THREE-COLOUR SELECTION 873. Three-colour Selection Filters. Formerly attempts were made to produce almost mono chromatic filters, each transmitting only a narrow spectral band. It is, however, a mistake that some groups of visible rays should escape record on one or other of the three negatives. Then use was made of filters with " vignetted " absorption bands and with ample overlap of spectral transmission regions, but later the filters used had cuts as sharp as possible with a slight overlap of the spectral regions transmitted.
If it were possible to make coloured filters with sharply defined absorption bands, photo graphs made in the spectroscope through such filters would have the same regions of densities whatever the illumination, whereas, in photo graphs taken through a filter with gradual absorption, the spectral region recorded on the plate extends farther (for an equal duration of exposure) as the intensity of the light is greater. Filters with " soft cuts " would seem to be a constant cause of errors, because lights and shadows occur in all subjects photographed. The quantity of light reflected by the bright portions being much larger than that reflected by the shadows, selection may not be identical for objects of the same colour but differently illuminated.
The very ingenious experiments of D. A. Spencer (1935), experiments limited, however, to subtractive synthesis, have shown that in the present stage of the dyes and pigments available for this synthesis, the form of the spectral trans- mission curves of the selection filters has only a negligible effect on the qualities of the three colour prints obtained, and that there are able, furthermore, very wide tolerances in the extent of their respective regions of transmission. The results given by various combinations of filters with increasing overlapping differ merely by the decreasing saturation of the colours in the resultant three-colour prints. In short, errors in filter transmission, when they are not exaggerated, are negligible compared with errors due to the colours used in synthesis.
The extreme limits have not been disputed. Under normal conditions the eye cannot see ultra-violet (up to 4,00o A.U.), nor infra-red (beyond 7,00o A.U.). The blue filter must therefore absorb the ultra-violet, while the limit of sensitiveness of ordinary panchromatic plates renders unnecessary any limitation of the trans parency of the red filter towards the infra-red.
As a rule, the following are the limits for each of the spectral regions which can act on the photographic emulsion through the three selec tion filters When it is necessary to reduce the exposure to the minimum, it is advantageous to associate filters having a very wide transmission with emulsions of restricted chromatic sensitivity, as such filters can be much more transparent in the useful region. For instance, the green and blue filters are replaced by yellow and pink filters, respectively used with an orthochromatic emul sion and an ordinary emulsion. Besides the useful spectral region these filters will transmit red, which is inactive on the emulsions in ques tion, but the one transmits much more green and the other much more blue than the normal selection filters would do (F. NV. Coppin, 1933).
The dyed gelatine films used for selection filters can be employed unprotected or sealed between two pieces of glass ; in the latter case each filter must form a cemented system with plane-parallel surfaces, and the thicknesses of the three filters must be such that the displace ment of the sharp image (§ 12o) must be the same in each case.
The coefficients (factors) of the selection filters must be determined experimentally (§ 214) for the panchromatic emulsion and light employed.' It is perfectly possible to make coloured filters all of which have the same factor for a given emulsion and light. This can be done by adding a neutral grey filter of appropriate density to the ordinary selection filters which have a factor lower than the maximum one. The factors will, however, be modified if the condi tions are changed.