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survey photography. Instead, "permissible heating" estimates have been
summarized graphically. These are based on the following "reasonable"
tolerances of the above mentioned film parameters.
For practical purposes, a +1/3 stop variation in speed (50.1 on a
log exposure scale) was considered permissible. According to the
1.C.A.S. Specification?, a processing variation of *10% in average
gradient is acceptable; hence, the same figure has been adopted as the
tolerance for heat-related fluctuations. Doubling of the fog level was
chosen as an allowable variation for negative films. For reversal
films the fog level was required to be at least 2.0 plus minimum
density. A change of 0.1 log exposure in Colour Balance was considered
acceptable. The tolerance for fog level was actually not required,
because deterioration of the other parameters always preceded a criti- A
cal variation in this property. (9
Using these guidelines, the maximum heating duration attained
before "failure" of each of the film variables was tabulated. When
plotted, each parameter defines "permissible" and "unacceptable" combi-
nations on the temperature/heating-duration graph. The "net curve"
consists of portions of one or more of these curves, and indicates
heating combinations for which all film properties vary within accept-
able limits (fig. 9). For clarity, only the net curve has been shown
for the heating-constraint graphs of the 6 emulsions (figs. 10 to 13).
3.4 APPLICATION
Caution must be observed in applying these heat constraint graphs
to the planning of practical procedures. The graphs apply only to the
sample films used representing single emulsion numbers and specific
histories. To obtain some indication of probable reproducibility,
heating time constraints from preliminary tests involving other 44
emulsion numbers were compared with those from the extended study as
reported here. This comparison indicated that permissible heating
times were likely reproducible only to within a factor of three times.
Thus times of permissible heating duration as shown in figures 10 to
13 should be divided by at least 3 before use as limits for practical
situations. Preferably a considerably larger safety factor should be
maintained.
3.5 RECOMMENDATION
It is suggested that heat constraint data of the type reported
should be available for all aerial films and should be based on studies
of a statistically adequate range of samples of each film type. Film
manufacturers appear to be in the best position to obtain such data.
