aberration

aberration

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.

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Aberration (optics)

A departure of an optical image-forming system from ideal behavior. Ideally, such a system will produce a unique image point corresponding to each object point. In addition, every straight line in the object space will have as its corresponding image a unique straight line. A similar one-to-one correspondence will exist between planes in the two spaces. This type of mapping of object space into image space is called a collinear transformation. When the conditions for a collinear transformation are not met, the departures from that ideal behavior are termed aberrations. They are classified into two general types, monochromatic aberrations and chromatic aberrations. The monochromatic aberrations apply to a single color, or wavelength, of light. The chromatic aberrations are simply the chromatic variation, or variation with wavelength, of the monochromatic aberrations. See Chromatic aberration, Geometrical optics, Optical image

The monochromatic aberrations can be described in several ways. Wave aberrations are departures of the geometrical wavefront from a reference sphere with its vertex at the center of the exit pupil and its center of curvature located at the ideal image point. The wave aberration is measured along the ray and is a function of the field height and the pupil coordinates of the reference sphere (see illustration).

Diagram of the image space of an optical system, showing aberration measures: the wave aberration and the transverse ray aberration

Transverse ray aberrations are measured by the transverse displacement from the ideal image point to the ray intersection with the ideal image plane. The chief monochromatic aberrations are spherical (aperture) aberrations, coma, astigmatism, curvature of field, and distortion.

Each surface in an optical system introduces aberrations as the light beam passes through the system. The aberrations of the entire system consist of the sum of the surface contributions, some of which may be positive and others negative. The challenge of optical design is to balance these contributions so that the total aberrations of the system are tolerably small. In a well-corrected system the individual surface contributions are many times larger than the tolerance value, so that the balance is rather delicate, and the optical system must be made with a high degree of precision. See Lens (optics), Optical surfaces

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.

aberration

1. (aberration of starlight) The apparent displacement in the position of a star because of the finite speed of light and to the motion of the observer, which results primarily from the Earth's orbital motion around the Sun. It was discovered in 1729 by the English astronomer James Bradley. Light appears to approach the observer from a point that is displaced slightly in the direction of the Earth's motion. The angular displacement, α, is given by the relation tan α = v/ c , where v is the Earth's orbital velocity and c is the speed of light. Using the Earth's mean orbital speed gives the constant of aberration, equal to 20.4955 arc seconds. Over the course of a year, the star appears to move in a small ellipse around its mean position; the ellipse becomes a circle for a star at the pole of the ecliptic and a straight line for one on the ecliptic. The maximum displacement, i.e. the semimajor axis of the ellipse, is 20.5 arc seconds. The aberration due to the Earth's orbital motion is sometimes termed annual aberration to distinguish it from the very much smaller diurnal aberration that results from the Earth's rotation on its axis. Compare annual parallax.

2. A defect in the image formed by a lens or curved mirror, seen as a blurring and possible false coloration in the image. Aberrations occur for all light rays lying off the optical axis and also for those falling at oblique angles on the lens or mirror surface. The four principal aberrations are chromatic aberration (lenses only), spherical aberration, coma, and astigmatism. Curvature of field and distortion are other aberrations. Chromatic aberration occurs when more than one wavelength is present in the incident light beam. For light of a single wavelength, only the latter five aberrations occur. These image defects may be reduced – but not completely eliminated – in an optical system by a suitable choice of optical materials, surface shape, and relative positions of optical elements and stops. See also achromatic lens; correcting plate.

3. A defect in the image produced by an electronic system using magnetic or electronic lenses.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

aberration

[‚ab·ə′rā·shən]

(astronomy)

The apparent angular displacement of the position of a celestial body in the direction of motion of the observer, caused by the combination of the velocity of the observer and the velocity of light.

(optics)

optical aberration

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

aberration

Optical diagram of a recorder camera with gyro gun sight (GGS). Aberration occurs if mirror or reflector is not in correct position resulting in out-of-focus image.

i. A condition in an optical system in which the images are imperfect or improperly located.
ii. Geometrical inaccuracy(ies) introduced by optical, IR (infrared), or similar electromagnetic systems in which radiation is processed by mirrors. In optics, a specific deviation from perfect imagery (e.g., spherical aberration, coma, astigmatism, curvature of field, or distortion).
iii. The displacement of the apparent directions of the stars resulting from the motion of the observer. Also called an atmospheric aberration.

An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved

aberration

1. Optics a defect in a lens or mirror that causes the formation of either a distorted image (see spherical aberration) or one with coloured fringes (see chromatic aberration)

2. Astronomy the apparent displacement of a celestial body due to the finite speed of light and the motion of the observer with the earth

Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005