Chromatic aberration

Chromatic aberration (also called chromatic distortion and spherochromatism) is a defect in a lens in which the different colors of the spectrum are not brought to the same focal point. It is caused by dispersion because the refractive index of the lens elements varies with the wavelength of light. In particular, the refractive index of most transparent materials decreases as the wavelength increases.

The result is fringing – the formation of a colored halo along the boundaries that separate dark and bright parts of the image. This problem seriously affected the performance of refracting telescopes for centuries, and was the reason why so many refractors were built with large focal ratios: lenses with longer focal lengths have less chromatic aberration.

A better solution came with the introduction of corrective elements using at least two different types of glass in a compound lens. An achromatic lens corrects red and blue light, while an apochromatic lens corrects at least red, blue, and green. Reflecting telescopes are free of this type of aberration.

Axial chromatic aberration

Axial chromatic aberration is due to the different refractive index of the lenses with respect to the different monochromatic radiation passing through them. Since the focal length of the lenses is a function of the wavelength of the light rays refracted by them, the lens will form as many different images of the same light source as there are monochromatic components from which it is made.

When the source is placed on the axis of the optical system, the different images are formed on that axis with a certain longitudinal (or axial) deviation from a reference image. Axial chromatic aberration can be measured by the distance between two given images (for example, between the image of radiation of the maximum observed wavelength and that of the minimum).

Chromatic aberration correction is achieved with achromatic doublets. Optical systems for which this aberration has been corrected are said to be achromatic.

Extra-axial Chromatic Aberration

This type of aberration affects systems in which the linear transverse magnification varies with the wavelength of the light used.

It is due, like axial chromatic aberration, to the different dispersion to which the different colors are subjected; by placing a polychromatic point source outside the axis of an optical system affected by this aberration, as many images of it are observed as there are monochromatic components of the source used.

Unlike axial chromatic aberration, these images are transverse to the optical axis. For extended polychromatic sources, the overall effect is an iridescent edge around the image.

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