diffuse surface would allow to get imagery which are only affected 
by lens and film properties. 
Different approaches were made in order to find a perfect diffuse 
surface, which covers the field of view of the camera totally +. First 
an opaque plate was put in front of a projection screen, which was 
illuminated indirectly. However the distribution of densities on the 
photographed opaque plate was not uniform, as proved by microdensito- 
meter profiles. Then the cloud - covered, hazy sky was taken for a 
diffuse target. The result of this approach is shown in Fig. 2: the 
negative photography obtained from the sky was used as a mask for com- 
pensation of the original light fall - off effect, but without success. 
Obviously the mask compensates only the left branch of the profile; at 
the right one the presence of the sun - though not visible - disturbes 
the mask effect. 
A Density 
  
  
   
  
   
   
   
    
    
    
     
   
  
  
  
   
   
  
  
  
  
   
    
Ny f 
Original Profile (positive) 
SA | 
Result 
^ AL 
A N 
Mask (negative) | 
| 
Image Center 
  
T 
lcm 
Fig. 2: Original light-fall-off effect superimposed by a negative mask obtained 
from hazy sky photography (Microdensitometer profiles through image center) 
Finally Bariumsulfat (Ba SO, ) > the "classical" substance for the design 
of reflection standards was tried in order to obtain a perfect reflecting, 
plate - shaped diffuser. A so - called "LAMBERT - Reflector" provides 
theoretically uniform diffuse reflection, so that 
Reflected Radiance Flux M 
Reflectance p = = 1 
Incident Radiance Flux $; 
  
In practise, LAMBERT - Reflectors have to be used basically for deter- 
mining the reflectance factor R, which characterises the reflectance of 
  
+ "ULBRICHT'S Sphere Photometers" used at the "Physikalisch - Technische 
Bundesanstalt" in Braunschweig, which provide diffuse illumination,were 
too small for the HASSELBLAD camera, though up to 130 cm in diameter.