While in the early days of photography practically the only source of light was the sun, the use of artificial illuminants is constantly increasing. Many kinds of light are available and they are being made in such convenient form that the studio that depends entirely upon sunlight has come to be the exception.

The advantages of artificial illuminants, which have caused their more general use, are their constancy and the fact that they permit the photographer to work at all hours of the day or night. Variation of the intensity of daylight makes the judging of time for which sensitive material must be exposed so difficult that the photographer must acquire considerable skill to avoid incorrect exposure with its resulting waste of material. With most artificial illuminants the matter of exposure can be reduced to a simple calculation of time.

The possibility of working at any time under evenly uniform conditions is certainly an advantage in portraiture, commercial work, printing, enlarging and photo engraving.

For portraiture and commercial work a large diffused light source is essential and only occasionally can a concentrated light source be used. A broad source of light; such as that given by the mercury vapor lamp, or a diffusing system must be arranged.

The lights best suited to portraiture are the white flame arc's mercury vapor, enclosed arcs and nitrogen tungsten. The scale, gradation and latitude of emulsions vary with the color of the light, so color is an important factor in the quality of results. The blue-violet light containing the greatest amount of ultraviolet will give the greatest gradation, while gradation will be less perfect with light containing more yellow and red.

As halation is caused by the penetration of light through the emulsion and its reflection from the back of the support, there is more halation from the light containing yellow and red because these penetrate the emulsion. The yellow color of the emulsion acts as a filter in stopping out the ultra-violet and blue-violet light and materially reduces halation, making such a light especially suitable for producing negatives of the best technical quality.

For color separation in commercial work, especially when a panchromatic plate must be used to secure a correct rendering of objects containing red, the mercury vapor light is useless, as it is completely minus red. As a consequence, red objects will not be recorded regardless of whether a filter is used or not. All red will appear as black.

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For such work nitrogen tungsten lamps are especially good, while the white flame arc is good and the enclosed arc fair.

For printing, most any of these lights are satisfactory, constancy being most important, but tungsten lamps are most generally used. When a very strong light is used and short exposures made, a lack of flicker is so important that arc lamps are not practical. We have heard of one large firm which uses mercury vapor lamps, which are held constant by a resistance and ameter, the current being watched and the exposures timed accurately as a result.

For enlarging it has been customary to use an arc lamp, except when condensers are used, in which case it is best to use a concentrated filament tungsten lamp with a suitable reflector, because of its great constancy. When arc lamps are used without condensers a diffusing screen should be placed between the light and the negative, opal glass being best for this purpose. If mercury vapor lamps are used a powerful diffuser is not needed, a sheet of ground glass being sufficient if several small tubes are parallel to each other a short distance apart. As Artura Carbon Black is more sensitive to the ultra-violet of the mercury light than other papers, and as a blue-tone negative transmits this light while a pyro stained negative partially absorbs it, the tendency with this paper and blue negatives is to over-expose and produce flat prints. This penetrating quality of the mercury light is deceptive, because the ultra-violet light is invisible to the eye but has a decided effect upon the photographic paper.