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aberration, in optics, condition that causes a blurring and loss of clearness in the images produced by lenses or mirrors. Of the many types of aberration, the two most significant to the lens maker are spherical and chromatic. Spherical aberration is caused by the failure of a lens or mirror of spherical section to bring parallel rays of light to a single focus. The effect results from the operation of the laws of optics, not from defects in construction. Spherical aberration can be prevented by using a parabolic rather than a spherical section, but this involves much greater complexity and expense in lens or mirror construction. Chromatic aberration results in the blurred coloring of the edge of an image when white light is sent through a lens. This is caused by the fact that some colors of light are bent, or refracted, more than others after passing through a lens. For example, violet light is bent more than red and thus is brought to a focus nearer the lens than red. No single lens can ever be free of chromatic aberration, but by combining lenses of different types, the effects of the component lenses can be made to cancel one another. Such an arrangement is called an achromatic lens. See reflection; refraction.
The type of error in an optical system in which the formation of a series of colored images occurs, even though only white light enters the system. Chromatic aberrations are caused by the fact that the refraction law determining the path of light through an optical system contains the refractive index, which is a function of wavelength. Thus the image position and the magnification of an optical system are not necessarily the same for all wavelengths, nor are the aberrations the same for all wavelengths. See Aberration (optics), Refraction of waves
chromatic aberration(krŏ-mat -ik) An aberration of a lens – but not a mirror – whereby the component wavelengths of light, i.e. ordinary white light, are brought to a focus at different distances from the lens (see illustration). It arises from the variation with wavelength of the refractive index of the lens material: red light is refracted (bent) less than blue light (see dispersion). False colors therefore arise in the image. Chromatic aberration can be reduced by using an achromatic lens. Before the introduction of achromats, objective lenses of very long focal length were used in telescopes to reduce the aberration; this led to the production of very cumbersome instruments.
a major aberration of optical systems, caused by the dependence of the refractive index of a transparent medium on the wavelength of light (seeDISPERSION OF LIGHT). Chromatic aberration may occur only in systems that incorporate components made of refracting materials, for example, lenses. Chromatic aberration is not characteristic of mirrors; in other words, mirrors are achromatic.
Two mutually independent types of chromatic aberration are distinguished: longitudinal aberration and lateral aberration. Longitudinal aberration is an image error whereby the images of a point that are formed by rays with different wavelengths lie at different distances from the optical system. In other words, the locations of the focal points on the optical axis do not coincide for different colors (Figure 1, the line segment O1O2). As a result of longitudinal aberration, a series of colored circles—rather than a single bright point—is observed perpendicular to the optical axis on a screen placed where the image is formed. Lateral aberration is an image error whereby the lateral magnifications of the images of an object that are formed by rays with different wavelengths may turn out to be different. The differences in lateral magnification result from the difference in the locations of the principal planes of the optical system (seeCARDINAL POINTS OF AN OPTICAL SYSTEM) for different wavelengths, even if the foci for the wavelengths coincide (however, in this case, the focal lengths will differ). As a result of lateral aberration, objects of finite size yield images with a colored fringe.
The larger the number of different wavelengths for which the focal points are made to coincide, the more difficult it is to correct the longitudinal aberration in an optical system. The simplest case is that in which the focal points are made to coincide for only two wavelengths (and the distance between focal points is reduced for other wavelengths). Optical systems, usually objectives, that are corrected for two wavelengths are said to be achromatic. More advanced optical systems in which the foci are made to coincide for three wavelengths are referred to as apochromatic. In an apochromatic system, the longitudinal aberration is corrected by increasing the number of components with different refractive indexes and by incorporating mirrors into the system. Apochromatic systems are widely used as, for example, photographic and astronomical objectives.
A more thorough correction of longitudinal aberration requires a more complicated system design. In this case, the number of lenses and mirrors is increased, the shapes of the lenses and mirrors are modified, or both. The larger the relative aperture and the wider the field of view of an optical system, the more complicated the corrected system. To correct lateral aberration, the principle planes also must be made to coincide for the largest possible number of wavelengths. Such correction entails major difficulties.
REFERENCESLandsberg, G. S. Optika, 5th ed. Moscow, 1976. (Obshchii kurs fiziki.)
Herzberger, M. Sovremennaia geometricheskaia optika. Moscow, 1962. (Translated from English.)
Born, M., and E. Wolf. Osnovy optiki. Moscow, 1973. (Translated from English.)