4
the photo images of the main subjects. For prints from three-layer films the criterion is
the quality of reproducing the natural colour tones of the objects photographed, i.e. the
colours of the images as compared to those of the objects viewed from the ground. This way
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Fig. 5. The scheme of the additive printer
(copying apparatus).
1 — the light source 10 — a lens used by the
2 — heatfilters measuring process
3 — condensors 11 — a stop
4 — lenses 12 — the film
b — optical wedges 13 — pressing glass
6,7,8 — filters; their spectral char- 14 — ground glass
acteristics are shown in curves 15 — photocell
9 — a diffusing plate 16 — galvanometer
of approaching the problem of selecting tones is justified by the fact that when the country
is viewed from some height some levelling-out of colour contrasts always takes place.
In processing prints the same solutions are used as in the negative process.
5. The entire process of colour photography is carried out under control. There is a
National standard for testing negative materials. By means of a special device a den-
sitometric wedge is printed on every film to be developed. When the film has been developed,
the wedge densities are measured with the aid of a photoelectric densograph; as a result,
characteristic curves are obtained for each layer, and the mean value of the contrast factor
(Gamma) is obtained. The over-all density of negatives that are representative of the film
under consideration is measured on a special densitometer. The specifications for good
quality three layer films are as follows: according to the type of the country, y may be from
0.9 in mountainous regions, where sharp differences in brightness values occur, to 1.8 in
areas of low relief.
The y value of spectrozonal negatives is 0.4—0.6 higher than that of three-colour ones.
The mean density of negatives may vary from 0.6 up to 1.2.
Investigations have shown that, though having a lower resolution, colour pictures are
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