Colour film is exposed in the FR80 by passing the white light from the cathode ray tube into a combined 16mm/35mm, camera having a built-in secondary, or subtractive, filter set. Each filter in this system removes one of the components red, blue or green from the white light incident upon it. To the eye the filters thus appear blue-green, red-green or blue--red respectively. These colours are labelled cyan, yellow and magenta. They form the complementary set to red, blue and green.
Colour film is more sensitive to variations in light level and exposure time than is black-and-white film and so the specifications for coloured output must be more precise and to some extent constrained.
In order to produce secondary colours on film, filters are inserted into the light path singly; to produce primaries requires the insertion of pairs of filters with a consequent increase in the attenuation of the light.
Obtaining similarly 'bright' images on film, whatever their colour or form, requires adjustment of the effective sources (CRT) brightness at each change; for example, closely spaced parallel lines appear less bright when their spacing is increased, there being light scattering processes at work both in the tube-face and in the film. The scattered light quite clearly increases the effective film speed by a 'pre-fogging' process (the level of brightness at: any point in the image depends on the light intensity at that point and the intensity of neighbouring points).
Although the FR80 has a number of parameters that can be be varied to achieve different intensities, in this paper the brightness of a line is defined by the intensity selected before the line is drawn and the number of times (hits) the line is drawn.
Experiments are still continuing on the FR80 to optimise the filter set. Consequently, it is possible that the suggestions made in this paper will change. Eventually it is hoped to define a complete range of colours rather than just the primary and secondary colours defined here.
The following suggestions are made assuming that the film stock being used is HS Ektachrome with E-4 processing (this is the standard used at the moment).
Users are recommended to draw all lines at the maximum intensity of 255. If an area of colour is required, this should be obtained by drawing parallel lines 20 raster positions apart from centre to centre.
Equal subjective brightness can then be obtained by drawing lines the number of times (hits) defined in the following table:
| SPROGS/SMOG Colour Number |
GROATS Colour Number |
Colour | Number of Area Hits |
Number of Single Line Hits |
|---|---|---|---|---|
| 6 | 2 | Red | 3 | 5 |
| 3 | 4 | Green | 3 | 6 |
| 5 | 6 | Blue | 3 | 8 |
| 1 | 7 | Cyan | 1 | 2 |
| 4 | 3 | Magenta | 1 | 3 |
| 2 | 5 | Yellow | 1 | 1 |
| 8 | 1 | White | 1 | 2 |
On this basis, the film (HS Ektachrome) should be correctly exposed and the colours appear saturated with acceptably slight halation. It may be necessary to have extra hits if a bright white area is required.
Due to the characteristics of the film, no easily expressible, colour-independent, relationship exists between intensity as set and repeated hits. Variations of intensity can be used to provide small adjustments in brightness (for shadow effects perhaps). It is worth noting that the above table provides subjectively equally 'bright' results for primary and secondary colours as well as white (clear).
As can be seen from the table above, the cost of producing the primary colours is expensive on the FR80 as each line has to be drawn a number of times. Often, the user is displaying a simple line drawing and the secondary colours together with white are an adequate set. In this case, it is possible to produce a set of colours having equal subjective brightness on the processed film with only a single hit for each line drawn but varying the intensity according to the colour required.
The possible sets are given in the table below. Their resulting 'absolute' brightnesses are lower than for the set recommended above.
| Colour | Intensity for one hit | Intensity for one hit |
|---|---|---|
| Cyan | 128 | 64 |
| Magenta | 256 | 128 |
| Yellow | 32 | 16 |
| White | 128 | 64 |
Owing to the logarithmic film characteristic, the second set is not 'half as bright' as the first.
