ensity 
ensity 
makes 
/ one. 
ly on 
ent is 
nding 
| from 
1e left 
nding 
range 
sh. 
gram 
lution 
ing to 
and 
  
resolution, and is inversely proportional to 
RMS. If the average contrast is too high, 
the RMS value will be larger the resolution 
Will be poor, and the information capacity 
will be smaller. If the average density is too 
low, the RMS value will be smaller, the 
resolution and information capacity will be 
reduced. 
4. Statistics of Density Range 
4.1 Visual assessment 
There have been selected thirty nine aerial 
negatives, which have different tonal gradations 
and different subjectives (eight of them 
supplemented with steropairs ). And there have 
been consulted twenty people consistmg of 
scientists, processors, interpreters, plotter and 
control extension operators, who make the 
evaluation of density and contrast for each 
sample picture. Finally, we obtained the statistic 
results of comprehensive evaluation for each 
sample picture according to fuzzy mono-factor 
judgement rules. 
4.2 Measurements of density index 
All sample pictures were firstly digitized by P- 
1700 scanner (the sampling aperture is 100 
micron), then density histogram of each image is 
obtained. And the density indices of each picture 
are calculated using the following formula : 
h h 
fog density Do-2niD;/ 2 ni 
ich i-fi 
D; D; 
average density D,-?niDi/?ni 
i-D, =D 
minimal density D, 
Dy D» 
?ni€[(1-T)2] / 2 
=D, i=) 
maximal density Dmax 
D, D, 
Sn; <[(I-T)En;] /2 
i=Dmax i-D, 
image contrast A D=Dyax IT Dan 
Where T is the reliable level, and its value is 99 
percent. 
4.3 Statistics of density range 
According to both the visual assessment results 
and the measurements of density indices, we 
have calculated the density range of three 
classes : the excellent, the qualified, and the 
unqualified. Thus the results in Tab. 2 were 
derived. 
5. The Establishment of Quality 
Assessment Criteria 
A set of quality assessment criteria for density 
index of aerial negatives has been proposed in 
the light of the above experiment results. 
5.1 Average density D, 
Basically, the average density D, should be 
near D,, which is the density corresponding to 
the highest visual resolution Rmax Because the 
sensitized material can reproduce the fine 
variation in tomal gradation and human can 
distinguish at the highest sensibility near by D.. 
In good conditions of exposure and 
development, D, is different in type of film. It is 
relatively stable, and approaches the left part of 
middle of characteristic's linear section for a 
given kind of film. Tab. 2 shows that the optimal 
value of D, is generally about 0.9. The image 
contrast is too small to reproduce the image of 
low luminance subjective when D, is less than 
0.6. And the image contrast will be degraded 
and the image of high luminance subjective is 
compressed when D,is larger than 1.5. From 
Tab. 1, it can be seen that the larger the average 
density is the larger the RMS, the lower the 
information capacity, and the smaller the 
acutance as well as the resolution. So the D, 
must not exceed 1.3 in order to obtain proper 
microcharacter. 
5.2 Minimal density Du, 
Dmin Should be in the linear part of curve, and 
close to the lower limit of density corresponding 
to 0. 8R,..«. Besides, the difference between Dy, 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996