ACTINIC POWERS OF ARTIFICIAL LIGHTS Standard candle . . .
Colza lamp, fin. wick . . 2 Paraffin lamp, 4-in. wick . . 4 „ I-in. wick . . 10Paraffin duplex lamp . . 3o 2-ft. gas burner . . . 12 5-ft. „• • • 35 i6-c.p. electric incandescent lamp 35 32-c.p• „ I, i Welsbach ncandescent gas . 8oSir William Abney gives the following table, showing the photographic value of some artificial lights in terms of the photographic value of a standard candle, the photographic value being taken as the effect on bromide of silver : Light of the Optical Value Photographic Value in of one Standard Candle Terms of Standard Candle Standard candle . . .
Ordinary paraffin candle . . 1.3 Oxyhydrogen light, blow-through jet . . . . . 2 Electric arc light . . . io Magnesium burnt at the rate of I gr. per minute. . . 15 Bright sun at noon in summer 21.6 Referring to the above, Sir William Abney says : " It will be noticed how the optical and photo graphic values differ. It might be thought that, although these differences do exist, yet, by increasing the smaller number, the same effect might be obtained. It must be recollected, how ever, that the electric light radiates from 1,000 to io,000 candles from a very small area, and that, to make the same photographic illumin ation, the number of candles would have to be the same, but multiplied by io. Thus, if an electric arc light radiated 1,000 candles, the number of standard candles that would have to be employed would be io,000, a number which would never be concentrated in any reasonable area. Sunlight may be taken as equal, optically, to 5,600 candles placed at one foot from the object illuminated." The electric arc and the mercury vapour lamps are the most powerful for photographic purposes, and largely used by professional workers because of their richness in those rays to which dry plates are the most sensitive. The light from incan
descent electric lamps is usually of a yellowish tint, serviceable enough for printing, dark-room illumination, etc., but hardly strong enough for portrait work unless used in large numbers. Acetylene gives a very serviceable and intensely actinic light for most photographic purposes. The artificial light that is most generally used for printing, however, is the incandescent gas mantle, which, when of an average quality and full size, gives a light of between 6o and 70 candle power. Magnesium, in the form of either powder or ribbon, is extensively used for portrait work outside the studio at night, its use dating from 1863. Ribbon and powder, weight for weight, give approximately the same illumination ; and one inch of ribbon in contact printing is equal to four minutes' exposure to an ordinary flat flame gas burner at the same distance (see also " Flashlight Photography "). In 1882 a lamp was devised for burning magne sium ribbon in oxygen ; a number of improve ments followed, and the lamp is now an article of commerce. Ordinary gas burners, oil lamps, and candles give lights that are of poor actinic value. As regards the cost of some illuminants, Leon Gaster, A.M.I.E.E., has given the following table :— Cost per r,000 hours per candle power.
Petroleum . . . . 9.08d.
Alcohol incandescent . . io•o3d.
Auer burner, incandescent gas . 3.17d.
Pressure gaslight, incandescent . 2- r 2d.
Carbon filament lamp . . 14.1i d.
Nernst lamp . . . . 8-82d.
Arc lamp. . . . . 5.18d.
Flame arc lamp . . . i.o6d.
(See also " Acetylene," " Flashlight Photo graphy," " Candle Power," Limelight," Artificial Light, Portraiture by," etc.)