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.