Chromatic Aberration

Chromatic AberrationWhen a non-chromatic light beam of a point source passes through a lens it splits into several rays. Each of them has a certain wave length. Thus all rays when crossing the optical system will propagate in different ways. That means they do not intersect in one point, i.e. focus. Some rays deflect in a certain way, depending on the lens type. As a result the image seems to be blurred. Such deflections are called aberrations. Chromatic aberration is considered to be one of the most significant types of aberration. It is conditioned by dispersion of the lens material. Lens focal length is determined by its refractive index. So it is dependent on the wave of incident light. This results in every chromatic constituent of the white colour having its own focus in various points on the major axis. There are two types of chromatic aberration. In longitudinal aberration focuses from red to violet are distributed along the major axis. In lateral aberration magnification changes depending on the wave length, and coloured contours appear on the image. In order to get a good-quality image it is necessary for both monochromatic and chromatic aberrations to be small. There are various methods of reducing chromatic aberration. One of them is to use two adjoining thin lenses of different glass with dispersion of different types. The simplest example here is a photographic lens. It consists of two lenses - a crown glass collecting lens and a flint glass diverging lens. Flint dispersion is much greater. Thus dispersion of a collecting lens is compensated by dispersion of a weaker diverging lens. As a result we get a collecting system called achromat. Of course, achromatization for two wave lengths does not eliminate the chroma error. The remaining chromatic aberration is called secondary spectrum.

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Science & Technology

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• lens
• optics
• physics