7
1.32 Film Thickness Variations
Comprehensive tests of film thickness variations have been made. Eighteen samples were taken from
7 different types of aerial film. Four samples were unexposed and not developed while 13 samples were
exposed and developed. One sample was measured before and after the photographic treatment. The
average thickness was 150 /um. The measurements were made with a measuring gauge which could be
read in units of /um. The film samples were covered with a mask with holes at the points to be measured.
See fig. 1.32:1 in the Appendix. The base for the measurements was a glass plate over which the film
and the mask were moved. The measuring gauge was supported by a rigid frame, and the pressure
against the film was constant from setting to setting within narrow limits.
Each film sample was measured in 25 regularly located points. Three independent repeated series of
measurements were made in each sample. The averages of the three observations were computed for
each point as well as the standard deviation of one observation and of the average. The root mean square
values of the thickness variations from the average were computed for each sample.
In separate series, each repeated three times, the points along the diagonals of six exposed and deve
loped films were measured. The points along diagonal 1 were denoted 161 through 188 and those along
diagonal 2 denoted 261 through 288. See fig. 1.32:1 and 1.32:2 through 7 in the Appendix. In one sample,
the measurements along diagonal 2 were repeated independently five times in order to increase the
precision of the averages, see fig. 1.32:8 (Appendix).
Finally, 462 points in one sample and 506 points in another were measured for the construction of
contourlines, showing the thickness variations over the entire surfaces. One of the samples was neither
exposed nor developed. In the same samples the thickness variations along the diagonals were measured
independently and the results from the two tests were then compared. See fig. 1.32:9 through 13 in the
Appendix.
The numerical results of the measurements are summarized in the following table.
Film
type
Undeveloped film
Exposed and developed film
Sample II
*m25
s
Sample I
*ra25
Sample II
s m25
s
Sample I
S m25
s mdl
s md2
1
2.0
1.7
0.6
1.6
1.8
1.8
0.4
2
—
—
—
1.3
1.6
1.6
2.1
0.5
3
—
—
—
0.9
0.8
0.8
0.8
0.5
4
—
—
—
2.0
1.5
1.9
1.8
0.6
5
1.6
1.4
0.5
1.0
1.0
1.2
1.1
0.5
6
—
—
—
1.5
1.7
1.7
1.7
0.6
7
0.9
—
0.5
1.0
0.8
0.8
1.0
0.6
All data are in um.
S m25 = the root mean square value of thickness variations in 25 points
S mdi = the root mean square value of thickness variations in diagonal i
s = the standard deviation of one observation (setting)
From the table and the figures the following conclusions can be drawn. There are considerable diffe
rences in thickness variations found between different makes of film (0.8 and 2.1 ¿¿m). In most samples
periodic thickness variations have been found in the longitudinal direction of the film roll (in the most
pronounced cases a ’’wave length” of about 40 mm) and sometimes, though less pronounced in the lateral
direction also.
In some cases (see fig. 1.32:11) considerable thickness variations (up to about 10 um) were found
within small areas. No correlation between film thickness and photographic density could be found.
Undeveloped and developed film samples of the same make showed nearly identical thickness varia
tion characteristics.