FIREWORKS EXHIBITION ON MAY DAY AT PANAMA-PACIFIC EXPOSITION
The daylight which enters buildings is often considerably altered in color by reflection from other buildings and from vegetation, and after it enters a room it is sometimes modified by reflection from colored surroundings. It may be commonly noted that the light reflected from green grass through a window to the upper part of a room is very much tinted with green and the light reflected from a yellow brick building is tinted yellow. Besides these alterations, sunlight varies in color from the yellow or red of dawn through white at noon to orange or red at sunset. Throughout the day the amount of light from the sky does not change nearly as much as the amount of sunlight, so there is a continual variation in the proportion of direct sunlight and skylight reaching the earth. This is further varied by the changing position of the sun. For example, at a north window in which the direct sunlight may not enter throughout the day, the amount of sunlight which enters by reflection from adjacent buildings and other objects may vary greatly. Thus it is seen that daylight not only varies in quantity but also in quality, and an artificial daylight, which is based upon an extensive analysis, has the advantage of being constant in quantity and quality as well as correct in quality. Modern artificial-daylight units which have been scientifically developed not only make mankind independent of daylight in the discrimination of colors but they are superior to daylight.
Although there are many expert colorists who require an accurate artificial daylight, there are vast fields of lighting where a less accurate daylight quality is necessary. The average eyes are not sufficiently skilled for the finest discrimination of colors and therefore the Mazda "daylight" lamp supplies the less exacting requirements of color matching. It is a compromise between quality and efficiency of light and serves the purpose so well that millions of these lamps have found applications in stores, offices, and industries. In order to make an accurate artificial north skylight for color-work by means of colored glass, from 75 to 85 per cent. of the light from a tungsten lamp must be filtered out. This absorption in a broad sense increases the efficiency of the light, for the fraction that remains is now satisfactory, whereas the original light is virtually useless for accurate color-discrimination. About one third of the original light is absorbed by the bulb of the tungsten "daylight" lamp, with a resultant light which is an approximation to average daylight.
Old illuminants such as that emitted by the candle and oil-lamp were used for centuries in interiors. All these illuminants were of a warm yellow color. Even the earlier modern illuminants were not very different in color, so it is not surprising that there is a deeply rooted desire for artificial light in the home and in similar interiors of a warm yellow color simulating that of old illuminants. The psychological effect of warmth and cheerfulness due to such illuminants or colors is well established. Artificial light in the home symbolizes independence of nature and protection from the elements and there is a firm desire to counteract the increasing whiteness of modern illuminants by means of shades of a warm tint. The white light is excellent for the kitchen, laundry, and bath-room, and for reading-lamps, but the warm yellow light is best suited for making cozy and cheerful the environment of the interiors in which mankind relaxes. An illuminant of this character can be obtained efficiently by using a properly tinted bulb on tungsten filament lamps. By absorbing about one fourth to one third of the light (depending upon the temperature of the filament) the color of the candle flame may be simulated by means of a tungsten filament lamp. Some persons are still using the carbon-filament lamp despite its low efficiency, because they desire to retain the warmth of tint of the older illuminants. However, light from a tungsten 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 glasses, colored caps, and sheets of colored glass may be used. Tints may be obtained by means of colored reflectors. Other colored media are dyes in lacquers and in varnishes, colored inks, colored textiles, and colored pigments.
Inasmuch as colored glass enters into the development of permanent devices, it may be of interest to discuss briefly the effects of various metallic compounds which are used in glass. The exact color produced by these compounds, which are often oxides, varies slightly with the composition of the glass and method of manufacture, but this phase is only of technical interest. The coloring substances in glass may be divided into two groups. The first and largest group consists of those in which the coloring matter is in true solution; that is, the coloring is produced in the same manner as the coloring of water in which a chemical salt is dissolved. In the second group the coloring substances are present in a finely divided or colloidal state; that is, the coloring is due to the presence of particles in mechanical suspension. In general, the lighter elements do not tend to produce colored glasses, but the heavier elements in so far as they can be incorporated into glass tend to produce intense colors. Of course, there are exceptions to this general statement.
The alkali metals, such as sodium, potassium, and lithium, do not color glass appreciably, but they have indirect effects upon the colors produced by manganese, nickel, selenium, and some other elements. Gold in sufficient amounts produces a red in glass and in low concentration a beautiful rose. It is present in the colloidal state. In the manufacture of "gold" red glass, the glass when first cooled shows no color, but on reheating the rich ruby color develops. The glass is then cooled slowly. The gold is left in a colloidal state. Copper when added to a glass produces two colors, blue-green and red. The blue-green color, which varies in different kinds of glasses, results when the copper is fully oxidized, and the red by preventing oxidation by the presence of a reducing agent. This red may be developed by reheating as in the case of making gold ruby glass. Selenium produces orange and red colors in glass.