Creating Archives of Digital Images 
149 
Since photographic negatives were to be 
scanned and than contrast-reversed, it was 
decided to develop a procedure allowing 
the recording of densities. As test image 
served a standard step tablet consisting of 
21 steps and covering the density range 
from 0,05 to 3,05 in steps of 0,15. 
Normally, the digital data obtained when 
scanning a step tablet produce a highly non 
linear histogram; our goal was obtaining a 
linear histogram. Fig. 2 (preceding page) 
shows part of the linear step tablet (a, 0,20 
to 2,00), its appearance on a monitor (b) 
and the histogram of the scanned image (c). 
The scanned image shows a change in the 
distribution of densities, and the differences 
between neighbouring fields decrease with 
increasing density. Both used scanners 
were investigated concerning their 
radiometric performance using a step tablet 
with steps of 0,15 density units and 
reaching from 0,15 to 1,95. Experimenta 
tion with the histogram black and white 
points yielded the following values for the 
step tablet: 
density range 
of 
scanned image 
reading for 
minimum 
density 
reading for 
maximum 
density 
1.65 
191 
16 
The reading for the maximum density 
corresponds to a maximum density of 1.87 
which the scanners can read. 
Fig. 3 (preceding page) shows that a differ 
entiation is no longer possible once the 
maximum value of 16 is reached. These 
results extended over the available dynamic 
range (2 8 ) allow the differentiation of 14 
increments indicated in figures 4 to 6 
(preceding page) by dots. 
The goal of obtaining densities as output 
from the scanner requires the introduction 
of a correction function in a suitable form. 
This function can be obtained by mirroring 
the curve shown in fig. 4 along the diago 
nal, as shown in fig. 5. This curve can be 
implemented in the used scan software 
FOTO LOOK. Fig. 6 shows the relation 
ship between original step tablet densities 
and achieved densities; it is nearly linear 
and was for the performance of the work 
digitally defined. Fig. 7 (preceding page) 
shows the histogram for corrected step 
tablet data (c). 
Fig. 8: Positive digital data in density mode as presented on a monitor 
3. TONE REPRODUCTION IN DENSITY AND TRANSMITTANCE 
The differences in tone reproduction when using either density or transmittance will now be demonstrated using as image an interior 
scene combined with a step tablet covering a density range of ~ 2.0. The image data obtained in density mode are taken from the 
archive data (fig. 10, following page). Image reversal results in a positive image (fig. 9). Display of these data on a monitor results 
in a tonal variation dependent upon its non-linearity characteristics (fig. 8). If the negative image had been scanned in transmittance 
mode, the resulting image data would have looked like fig. 11 (following page) as negative image and like fig. 12 (final page) as 
positive image after a contrast reversal.