294. Mercury-vapour Lamps. These lamps, which were invented in 1901 by P. C. Hewitt, could only be used at first with direct current, and could not be started from a distance, because they had to be tilted in order to strike the arc. Owing to the many improvements which have been made in their manufacture, special lamps can now be supplied, which run on alternating current and are lighted merely by closing a switch.' They arc generally in the form of fairly thick glass tubes about 4 ft. long and an inch in diameter. The whole length of the tube emits a bluish-green light composed of separate spectral lines ranging from yellow to ultra-violet, the green line possessing the highest visual intensity. Ordinary emulsions are of course insensitive to this radiation, since their sensitivity is mainly confined to the violet and ultra-violet regions. The very high actinic value of these lamps towards ordinary emulsions allows of an appreciable economy in electrical energy, besides which they give off less heat than any other source of light, and, owing to the low visual intensity from the luminous tube, all glare is avoided. Unfortunately, this type of lamp cannot be used for orthochromatic photo graphy, for the use of a yellow filter, even of very low absorption, causes a deplorable fall in the efficiency of the lamp.
To correct the disagreeable effect of this green light, which gives people a peculiar ghostly appearance, the only effective means is the addition of lamps emitting mainly red light, banks of under-run incandescent lamps, or neon the respective intensities of the two lights being adjusted to suit the chromatic sensitivity of the emulsions used.
In the cinematograph industry, where special make-up and appropriate choice of the colouring in the costumes and decorations render the use of orthochromatic materials unnecessary, these lamps were used to a large extent, more especi ally for the general lighting, obtained from a number of frames arranged horizontally above the scene, each frame carrying from four to eight tubes. The contrast lighting (for the model ling of reliefs) was supplied by batteries of lamps mounted vertically on trolleys.
In a portrait studio, two tubes mounted on movable or fixed platforms, for general lighting, and another arranged so that it is movable in all directions, can be made to give a great variety of lighting effects. The numbers of lamps can be reduced to two for purposes of economy, or increased if it is desired to work with very short times of exposure.
Numerous attempts have been made to in crease the efficiency of mercury lamps by increasing the pressure and the temperature of the mercury vapour inside the lamp and de creasing its volume in order to increase brilliancy.
The bulb must then be made of quartz and, therefore, transmits with considerable intensity ultra-violet rays of short wave-length. The bulb, unless protected by an envelope of rather thick glass to absorb these rays, would, even in a very short time, cause grave injury to eye sight.' By way of example we will indicate the chief characteristics of two very high pressure lamps (C. Bol, 1935), one cooled by water circulating in a jacket and the other air-cooled, both for alternating current with auto-transformer and self-induction coil. The emitted light is at first formed only of mercury lines, somewhat broadened, on which there is gradually super imposed a continuous spectrum that becomes more and more intense, the light being then almost white, with green predominant. These lamps can be re-lit only after complete cooling, and this restricts their photographic uses.
295. Illumination of Flat Originals. While the distribution of the lighting over a person, or group of objects is almost entirely a matter of taste, and is therefore outside the scope of this work, the illumination of flat originals, for the purposes of photographic reproduction, is controlled by definite rules, which have been described in full in text books on photo-mechanical reproduction. We shall just briefly outline the essential facts.
The light-sources should be preferably mer cury arc lamps for all originals in black and white, and incandescent 1 lamps for all colour work.
The desired uniformity of illumination cannot be obtained with a single lamp, unless it is moved, after appropriate fractions of the exposures, successively to the positions which are later recommended for several separate sources of light.
The positions chosen for the lamps should be such that, in the first place, the whole area of the original is uniformly illuminated, 2 and, secondly, that no reflections of the lamp appear on the original, a condition which is much more readily fulfilled if a lens of long focal length is employed.
Even illumination of the original can be obtained by using four incandescent lamps or two mercury tubes.
Suppose it is necessary to illuminate a square by means of four lamps, which roughly may be assumed to be point-sources. By placing a lamp opposite each corner of the square at a distance equal to half the diagonal of the square, or slightly nearer, even illumination is obtained to within 4 per cent, the mean value being about 1-45, the illumination from a single lamp, falling on the original at a point directly opposite it, being represented by 1.