
WELANDER 
31 
stam [7]. Fig. 2 shows such a function for the image centre of an air camera 
lens. 
The T-functions supply the explanation why the RP measurement does 
not correspond to practical quality judgements of a photograph. The RP 
concept only takes care of the smallest visible details in the photograph. Ob 
viously the contrast transferred from the larger objects is important. Accord 
ingly it is necessary to know when judging a photograph, if optical enlargement 
is used or not. The eye also has a T-function, studied by, among others, Rosen- 
bruch [8], although non-linear perception factors complicate the evaluation. 
For high contrasts the eye detects details corresponding to about 10 lines 
per mm. 
Laboratory test methods 
A number of methods for the measurement of T-functions are developed, 
principally in optics. Such a method, specially adapted for emulsion measure 
ments, will be briefly described here. 
The principle is based on the imaging of an infinitely narrow slit. The slit 
image is measured by a micro-photometer. The photometer trace shows among 
others that the slit image is broader than the narrow original slit. For the