292. Incandescent Lamps. The first notable use of the incandescent lamp for the illumination of sitters and originals to be photographed dates from 1913, when I. Langmuir first made lamps with a tungsten filament enclosed in an atmo sphere of inert gas. In these half-watt (or gas filled lamps), as they are called, the pressure exerted by the gas retards the volatilization of the metal, making it possible for very high temperatures (exceeding C.) to be reached, at which the light emitted approximates very closely in composition to that of the best arc lamps. Very high intensities (up to 30,000 watts) have been attained, exceeding those of the most powerful arc lamps, which can now be replaced for nearly every purpose by the new lamps.
The luminous intensity and the photographic efficiency of a given lamp increase as the fila ment is heated to a higher temperature, which is effected by running the lamp at a higher voltage. Such over-running is limited by the increasingly rapid volatilization of the incan descent metal, causing rupture at the weakest point of the filament.
The rating adopted for ordinary lamps for domestic and public lighting gives them an average life of Loop hours (which may, how ever, be considerably shortened by fluctuations in the supply voltage'). To increase the efficiency recourse was first had (Trevor and Salt, 1914) to overrunning, during the actual exposure, lamps of a lower voltage than that of the mains (for instance, 8o volt lamps on no volt mains) by short-circuiting a resistance (or transformer), the switch being often actuated simultaneously with the shutter of the photographic camera.
The following table, prepared from measure ments 1w Luckiesh (1915) and L. Lobel (1921), shows approximately the variations in the visual intensity and actinic intensity (towards an ordinary emulsion) of gas-filled metal-filament lamps when over-run or under-run, the value of roo being arbitrarily assigned to the intensity of the lamp in question when working under conditions such that the lamp has a life of r,000 hours.' With orthochromatic, and especially with panchromatic, emulsions, the changes are not so marked, since the actinic and visual power of the lamp approximate more closely.
Subsequently, makers of incandescent lamps placed on the market special lamps for photo graphy and cinematography, specially devised to withstand overrunning, and which must not be worked beyond the stated rate. These lamps are of three types, corresponding respectively to continuous use for 100 hours or ro hours at the maximum rate, or to intermittent use of an average total duration of 2 hours at the said rate. While preparations are being made these
lamps must be In the table below, the last column shows the voltage at which lamps supplied for Ito volts must be run so as to have a life of about r,000 hours, and the The results differ considerably according to the very different qualities of lamps of various makes. Some lamps, especially those of low power, will with resistance put in must reduce the voltage to about that shown in order not to shorten appreciably the life at the maximum rate.
Rather than use a small number of lamps of high intensity,' which give very little even by using matt or " opal " bulbs, it is sometimes preferable to employ a battery of from 12 to 20 lamps, of smaller intensity, mount ed on a frame 6 ft. by 4 ft., behind a diffusing material. The area of illumination can then be adjusted by lighting a suitable number of lamps.
293. At a time when portrait photographers and cinematographers used only ordinary or slightly orthochromatic emulsions and long life lamps emitting chiefly rays inactive as regards these emulsions, they were led to use high light intensities. In order then to avoid the discom fort and fatigue due to heat and glare, it was suggested by Luckiesh to use blue bulbs made of a glass absorbing the greater part of the inactive rays. Dealers, who probably had more knowledge of electrical than of photographic technique, have seriously affirmed that these lamps, emitting blue light, furnished a more actinic light than the white lamps of the same consumption, and these statements have been received with enthusiasm by photographers, who have tinted the glass in their studios blue in order to increase the actinic value of the light ! It cannot be repeated too often that the action of a filter is not to tint the light as a skein of wool is dyed by a coloured solution, but to cut out a certain variable fraction of the incident radiations, so that it is obviously impossible to improve the actinic value in this manner.
Not only is there a loss, from the point of view of the exposure necessary, but this loss becomes considerable when panchromatic ernul sions are used, for, having absorbed the red by the blue filter incorporated in the lamp, it is then necessary to use a yellow filter, in order to absorb in its turn the excess of blue and violet which now predominate. The marked. advantage which the incandescent lamp pos sesses for the photography of coloured objects is therefore lost.