International projectionist (Jan 1963-June 1965)

picture projection is ordinarily described as "daylight white." An illuminant will give this quality of light when it emits all visible wavelengths about equally. If the light is weak in the shortwave end of the spectrum (violet, blueviolet, blue), it will be yellowish in color. If weak in the longwave end (red scarlet, orange), the light will be bluish. If weak at both ends, the middle green and yellow rays will preponderate, and the light will look greenish. Less frequently, the middle part of the spectrum may be weak. In this case, the preponderance of red and violet rays will impart a purplish or pinkish color to the light. For the purpose of measuring light, particularly with reference to its color, scientists have agreed upon three standard illuminants designated as Source A, Source B, and Source C. Source A is amber-yellow tungsten-lamp illumination. Source B is direct noon sunlight. Source C. the one we are most interested in, is diffused daylight (direct sunlight plus blue skylight). Compared with lanmplight. Source C illumination has a bluish cast, but it is actually the near-neutral whiteness of skylight on a heavily overcast day. Unless they are perceptibly purplish, pinkish, or greenish, illuminants may also be rated on the basis of the temperature to which a perfectly absorbing "Planckian-type radiator" must be heated to match them in color. Thus the light of a candle flame may be said to have a "color temperature" of 3,015° Fahrenheit, or 1,657° Centigrade; and because the glowing carbon particles in a candle flame actually have this high temperature, carbon may be said to be a near-perfect Planckian radiator. In practice, color temperatures are specified on the absolute, or Kelvin, thermometer scale, which is the same as the Centigrade plus 273 degrees. In the example above, 3,015° F = 1,657° C = 1,930° K. Zero on the Kelvin scale is particularly significant: it is the temperature at which all heat vanishes — the absolute and unsurpassable cold. (0° K = -273° C = -460° F.) It can readily be understood that reddish and yellowish light sources have lower color temperatures than pure white and bluish white sources. The standard tungsten mazda lamp ( Source A ) , for example, has a color temperature of 2,854° K, while a 1000watt mazda projection lamp has a color temperature of about 3.200° K, Ultra violet -<— VIsib e — Infrared H.I .AC \c f I \ V. 1 XI :nc )N :■ : C 1 \'i> \ *I _C 1 ; :1: ; ?• ij .-'* •+*•**—' >.. ,».. i X /, X ENC )N **. '•4 d^*3 / "c •-"** ^*as *•—•" !.!»» 300 -4O0 500 600 70O 800 900 100O 120O 140O 1600 Wavelength in nanometers (millimicrons) FIG. 4 — Ultraviolet, visible, and infrared energy distribution for the high-intensity carbon arc and the xenon short-arc bulb (dotted line). The energy distribution of xenon light is continuous and nearly level in the visible spectrum. Xenon light is therefore daylight-white in color and very satisfactory for the correct rendition of the colors in color-film prints. 10 and hence emits a somewhat whiter light. Nevertheless, the "white" light of a mazda projection bulb is definitely yellowish in comparison with direct noon sunlight (Source B = 4,870° ) or diffused daylight (Source C = 6,740° ) . The color temperature of clear blue sky is said to be about 25,000°, while an "infinitely hot" body would emit a light just perceptibly bluer than the clear blue sky. (Even this is far from a "saturated" spectrum blue ! ) Co9or Temperatures of Carbon Arcs The color temperature of the low-intensity carbon is 3,900°, which is, in fact, the actual temperature of solid carbon heated to the temperature at which it vaporizes. (3,900° K = 3,600° C = 6,800° F in round figures. ) The LI carbon arc is thus whiter than a tungsten-filament bulb, though slightly yellowish in comparison with the high-intensity carbon arc or with daylight. Except for solid carbons, tungsten-lamp filaments, and other Planckian radiators, color temperature has no connection with the actual temperature of light sources. Thus clear blue sky, which has the enormously high color temperature of 25,000°, is actually freezing cold! But a lump of matter heated to a temperature of 25,000° (as in certain stars) would have exactly the same bluish color. Unlike the low-intensity arc. the high-intensity carbon arc does not obey Planck's rule. It is a non-Planckian radiator. The actual temperature of the crater of a HI positive carbon is the same as that of the crater of a LI positive carbon (3,900°). but the color temperature of a HI arc is quite a bit higher than that of a LI arc. This means that the HI carbon arc gives a less yellowish, or "whiter," light than the LI carbon arc does. The blue-white component of the light of a high-intensity carbon arc comes mainly from cerium atoms in an electrically excited state. As a matter of fact, the pure white light of a properly operated HI carbon arc is similar to direct sunlight (color temperature about 5,000°) both in appearance and in spectral energy distribution — no empty "gaps" or sharp emission "peaks" in the spectrum. This makes the HI carbon arc very satisfactory for color-film projection. Unfortunately, however, two important operating factors familiar to all projectionists militate against the achievement of an invariably constant color temperature for HI carbon-arc screen illumination. Color Temperature of HI Arc Varies One of these carbon-arc operating factors is electrical load. If HI carbons are burned at the minimum recommended currents, the color temperature may be as low as 4,500°. This represents a distinctly yellowish light, and only just a bit "whiter" than LI arc radiation. On the other hand, HI carbons burned at the maximum recommended currents emit a blue-white light of about 6,500° in color temperature. This is substantially the same as diffused daylight (Source C) in color. Readers of IP mav recall that we have always recommended that HI positives be burned at. or very close to, their maximum current ratings in order to obtain this snow-white daylight quality of light on the screen. The picture will also be brighter, the screen illumination more uniformly distributed, and the reproduction of color films more vivid and lifelike. The second carbon-arc factor affecting the color temperature of the light on the screen is arc focus. If the positive crater advances too far toward the mirror, the light will become brownish — a very low color temperature. If the crater recedes too far away from the (Continued on Page 16) International Projectionist June 1963