5 
Fig. 3 
Typical Horizon 
Photographs 
Therefore, the film for horizon pictures must yield maximum 
contrast and be of fine grain. 
In Nigeria and in Canada we used the Kodak High 
Speed Infra-red Film. This film is of medium-class contrast, 
resolution of 69 to 95 lines/mm., and of medium grain. To 
control exposure time, neutral density filters were used in 
addition to infra-red filters. This film provided acceptable 
horizon photographs. Later, after considerable experimenta 
tion, we selected a new type of film, the Kodak Spectroscopic 
Film, type 4N. This film is of very high contrast, very high 
resolution (136 to 225 lines/mm.), and of fine grain. 
Spectral sensitivity of this film is shown on Fig. 4. Using 
this film in Canada and Nigeria, high-quality horizon photos 
were obtained under average atmospheric conditions, and 
photos of acceptable quality under marginal weather 
conditions.