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.