In portrait and still-life photography, various means of control, particularly in a well-fitted be stated in passing that at the time photo graphy was first practised, when exposures, even in the most favourable cases, were counted in minutes, as much light as possible was admitted into the studios, with entire disregard of light and shade, by constructing them entirely of glass.' Owing to the considerable progress which has been made in the manufacture of sensitive materials and photographic lenses, it is possible nowadays to use an artist's studio, or even an ordinary room. The modelling in a face is much better rendered if the subject is illuminated only by a small area of glass window.
29o. Artificial Light Sources. The three fac tors to be considered in an artificial lighting installation for photography are quantity, quality, and the degree of diffusion.
studio, are available. A large variety of effects, particularly with the aid of artificial lighting, may be obtained, by means of opaque blinds, diffusers of semi-transparent fabrics in the form of blinds or stretched over movable frames, and reflectors made of white opaque materials. A detailed description of the arrangements in a portrait studio can be readily obtained. 2 It may puality depends essentially on the nature of the illuminant body. When this is a solid (par ticles of carbon in an illuminant flame, metal filament, crater of an arc, etc.) the spectrum of the emitted light comprises all the radiations in a wide interval (continuous spectrum). Com pared with sunlight there is always a pre dominance of infra-red and red, the extent of the spectrum and its intensity in blue-violet and especially in ultra-violet being all the less, as the temperature of the incandescent body becomes As a rule, a much larger part of such an artificial light is effective if the photographs are taken on orthochromatic or, better, on pan chromatic plates or films (216 footnote).
\Vhen the luminous body is a gas or a vapour made incandescent by heating to a very high temperature (the true electric arc) or rendered luminous by electrical discharges, the spectrum of the emitted light is discontinuous. Mono atomic (mercury, sodium, neon, helium, etc.) gases or vapours thus give a line spectrum on which in certain conditions (high pressure, high temperature) may be superposed a continuous spectrum, the other gases (hydrogen, carbonic gas, etc.) emitting a band spectrum.
The table on page 193, the data of which has been taken from various papers by Hiibl, Ives, Jones, Hodgson, and Huse (1913 to 1916), shows the approximate compositions of various light sources relative to sunlight, which is taken as the standard for comparison (on the assumption that sunlight may be divided into three equal parts of red, green, and blue-violet, the partial intensities in each case being brought to a total of 'co). Their relative efficiency towards three
types of sensitive emulsions is also given, and for this purpose the value of loo is arbitrarily assigned to sunlight, the efficiency of the other sources being measured for equal visual mien sity.' Lamps usually cast light in all directions, so that their efficiency is notably increased by reflectors bringing back to the subject the light emitted in other directions. When high intensity lamps are used for living subjects they must be fitted with diffusers preventing dazzle. From these various points of view the efficiency of a lamp can be of very different values according to the apparatus in which it is With the exception of the light obtained by the combustion of magnesium, which we shall study later, we have only, from the point of view of practical application, to deal with the various forms of electric lighting.
291. Arc Lamps. The arc lamp has for some time been the only luminous source from which high intensities can be obtained, tinder advan tangeous conditions with regard to the consump tion of energy.' When used for the lighting of photographic subjects this source introduces the defect which is common to all point-sources. The shadows are so sharply defined, with no penumbra, that it is very difficult to obtain a good rendering of the form of the subject. To avoid this it is often necessary to point the arc away from the model and to use the reflected light from an adjustable diffuser, often in the shape of a large umbrella.
Since the introduction of the luminous tubes called mercury arcs, and the great improvements which have been made in the manufacture of incandescent lamps, arc lamps are much less used in portrait photography but still find employment in cinematograph and process studios, more particularly for wet-collodion photography. As these applications lie outside the scope of this book, we shall not attempt to describe the various types of arc lamps. 2 In the use of alternating or rectified current, the alternations of light and dark (two extinctions per " period " of the current), give rise to alter nating bands of different density when exposure is made with a focal-plane shutter ; in cine matography the effect may be insufficient exposure of some pictures.