ARC LAMPS (Fr., Lampes d arc ; Ger., Bogen larnpen) Lamps in which a powerful light is obtained by passing an electric current through a pair of slightly separated carbon pencils. Where the current is interrupted by the gap an intensely brilliant arc is created. Arc lamps are of two principal kinds : (a) open, in which the carbons are exposed to the air, and (b) enclosed, in which the carbons are almost hermetically sealed in a glass-covered case, and burn in a mixture of carbon monoxide and nitrogen formed by their own combustion. The enclosed type is coming to be preferred, on account of its increased actinic power, due to its larger arc, the greater proportion of violet rays, and the fact that the life of the carbons is much longer, owing to the practical absence of air, and consequently slower combustion. Some excellent open-type arc lamps are, however, obtainable, in which several pairs of carbons are used ; these obviously yield a more powerful light than the single carbon pattern.
The arc light is employed in studio portraiture, printing, copying, enlarging, and photomicro graphy, also in the optical lantern and cinemato graph.
In studio work the lamp should be supported on an adjustable tripod or bracket, so that it may be raised as high as 8 ft. or 9 ft. or lowered at will. The direct light of the arc is liable to produce hard lighting and heavy shadows ; and it is therefore advisable either to cut this off by a small opaque disc placed in front of the carbons and to use only the light obtained from an umbrella reflector or screen at the back of the lamp, or, as an alternative, to use a translucent diffusing screen of muslin or tracing cloth in front of the lamp. Sometimes these two plans are combined. A illustrates the use of a diffusing screen c with the Boardman multi-carbon arc lamp (open type). The metal shield D screening the direct light of the arcs will be noticed, and also the large umbrella reflector E. B shows an extensively-used pattern of enclosed arc (the " Westminster ") designed for general photo graphic use. For low studios, a reversed
model is made, having the arc chamber on top instead of beneath. Whatever kind of lamp is employed, the lens must be shielded from the direct rays by means of a projecting hood.
For printing, the lamp should not be too near the frames, a good average distance being z ft. or 3 ft. Various stands are obtain able to support a number of frames in circular tiers, the lamp being suspended in the centre.
In lantern and cinematograph work the open arc at present holds the field, though the enclosed arc is coming into use. Lantern arc lamps are so designed as to economise space, and are fitted to a sliding tray. A model having the carbons at a right angle is claimed to give a better and more direct light, and to take up less room than those types in which the carbons form a very obtuse angle with each other.
In process work, arc lamps play an important part, practically all work being done by electric light, because of its uniformity and the certainty of the exposures as compared with daylight. The " open " arc has been almost entirely super seded by the " enclosed " arc. For copying, d. pair of lamps is usually employed taking about Jo amperes, though for large work four lamps may be necessary. The lamps are enclosed with semi-cylindrical reflectors, which are whitewashed inside. For colour work 15 ampere lamps are frequently adopted, and coloured flame carbons utilised to aid the expo sure through the colour filters. Red flame car bons, for instance, are found beneficial with the red filter for the blue printing plate. Search lights of high amperage are also employed in some studios for securing very powerful illumin ation. For printing on the metal, lamps of from 15 to 20 amperes are employed. Arc lamps for process work are usually run in " parallel " ; and a high voltage—zoo volts or more—is preferable with enclosed lamps so as to obtain about 140 volts across the arc, by which means a long flaring arc of great actinic power is obtained.