sten lamp may be filtered
to obtain the same quality of light as is emitted by the carbon
filament lamp by absorbing from one fifth to one fourth of the light.
The luminous efficiency of the tungsten lamp equipped with such a tinted
bulb is still about twice as great as that of the carbon-filament lamp.
Thus the high efficiency of the modern illuminants is utilized to
advantage even though their color is maintained the same as the old
illuminants.

All modern illuminants emit radiant energy, which does not affect the
ordinary photographic plate. This superfluous visible energy merely
contributes toward glare or a superabundance of light in photographic
studios. A glass has been developed which transmits virtually all the
rays that affect the ordinary photographic plate and greatly reduces the
accompanying inactive rays. Such a glass is naturally blue in color,
because it must transmit the blue, violet, and near ultra-violet rays.
Its density has been so determined for use in bulbs for the
high-efficiency tungsten lamps that the resultant light appears
approximately the color of skylight without sacrificing an appreciable
amount of the value of the radiant energy for ordinary photography. This
glass, it is seen, transmits the so-called chemical rays and is useful
in other activities where these rays alone are desired. It is used in
light-therapy and in some other activities in which the chemical effects
of these rays are utilized.

In the photographic dark-room a deep red light is safe for all emulsions
excepting the panchromatic, and lamps of this character are standard
products. An orange light is safe for many printing papers. Panchromatic
plates and films are usually developed in the dark where extreme safety
is desired, but a very weak deep red light is not unsafe if used
cautiously. However, many photographic emulsions of this character are
not very sensitive to green rays, so a green light has been used for
this purpose.

A variety of colored lights are in demand for theatrical effects,
displays, spectacular lighting, signaling, etc., and there are many
superficial colorings available for this purpose. Few of these show any
appreciable degree of permanency. Permanent superficial colorings have
recently been developed, but these are secret processes unavailable for
the market. For this reason colored glass is the only medium generally
available where permanency is desired. For permanent lighting effects,
signal