acter, 
ler as 
tives. 
been 
itives 
h as 
MTF, 
ctical 
S has 
tistic 
used 
ion is 
what 
n, in 
tical 
ation 
eyes, 
ntists 
tness 
the 
1ge. 
  
® The capability to distinguish the fine 
contrast depends upon not only the 
brightness difference, also the background 
‘brightness. 
2.2 Visual threshold of contrast 
The image visual threshold of contrast is defined 
as the density difference which is large enough 
for human to observe. It changes with the 
density variation, and reaches minimum at a 
suitable density value, apart from which it will 
grow larger with the decrease or increase of 
density. 
2.3 An experiment on vision response to 
density variation 
2.3.1 Experiment 
By using different sensitivity negatives, thirty 
wedges have been made under different 
conditions of exposure and development. And 
twenty people with good eyesight and 
experience in photogrammetry took part in the 
experiment. 
Firstly, each density level has been measured 
with densimeter for each wedge. Then the limit 
levels of high and low densities, which all tested 
people could just distinguish. were put down in 
writing. Finally, six hundred groups of data have 
been obtained. From the statistic results, we 
draw some conclusions of vision response to 
density variation. 
2.3.2 Conclusions 
€ The minimal and maximal densities, which 
people can just recognize are 0.11 and 2.47 
respectively. They fall into the toe and 
shoulder of characteristic curves. This result 
shows that the density range which people 
can adapt to overpasses the linear part of 
the curve. 
® The maximal distinguishable density at the 
low limit of density is 0.19, and the minimal 
one at the high limit of density is 2.12. That 
111 
means that, in order to recognize the details 
of object, the minimal density should be 
about 0.20 at lower limit, and the maximal 
one should be about 2.10 at upper limit. 
®The maximal distinguishable contrast 
threshold at the lower limit of density is 
0.07, and at the upper limit of density is 
0.15. That indicates that the lowest contrast 
needed when to recognize the higher 
density image is twice that of the lower 
density image. Even though the toe and 
shoulder of the curve should have the image 
compressed, the object in the toe is better 
distinguished than the object in the shoulder. 
Moreover, the difference between the 
minimal density and the fog density for 
negatives should be two times or three 
times as large as 0.07 in order to recognize 
the details in shadow. 
@In the middle range of density, the visual 
contrast threshold is less than 0.02. 
Therefore, much information can be 
obtained in the middle range of density. 
3 Density and Microcharacter 
The image microcharacter is used to describe 
the reproducibility of the fine image. It 
dominates the measurement and interpretation 
accuracy of aerial negatives or their copies. 
When seen with eyes, the microcharacter 
behaves as sharpness and grainness. The 
microcharacter indices, such as granularity, 
acutance, MTF and information capacity, are no 
doubt related to density. In fact, they are the 
function — of = density. Therefore, the 
microcharacter of image becomes a critical 
aspect of density index evaluation. 
With the help of PDS data collection system, we 
have calculated the resolution. acutance. RMS 
granularity, MTF, information capacity of aerial 
negatives, which are acquired by the same 
camera chain and developed under the same 
conditions (see Tab. 1) . The results show as 
follows : 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996