NON-ACTINIC LIGHTING DARK-ROOM LAMPS AND SAFELIGHTS 246. General Principles. The choice of illumina tion to be used when handling sensitive material, either (luring manufacture or during the various operations prior to and including fixation, must be guided on the one hand by the distribution of the spectral sensitivity of the emulsion under consideration, and on the other by the sensitivity of the eye to various colours.
It may be laid down that there is no really illumination in the true sense of the word that is to say, that there is no light, whatever may be its spectral distribution and however weak in intensity it may be, which will not fog a photographic emulsion if allowed to act for long enough.
It will thus be plain that in choosing a light for a dark-room we have to work out a corn promise between somewhat incompatible condi tions. We have to arrange, at least for a part of the time, for a light sufficient for the effective control of the operations being carried out, without at the same time appreciably fogging the sensitive material during the normal time required for the necessary manipulations.
To give shape to these ideas, suppose, to begin with, that we have to handle a panchromatic emulsion equally sensitive to all the colours of the spectrum. 1 Three filters, coloured respec tively blue-violet, green, and red, each trans mitting a third of the range of the visible spectrum and in the same proportions, could be used indifferently as filters for the dark-room lamp if only the actinic values of the three lights thus transmitted were being considered, in that they would each cause the same fog density if the emulsion were exposed to each for equal times. But, considering the physiological activi ties of the three lights thus transmitted, the blue-violet is a poor illuminator, the red little better, while the best visibility is obtained by the use of the green. This is in agreement with what has already been stated with regard to the spectral sensitivity of the human eye (§ 207, Fig. 141). Moreover, if it be supposed that, by
suitable regulation of their respective intensities, two filters, coloured red and green, may be made, such that they have the same visual intensity, then it will be found that if the source of light common to both be reduced in intensity, the red will appear less intensely illuminated than the green (Purkinje 1 phenomenon). This is a further reason for preferring a green light for, by allowing the pupil time to accommodate itself to very feeble illumination, 2 it is possible to use a green filter which, transmitting only one-thousandth of the visual intensity trans mitted by an ordinary ground-glass screen, at first sight appears to be absolutely opaque.
The same physiological considerations still apply even when the emulsion under considera tion is not equally sensitive to the whole of the spectrum. For example, an emulsion sensitized with pinachrome may be regarded as insensitive to the extreme red (beyond AU.) but a coloured filter transmitting only these radiations would appear so feebly luminous that a green filter, transmitting much less luminous energy but giving the same visibility, would give, as a rule, an equal degree of safety. For the same reason, although an orange filter usually trans mits all the red, there is often less risk of fog with an orange light than with a red one, since, for the same visibility, less total luminous energy is necessary. The advantage of an orange light is particularly noticeable when working with very rapid emulsions which, as we have seen, are often slightly sensitive to the extreme red (§ 223, note), as the proportion of these feebly active radiations is much less than it would be in the case of a red light.
The source of light and filter combined must not transmit either ultra-violet," which is very active on all sensitive emulsions, or near infra red, which is known to act, after a fairly long exposure, on several emulsions.