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plates and Cronar base films as part of his calibration study. He shows that the
residual distortions on both have the same standard error of 3 /x, so that there is
little reason to believe that plates are necessary for the highest possible performance.
11. Environmental Conditions
So far, we have considered the location of the image when recorded under
ideal conditions in a laboratory, where the temperature is likely to be controlled
to ± 2°C., the atmospheric pressure will only vary by an inch or two of mercury and
where the camera is rigidly held and free from vibration. In an aircraft such ideal
conditions are not often met and it is no wonder that there are many recorded
complaints from photogrammetrists that their negatives are worse than would be
expected from the calibration certificate.
12. Temperature
During calibration, the camera is usually at a uniform temperature in equili
brium with its surroundings. For a camera at a uniform temperature, and made
throughout of the same material, one would expect a temperature change to expand
the whole camera uniformly, so that only the scale of a negative would be affected.
In practice, camera and lens materials have different expansion coefficients and there
is also the change of refractive index of the lens components to be added. The effect of
temperature changes is therefore not easily calculated, but experience shows that
there is usually only a scale change, i.e. the principal distance alters but the distortion
is constant.
Proctor 121 quotes results for laboratory and field calibrations of a réseau camera.
Both gave similar results for distortion, although the field calibration gave a principal
distance some 29 p smaller. He estimated his camera coefficient at 2-5 p in principal
distance per 1°C., so that the 10°C. measured temperature difference between
laboratory and range could be accounted for fairly accurately. Tn practice, uniform
changes in temperature of the camera will become apparent during plotting as
small aircraft height errors.
Temperature differentials within the camera are of much greater significance.
The various components of a camera have widely differing thermal conductivities and
thermal capacities. Moreover, some parts are not in immediate contact with the
ambient atmosphere and must rely on conduction or radiation for any heating or
cooling. One can imagine that the centre of one of the lens components, of consider
able thickness, transparent to radiation, held by its edges, and shielded from convec
tion currents by the body of the camera and the filter, will take a long time to warm
up or cool down. Tests carried out at the Royal Aircraft Establishment many years
ago showed that a camera needs many hours to reach its final equilibrium condition
if taken from a warm laboratory and flown to 20,000 feet. Changes of focus and
geometry were going on continuously for 3 to 6 hours after take-off, in spite of the
aircraft heating system.
in many installations the camera is kept warm by circulating hot air around the
camera and between the aircraft window and the camera. Should the hot air inlet
be too close to the camera or be insufficiently baffled, this may give rise to hot spots
on the camera, with perhaps one side of the lens being hotter than the other. Changes
of glass refractive index, deformations of the surfaces of the lens components, and so
on, will then cause random asymmetric distortion in the image, even when equilibrium
has been reached.
