NEGATIVE MATERIALS 
The negative materials now evailable for air photography are a 
distinct advance on those of ten years ago. They are faster for the 
same graininess, and,more important; retain their speed better on keep- 
ing; they are somewhat more resistant to fog, and are physically more 
robust, enabling processing and drying to be carried out at higher 
temperatures. 
Sensitometric Characteristics 
  
It may be of interest to give sensitometric information for a num- 
ber of negative emulsions used by British organisations for regular or 
experimental air photography. The figures quoted in Table | and the 
curves of Fig.l are representative averages for material supplied to 
Ministry of Supply specifications; where commercial equivalents are avail- 
able their properties are very similar, subject to the inevitable produc- 
tion tolerances. The test conditions are listed at the bottom of Table |, 
and the necessity to do so emphasises the lack of international standard 
for the sensitometric evaluation of materials for air photography. It is 
suggested that such a standard could well be drawn up in the present state 
of knowledge, and that its use would greatly facilitate discussions of 
this nature. |n the absence of such a standard even for speed, ASA 
figures.are given to indicate broadly what kind of emulsions are being 
considered, but it is emphasised that the test conditions appropriate for 
air photography are so different that the ASA classification is somewhat 
fictitious. 
The curves and figures call for little comment, but attention is 
drawn to the wide range of speed and resolving-power available with small 
differences in contrast and curve shape. The L3 class is mainly intended 
for low-altitude work where its lower contrast is en advantage. For 
many years the L class has been standard for general and high altitude 
photography, but with the advent of improved lenses having better 
resolving-power and no loss of marginal illumination there is a case for 
regarding the A class as the standard type in future, at least for civil 
survey work. Its speed is in fact little less than that of the L class 
of several years ago, the latter having been improved both in initial 
speed and retention of speed during storage. Results of some keeping 
tests on a Class L3 emulsion in tropical and temperate conditions are 
given in Table 2; very similar results are found for Class L. 
The sample films were stored indoors without air conditioning in 
their normal packing. (Tin sealed with adhesive tape, cardboard outer 
container.) The accelerated keeping test consists of one month at 41°C 
and 70% R.H. The 'dry' tropical storage was at Kano, Nigeria; average 
maximum shade temperature 34.5°C, R.H. from 7% to 99%. The 'humid' 
tropical storage was at Port Harcourt, Nigeria; average maximum shade 
temperature 23°C, R.H. from 53% to 97%. 
These figures, relating to one of the better samples from a number 
tested, show that modern emulsions will keep surprisingly well under con- 
ditions that a few years ago would have been regarded as severe. This 
resistance to difficult conditions must prove of great assistance to the