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Ln interest
of the investigation lay in finding the magnitude of the departures from
flatness irrespective of sign, monochromatic light was used in most of the
records.
A count of the fringes between A and B in Fig. 3 will show that the height
difference is about 60 half-wavelengths, i.e. 16 microns. On some inter-
ferograms places have been found with as much as one hundred and ten fringes
height difference (i.e. 30 microns) between points within the dimensions of one
reseau square. It is clear therefore that there is scope for improvement in
the arrangements for film-flattening in the register glass type of camera.
The illustration in Fig. 3 is of the kind of pattern that is established
some considerable time after the film has been moved into position and the
pressure applied. It was noticed at an early stage in the experiments however,
that the fringe pattern had not become completely steady even several minutes
after applying the pressure. It was therefore decided to make a cine record
of the course of events during a complete cycle of operation of the camera.
A 16 mm film* was made and reveals that no interference pattern whatever is
visible, even in monochromatic light, for a second or so after pressure is
applied to the film. During this interval, therefore, the air space is
appreciably greater than the figures already mentioned, although the degree of
flatness of the film is not known; it is probably quite poor. After about
three seconds from completion of film transport the general nature of the inter-
ferogram is established, but considerable changes in detail continue for several
seconds longer; smaller changes are still occurring after many minutes.
These experiments point to the presence of air trapped under the film as being
responsible for the major departures from flatness.
It was decided, in view of these results, to investigate the effectiveness
of the suction method of holding the film flat. In this case it is necessary,
in order to use the interferogram technique, to arrange for an optically flat
glass surface to be suitably supported at such a distance from the emulsion as
to give a good fringe pattern without at the same time disturbing the film in
any way. This is not easy, but the E.IoD. Laboratories at Harefield came to
our assistance and succeeded in obtaining suitable interferograms• Meanwhile,
an exploration, over small areas, of variations in the level of the film surface
was made at R 0 A 0 E 0 with a depth measuring microscope. The results of the two
investigations agreed satisfactorily, the interferograms showing maximum
deviations of about 18 microns and the microscope 17 microns.
There are one or two points of interest about the results of the suction
methods of flattening. The first is that the interference patterns obtained
are remarkably consistent from one exposure to the next, whereas with the
pressure pad no such consistency is observed. The second is that the flattening
is established within one second. It was noted also that, using a plate with
many small holes, but no communicating channels, the areas of film between the
holes were bowed in such a way as to suggest the presence of trapped air.
!, "‘To be shown at the Congress