The flame-arcs afford a means of obtaining a variety of illuminants differing in spectral character or color.  By impregnating the carbons with various chemical compounds the color of the flame can be widely altered.  The white flame-arc obtained by the use of rare-earth compounds in the carbons provides an illuminant closely approximating average daylight.  By using various substances besides carbon for the electrodes, illuminants differing in spectral character can be obtained.  These are usually rich in ultra-violet rays and therefore have their best applications in processes demanding this kind of radiant energy.  The arc-lamp is limited in its application by its unsteadiness, its bulkiness, and the impracticability of subdividing it into light-sources of a great range of luminous intensities.

The most extensive applications of artificial daylight have been made by means of the electric incandescent filament lamp, equipped with a colored glass which alters the light to the same quality as daylight.  The light from the electric filament lamp is richer in yellow, orange, and red rays than daylight, and by knowing the spectral character of the two illuminants and the spectral characteristics of colored glasses in which various chemicals have been incorporated, it is possible to develop a colored glass which will filter out of the excess of yellow, orange, and red rays so that the transmitted light is of the same spectral character as daylight.  Thousands of such artificial daylight units are now in use in the industries, in stores, in laboratories, in dye-works, in print-shops, and in many other places.  Currency and Liberty Bonds have been made under artificial daylight and such units are in use in banks for the detection of counterfeit currency.  The diamond expert detects the color of jewels and the microscopist is certain of the colors of his stains under artificial daylight.  The dyer mixes his dyes for the coloring of tons of valuable silk and the artist paints under this artificial light.  These are only a few of a vast number of applications of artificial daylight, but they illustrate that mankind is independent of natural light in another respect.

There are various kinds of daylight, two of which are fairly constant in spectral character.  These are noon sunlight and north skylight.  The former may be said to be white light and its spectrum indicates the presence of visible radiant energy of all wave-lengths in approximately equal proportions.  North skylight contains an excess of violet, blue, and blue-green rays and as a consequence is a bluish white.  Noon sunlight on a clear day is fairly constant in spectral character, but north skylight varies somewhat depending upon the absence or presence of clouds and upon the character of the clouds.  If large areas of sunlit clouds are present, the light is largely reflected sunlight.  If the sky is overcast, the north skylight is a result of a mixture of sunlight and blue skylight filtered through the clouds and is slightly bluish.  If the sky is clear, the light varies from light blue to deep blue.