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.