International projectionist (Jan-Dec 1947)

What COLOR-CORRECTION Means COLOR-CORRECTION in lenses is not a new wrinkle introduced as a consequence of the growing popularity of color film. Lens designers and opticians have heen laboring over the color-corrections since the first lenses were assembled into optical systems. The term "color-corrected" is not so profound that, like "abracadabra" or "open sesame," it should become a conjurer's word to call up the perfect lens. "Colorcorrected" to the optical designer and optician has a very definite meaning, much as it may have been corrupted to include application to almost any type of lens. A color-corrected lens, in the language of the designer, is one satisfying two rather stringent specifications, and no lens failing to fulfill these two requirements can fairly be called "fully color-corrected." Historically, unwanted color in images was recognized very early, and constituted one of the first challenges to the lens designer's skill. The researches aimed at discovering the whys and wherefores of this annoying color led to a better understanding of lenses in general, and to new glasses, and even today are not completed. The state of the color-corrections in a lens system is fundamental in the performance of the system and constitutes one of the first considerations in design. The subject is rather complicated, as there are in reality two distinct color aberrations the designer must eliminate before he is entitled to say that his system is color-corrected. The Behavior of Light But before we undertake to amplify this statement, we must refresh our memories with a few facts of how light behaves. Light travels in vacuum at the astounding rate of 186,000 miles per second: that is, all light is conjectured to do so — blue, yellow, red, infra-red, etc. — it all skips merrily along at this dizzy rate in empty space. But something happens to this light when it reaches a region filled with a more tangible substance. What happens is exactly the same thing that occurs when a train hits a snow bank, or a football player enters a broken field — a reduction of speed. At the boundary of the optically denser medium, the light beam is bent, or refracted, if the angle at which the beam hits the denser medium is other than 90°. Specifically, as Fig. 1 shows, on entering a medium in which the speed is less By ALLEN E. MURRAY Scientific. Bureau Bausch & Lomb Optical Company the beam of light is bent toward the normal, an imaginary line perpendicular to the surface. "The reverse is true on leaving. The crux of the color effects is that in spite of the fact that all colors are transmitted through empty space with the same velocity, they insist on being FIGURE 1 treated differently when traveling through ponderable matter. In glass, for instance, red light will travel about 3,000 miles per second faster than blue light. This speed differential has as its consequence the greater bending of blue light over red light, causing the dispersion shown in Fig. 2. Achromatic Lens Combinations In the design and manufacture of photographic objectives, several different types of glass are used whose basic action is illustrated in Fig. 2. One type is of low index of refraction, i.e., it retards light little in passage. This glass, in general, will retard the blue only slightly more than the red. At the other FIGURE 3 extreme are the glasses of high index, in which the velocity of light is lower, and this in turn means a greater angle ot deviation whereby the blue is affected much more than the red, so that the angular dispersion is greater. Sir Isaac Newton, who founded much of optics as we now know it, from his extensive experience with the glass prisms of his day, concluded, erroneously, that dispersion is always proportional to the deviation and that, as a FIGURE 2 ANGLE OF DEVIATION RED GREEN BLUE ANGLE OF DISPERSION White light entering material of low refractive index. ANGLE OF DEVIATION ) — ANGLE OF DISPERSION White light entering material of high refractive index, at the same angle as above. consequence, achromatic combinations are impossible. Sir Isaac committed one of his rare mistakes in concluding that achromats are impossible and that the reflecting telescope is the best answer to the color problem. Not long after Newton's death, the first achromats were made in England by combining a BLUE CENTER RED FRINGE RED CENTER BLUE FRINGE RED 10 INTERNATIONAL PROJECTIONIST • January 1947