ELECTRIC LIGHT (Fr., Lumiere electrique ; Ger., Elektrisches Licht) The electric light is the most practically useful artificial illuminant for photographic purposes. Where public mains are not accessible, a small dynamo, driven by a gas-engine, offers the most convenient means of installation. Apart from the initial expense of the dynamo and engine, this is naturally more economical than the public supply, though involving greater trouble and attention. The switches should be such as will stand hard wear and rough usage, and an adjust able resistance is an advantage. There are four principal types of lamps—the arc, the incan descent or glow lamp, the Nernst lamp, and the mercury vapour lamp. In portraiture, the arc lamp is most used. For studio employment and for ordinary black-and-white copying, the enclosed arc is best, but for photographing colour the open arc is superior. These two patterns are fully dealt with under the heading Arc Lamps." Flame arc lamps, in which a brilliant golden yellow light is obtained by incorporating metallic salts in the carbons, or by introducing a central core of such salts, have also attracted some attention among photo graphers, it having been stated that colour values are better reproduced with them. A plate sensitised for yellow has, of course, to be employed. An arc lamp in use should not be examined or adjusted without a piece of smoked yellow glass before the eyes, or goggles of the same material.
The incandescent glow lamp is yellower and less actinic than the arc, but a number of these suspended inside a dead white spherical or para bolic reflector, having a muslin diffusing screen in front, can be used for portrait work, giving a soft and pleasing lighting. Several lamps of
this kind are on the market. Ruby and orange glow lamps are much used in the dark-room. The new metallic filament lamps have greatly improved the efficiency of incandescent electric lighting, yielding better illumination with less current than carbon filament lamps.
The Nernst lamp makes use of a thread of yttrium and zirconium oxides, and similar metallic earths. These lamps are employed with the optical lantern and for enlarging ; they are economical of current and carry their own resistances.
The mercury vapour lamp is dealt with under its own heading. It requires very little current, and is a useful light for printing, copying, or enlarging. A certain prejudice exists against its employment in portraiture owing to its unpleasant colour, due to the absence of red rays, but this is easily remedied by using one or two ruby glow lamps in addition.
For copying, etc., the nearness with which the lamp can be placed to the easel is of import ance from an economical standpoint, other considerations being equal. Thus, the enclosed arc can be brought closer than the open arc, the flame arc still nearer, and the mercury vapour lamp nearest of all.
In process work, the electric arc light is invari ably used for copying in preference to daylight. (See " Arc Lamps.") Mercury vapour lamps are also used to some extent. Incandescent electric lamps are not sufficiently powerful.