Changes in Fiim Tniekness 
Because the back of the film is used as a reference 
surface, changes in film thickness may cause positional 
errors. Figure 13a shows a famous examples [22], with one 
profile in film direction. Another eleven profiles were 
determined from the contour lines and again, the positional 
displacements resulting from the film thickness changes were 
plotted for points of the profiles - Fig. 13b. The displace- 
ments were so small that almost all are less than half the 
length of the diameter of the little squares indicating the 
point positions. New results on film thickness measurements 
have recently been published [46] which compare favourably 
with earlier results ([#1, [22]). 
Film thickness measurements are usually carried out 
using mechanical gauges requiring a certain minimum pressure 
in their use. Certain local film protuberances of 5 to 
20 um height were reported in [4]. However, they could not 
be measured mechanically. 
Another method, which uses a grid parallel to a fiim 
surface, exposed once with perpendicular light and a second 
time with light incident at 15°, as well as results were 
reported in [21]. These results are worse than those 
referred to earlier. Figure 14 shows an example 15 x 15 cm 
large with positional errors resulting when a camera with a 
foeal length of 115 mm is used. Figure 15a gives profiles 
through an 8 x 8 em segment of a film which had abnormally