New trends in physics teaching, v.4; The ... - unesdoc - Unesco

Colour
screen, that taken through a blue filter being projected through a blue filter, green through green
and red through red, as shown in figure 16. An accurate rendering of the scene would have been
obtained were it not for the fact that the plates in use at that time were not red sensitive! This is
an additive colour process, but it is obviously very cumbersome. The simple Dufay autochrome
process yielded delightful results at the turn of the century, the principle being the same.
Minuscule granules of starch (figure 17) were dyed blue, red and green, and acted as primary
filters superposed, side by side, over the film and pressed into it. The film was exposed to the
scene, developed and reversed so that, if exposed to a red scene, the silver deposited under the
red granules would be dissolved away, but not under the blue or green granules. The filmwould
then appear red, as it should. The big disadvantage of this process was that, even for a white
scene, two thirds of the light incident on the film was absorbed, since each granule absorbs twothirds
of the light incident on it, the red, for example, absorbing blue and green. The more
recent photographic processes avoid this problem. The principle of modern colour photography
involves a three-layer emulsion, the outer layer being sensitive to blue light, the second layer to
green and the third to red. Let us take a picture of a flag having blue, green and red stripes, as
shown in figure 18. The film is developed, removing the silver from the exposed silver halide. A
second stage of development then takes place. The conversion of the remaining silver halide, left
in regions of low exposure, to silver occurs, with the release of dyes in the emulsion which are
complementary to the colours to which each layer was sensitive. The silver is now dissolved away
leaving only the dye. Now, for white light, all layers would be exposed, and the silver removed
at the first development, leaving none for the second, so no dye would be released, and the film
would be transparent and colourless. For the flag we took, the colours wil be produced as shown
in the figure, for example, the top layer wil be exposed by the blue stripe, but not the bottom
two layers, so dye will not be released in the top layer, but wil be released in the layers dyed
magenta and cyan, which only allow blue through. For the green stripe, the centre, magenta
layer, which absorbs green, wil be clear, and the red stripe wil expose the bottom layer, which
wil be clear, allowing the yellow and magenta layers to be dyed, so only red gets through.
blue light
blue transparency
transparent backing
light
Figure 17. The autochrome colour photography process. Light traverses small, dyed starch granules.
It is clear from this example of colour film why this system is called a subtractive process.
Whereas, with the additive process, we start with a dark screen, and illuminate it with blue, green
and red primaries to obtain our match, with the subtractive process we start with a white light,
as in a projector, and subtract blue, green and red with yellow, magenta and cyan filters respectively,
until we obtain the colour we desire. Note again that earlier colour processes, such as the
autochrome discussed above, only allow a maximum of a third of the white light incident on the
colour slide to pass through, whereas the newer processes can let almost all of it through.
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