247. Choice of Dark-room Illumination. In practice, the following lighting is for various types of sensitive material— Slow positive silver chloride emulsions (for development) . . . Yellow Rapid positive silver bromide emul sions . . . . . Orange Rapid negative emulsions, not ortho chromatic 3. . Orange-red or yellow-green Orthochromatic emulsions, sensitive to green ..... Ruby-red Panchromatic emulsions . . Bluish-green offeeble inten sity Emulsions sensitive to infra-red, but not to green . . Pure green Not so very long ago the sensitivity of photo graphic plates was so small and so restricted to one part of the spectrum that it was always safe to use ruby-red glass (manufactured for windows or for railway signals) and, frequently, even fabric impregnated with lead chromate and varnished. But while the general and the chromatic sensitivity of photographic emulsions has constantly been improved, the manufacture of red glass has been more and more neglected,' the glass manufacturers having considered only the visual effect, and not the spectral trans mission. Many red glasses, sometimes even the deepest, transmit a considerable proportion of violet which is usually not absorbed by the yellow-brown glass, sometimes superimposed on the red one as a precaution. The green glass of commerce transmits almost the whole of the spectrum and has merely a predominant trans mission in the green. It is only very exception ally that it is possible to find a green and a yellow glass which, when superimposed, gives a non actinic filter which is satisfactory for the manipu lation of ordinary emulsions.
The inability of glass makers to satisfy the needs of photographers has resulted in the manufacture of special filters for the illumination of photographic dark-rooms which are generally described as safelights, and are prepared either by dyeing papers 2 in solutions of dyes of suitable strength or by coating coloured gelatine. The actual safelight is made by the superposition of two or three similar or dissimilar elements.
Safelights applicable to each case have been worked out by determining each time the maximum general dark-room illumination which is compatible with safety, and not the amount of direct light which may be allowed to fall on to the sensitive material. The operator thus avoids the painful sensation of groping about in almost complete With the traditional deep-red glass safelights, which limited the illuminated zone to a square foot or so in the immediate neighbourhood of the lamp, the dishes had to be brought into contact with the lamp itself, whereas a lamp fitted with a modern safelight may be placed some distance from the baths, and the dark-room will be well illuminated in its entirety without the sensitive material being more strongly illuminated than it would have been under the old conditions. Neverthe less, it is still well to cover the bath during the time when it is not necessary to watch the development.
The use of a desensitizer, either as a prelim inary bath or in the developer itself, allows a very bright light to be used from the moment the emulsion has been effectively desensitized., so that the remaining operations may be car ried out in a yellow or white light without the slightest risk of fog whatever the original sensitivity of the materials in use.
To ensure good general illumination of the dark-room, the walls and ceiling should be light in colour ; the diffused light from the walls cannot be more dangerous than the light which issues directly from the dark-room lamp.' 248. Light Sources. Daylight, owing to its continual fluctuation, is the least serviceable light source for dark-room illumination ; it may, however, be used for general lighting of the dark-room, if it is possible to exclude it partially or wholly when desired by a shutter or blind.
In all rooms equipped for permanent use where illumination is restricted to combustible solids (candles), liquids (paraffin, petrol, oil), or gases (acetylene, incandescent gas mantles), it is well, when space permits, to place the source of illumination outside the dark-room, allowing the light to enter through a window. In front of this window grooves should be provided to take interchangeable frames fitted with safe lights corresponding with the different photo graphic materials in use. With such an arrange ment the products of combustion are kept from accumulating in the dark-room, vitiating the atmosphere and making it uncomfortably hot in warm weather. Also a greater choice of lighting appliances is possible. The burners usually provided in dark-room lamps are seldom well made. Finally, there is avoidance of the risk of white light leaking into the room, as it frequently does through the badly-constructed ventilation holes of dark-room lamps_ Failing this arrangement of external lighting, the products of combustion should be allowed to escape by metal pipes.
As regards the simplest kinds of lamp, it should be borne in mind that a candle, when it is enclosed in a lamp, melts more quickly than it burns.' For this reason a stearine night-light in a glass saucer is to be preferred if no other illuminant is available. These night-lights can be renewed to any extent by using wax matches as wicks. The reservoir of a petrol (gasoline) lamp should never be inside the lantern, which should enclose only the flame.
Preference will obviously always be given to electric lamps supplied from the or by a storage battery. In view of the fact that the higher the voltage is across the lamp the greater is the emission of the blue and violet radiations which have to be absorbed by the safelight, it is worth limiting these useless portions of the radia tion by running the lamps at a voltage lower than normal. Thus, a lamp intended for 125 volts may be used on a supply with a voltage of rio To obtain full illumination in the dark-room, after desensitizing negative emulsions or when using positive emulsions, it is useful to be able to modify the amount of current used by the lamps. For this purpose the arrangement shown diagrammatically in Fig. 150 (Clerc, 1q13) may be used. A double-pole switch is arranged to connect the lamps either in parallel (maximum light) or in series (minimum light of intensity, measured after the light has passed through the safelights, about 25 per cent of the maximum). Alternative arrangements involve the use of a resistance in the lamp circuit ; or, with alternating current, a special transformer, with tappings for two different voltages (e.g. 6o and 90 volts for a no volt lamp) 1 controlled by a switch, may be used.