Achromatism

ACHROMATISM (from Greek a- negative particle, and chroma-color), the property of an optical system, for example, an objective, consisting in that it does not produce chromatic aberration (see). For achromatism, it is necessary that upon exiting such a system, rays are only refracted, but there is no dispersion (i.e., rays of all colors that make up white light would emanate in the same direction). This can be achieved by composing a lens from two different parts: a biconvex crown glass lens (see Figure 1) and a convex-concave flint glass lens. Since the latter type of glass produces very high dispersion, a weakly refracting flint glass lens can eliminate the dispersion of the strongly refracting crown glass lens. Rays passing through the system shown in Figure 1 converge somewhat further than the point at which they would converge if the flint glass lens were absent. At the same time, the entire lens system becomes achromatic. An achromat is an objective that provides an uncolored image, consisting of a biconvex and a concavo-convex lens and not having chromatic aberration. Achromats consist of one or more systems of glasses, which in turn are made of differently ground lenses (of different types of glass) with different light-scattering properties. One of the lenses is usually made of light flint glass, and the other of heavy crown glass; in strong achromatic systems in recent times, one combination containing a lens of fluorite (fluorspar) has also been introduced. By combining different types of glass, it is possible to bring two different rays to a single point on the optical axis. Spherical aberration is eliminated for one ray that appears brightest to the eye. Achromats have a flat field of view and are therefore especially suitable for surveys and photomicrography. Long-focus achromats are also used as photographic lenses.

Figure 1.

Long-focus achromats are also used as photographic lenses.