International projectionist (Jan 1963-June 1965)

FIG. 1 — Two sizes of xenon bulb manufactured by the firm of Osram in Germany. The bulbs are made of heatresistant quartz, the metal electrodes of tungsten. The bulbs are filled with pure xenon gas at a pressure of from 8 to 10 atmospheres. to incandescence by an oxyacetylene flame which, by itself, is only feebly luminous in spite of its high temperature. The "mazda" light bulb used in small projectors emits its radiation from a solid tungsten-wire filament heated to incandescence by an electric current. The low-intensity carbon arc produces its light from the white-hot tip of a solid carbon rod heated, like an incandescent-bulb filament, by the passage of electricity. The solid carbon does indeed slowly vaporize and burn away, but the "arc" of current-conducting gas between the tips of the two carbons, although luminous, is much less bright than the glowing positive crater. Even the high-intensity arc is a solid emitter. Some of its light comes from the solid carbon of the crater, but most comes from a cloud of electrically excited cerium oxide particles suspended in a film of carbon gas covering the crater floor. Inasmuch as the low-intensity crater light is yellowish, and the high-intensity cerium light is bluish, varying the electrical load varies the color of the highintensity carbon arc. The xenon light source, on the other hand, is a true gaseous-discharge type of illuminant, and "arc light" in the true sense of the term. All of the radiation is emitted from an arc of glowing gas between the two metal electrodes, not from the electrodes, themselves. Bluish violet 400 436 Yellow „ £reenYellow 546 578V 700 SPECTRUM OF MERCURY 400 436 546 578 700 SPECTRUM OF MERCURY (VAPOR UNDER PRESSURE) FIG 2 — The spectrum of mercury vapor consists of bright colored lines on a dark background. The three most intense lines are bluish violet, yellowish green, and yellow. If the pressure of the vapor is greatly increased, the lines broaden to give a more satisfactory light for illuminating purposes. Nevertheless, the absence of red rays in mercury light causes all red objects illuminated by it to appear dark brown or black in color. This diagram is for illustrative purposes only: there is no mercury in a xenon bulb. Common Discharge Lamps Colored Most of the common gaseous-discharge illuminants, such as the neon-type tubes used for illuminated signs, are manifestly unsuitable for projection purposes. First, they aire not sufficiently concentrated (low luminosity per unit area ) . and hence cannot be focused to a small, intensely bright "spot" by mirrors or lenses. Second, they emit a strange sort of light which usually appears more or less colored and which, when analyzed with a spectroscope, is seen to consist only of a few intensely colored "emission lines" (Separate wavelengths of light) on an otherwise dark spectral background. The mercury-vapor lamp used for blueprinting is another common example of an electric gaseous-discharge light source. Electricity passes through mercury vapor and excites the mercury atoms into emitting radiation. The light of a mercury-vapor lamp has a ghastly greenish white color, but this is not its worst feature as an illuminant. It makes most colored objects illuminated by it, look horribly discolored and blackish. The discoloring effect of mercury-vapor light is due to the absence in its spectrum of all but a very few colors. The mercury spectrum consists chiefly of only three bright colored lines on a black background — yellow, yellowish green, and bluish violet. (The strong ultraviolet lines, useful in a few special applications, are invisible. I In mercury light, all objects which are bright blue, bluish green, orange, or flesh-colored appear grav or brown; and all red objects look black for the simple reason that mercury light is almost completely devoid of red rays! An improvement in both the illuminating intensity and visual quality of mercurv light results when the pressure of the electricity-conducting vapor, or gas, is greatly increased. Not only does the mercury-vapor tube then give more light, but the spectral emission lines (yellow, green, and violet) widen into relatively broad bands which include more colors. The light then has a less ghostly appearance and gives a somewhat better (though still very far from perfect) rendition of colored objects illuminated by it. This is why the mercurv vapor in modern streetilluminating lamps and in the "pulsed-light" tube for shutterless motion-picture projectors is excited under high pressure. But even so, the extreme weakness of the red region of the mercury spectrum and the absence of pure blue are serious handicaps from the visual point of view. The light from a high-pressure mercury bulb may have a daylight-white appearance, but it is spectrally too selective to behave in a natural manner as an illuminant. This is true even when cadmium has been added to the mercurv to supply the missing red wavelengths, or rubidium and cesium to supply both red and blue. The mercury arc and its spectrum have been described here only to clarify the basic spectro-colorimetric prob International Projectionist June 1963