Despite the considerable progress which has been made in electric lighting, most of the energy consumed is converted into heat, and so the ventilation of electric lamps is just as necessary as it is for other types of lamp. The life of an electric lamp is always shortened if the lamp is used at a higher temperature than would be attained if it were not enclosed.
Neon pilot lamps which emit a very feeble orange glow (about i candle-power) may be used with advantage in all cases where only a very faint light is required (e.g. for reading a dark room watch or clock placed on the work bench, or for illuminating passages between several dark-rooms. Whenever these lamps are used on the work bench a yellow safelight should be employed to cut off the small proportion of violet rays emitted.
The reader should be cautioned against the use of lamps with red bulbs (red glass or glass covered with red varnish), which are more useful for decorative lighting at fetes than for photo graphic purposes. The few lamps specially manufactured are very much higher priced than ordinary lamps, and the difference in price, after several replacements, amounts to more than the cost of a good dark-room lamp.
249. Dark-room Lamps. The chief quality demanded of a dark-room lamp is that it shall stop all light other than that which passes through the safelight. It must be sufficiently well ventilated not to cause damage through over-heating of the filters of paper or gelatine (charring, melting, etc.), which are obviously more susceptible to injury of this kind than the coloured glass which, at one time, was the only material used. In some lamps the safelights are separated from the lamp itself by a sheet of glass, and the two compartments thus formed are separately ventilated.
From every point of view work is considerably facilitated if the lamp gives only a diffused light. This may be achieved either by the safelight being itself a diffuser, or by arranging the source of light so that the light which falls on the safelight comes from the matt-white surface of the interior of the lamp, which can be regarded as a source of diffused light.' Lastly, it is very desirable that the luminous surface of the lamp should not be visible to the operator in his normal working position. This may be achieved by using a hinged shutter or opaque screen, so that, when required, the negative can be examined by transmitted light 2 in the course of its development.
We will refrain from describing the innumer able types of dark-room lamp, more or less satisfactory in design and construction, which have been manufactured. A description of those at present available will be found in the dealers' catalogues. We will content ourselves with a mention of lamps with liquid filters. These consist generally of two concentric bowls of clear glass, between which is poured a solution of mineral salts or of dyes, the composition and concentration of which are chosen according to requirements. 3 250. Testing of Safelights. The spectroscopic examination of salelights, which is frequently recommended and sometimes used as a guaran tee of these goods, is entirely illusory. The harmful rays, and particularly the violet rays transmitted by many red glasses, when dispersed by the spectroscope, affect our eyes so little that they are often unnoticed even when the source of light is the sun or a strong electric arc. The only way in which a spectrographic exam ination can be of any use is when the eye is replaced by a photographic plate of the same kind as that for which the safelight is intended.' The only visual examination capable of giving any safe indication as to the quality of a safe light is that in which various filters transmitting only one portion of the spectrum (monochromatic filters) are superposed on to the safelight to be examined ; in these circumstances the combina tion should in no case appear to transmit rays which should be absorbed by the safelight.
The best, and, incidentally, the simplest, method of testing a safelight is to make a practical test under reasonable conditions. If it be remembered that the loading of a dark slide takes place partly in darkness and partly in indirect light, that a plate loses from 50 to 70 per cent of its sensitivity (quite apart from previous desensitizing) when it is immersed in a developer, and that the bath or dish in which development takes place can be covered during the greater part of the development, it will be readily understood that if the dry surface of a sensitive emulsion be exposed to the light from a safelight, placed 20 in. away, for a period of 30 seconds 2 without the emulsion becoming appreciably fogged (2 minutes if the safe-light tested is intended for use with positive emul sions), the safelight is a satisfactory one for the particular plate. To make the test, a plate (or film) is placed in a printing frame, and about a third of it is covered with black paper. A piece of cardboard, which at the start covers the whole of the sensitive surface, is moved every ten seconds, so that successive strips of the material are exposed to the light ; these strips will thus receive exposures of Jo, 20, 30, 40, etc. seconds respectively. After develop ment, the plate is examined, and if the 30-second strip and the portion which has not been ex posed at all do not appear to differ in density, then the safelight may be regarded as suitable for the material in question. If, on the other hand, fog appears on the io- or 20-second strip, it is necessary either to change the lamp for a weaker one or to darken the safelight by adding one or more sheets of paper, coloured yellow, green, or red, as the case may be. If, however, the first appearance of fog on the test strips indicates that a much longer exposure than 30 seconds is safe, then it is possible to increase the strength of the lamp. In either case, a fresh test should be made after the alteration considered to be necessary has been carried out.
251. Preparation of Safelights. It is beyond the scope of this work to describe the commercial methods of manufacturing coloured screens, and we will content ourselves with indicating a method of preparation which may, on occasion, be used by the photographer.
Procure some parchmentized paper (or sul phurized paper) and immerse for a few minutes in one of the tinting solutions given below, heated to F A. 4% solution of tartrazine. (Yellow.) B. 1% solution of chrysoidine. (Orange.) C. i% solution of naphthol green. (Green.) D. 2% solution of carmine blue or Patent Blue. (Greenish Blue.) E. i% solution of methyl violet. (Violet.) According to circumstances, the safelight is made by using or combining the dyed sheets, as follows— Slow development papers . i or 2 sheets of A (Yellow) Bromide paper. . i sheet of A and r of B (Orange) Ordinary slow plates or films 2 sheets of A and i of ID (Green) Ordinary or orthochromatic i sheet of A and i of E plates or films . . (Red)' Panchromatic plates or filrns 2 sheets of A and z of C and 1 of D (Dark green) Infra-red plates or films . 2 sheets of A and i of D (Green) with a Prus sian blue filter added..
Waste plates, which have been fixed (without having been developed), washed and dried, may be dyed in the same way. Safelights thus pre pared are very transparent, and one or more sheets of coloured or translucent paper should be placed between the two sheets of glass unless the lamp is to be used for indirect lighting, or is so constructed that the light-source affords the lighting entirely by reflection from a matt white surface in the lamp.