Optics

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Chromatic Aberration (CA)

Longitudinal chromatic aberration

Lateral colour aberration, also known as chromatic aberration, is a normal effect found in all spherical lenses. The refraction of a lens depends on the wavelength (= colour) of the incident light; the focal point of a lens shifts with the wavelength.

Colour fringes

This difference in focus leads to colour fringing in the image, especially with extremely hard contrasts of a light-dark pattern. It results in colour gradients at the edges of the image, as well as a reduction in contrast due to the blurred image.

Chromatic aberration

White light coupled into an uncorrected lens system. There is a lot of chromatic aberration, especially at the edge of the image.

Chromatic longitudinal aberration

Focus difference creates colour fringing

Options for eliminating the error

Particularly when using extreme wide-angle optics and differently coloured illumination, this leads to significantly diminished or enlarged images, as the focal points (and thus the focal length of the lens) shift.

The error can be remedied by stopping down the lens, as this generates an enhanced depth of field and reduces the blurred zones.
By way of design, this error can be corrected by grouping two lens elements, so-called dichromates. The longitudinal aberration effects of a highly refractive and a lowly refractive diverging lens neutralise each other. As an alternative, very expensive aspheric lenses can be used, too.

The lateral aberration has the same cause as the longitudinal aberration and occurs with rays of light close to the margin. Differently coloured parts of the image appear in different size. This error cannot be corrected by stopping down.

Correction of chromatic longitudinal error

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Color corrected lenses

Especially in colour applications, it is important that there are no colour errors in the image. Colour corrected lenses are designed to eliminate chromatic aberration.

Achromats

If the lens is corrected for the two colours blue and red, it is called an achromatic lens.

Apochromats

If the lens is optimised for blue, green and red light to avoid chromatic aberration, it is called an apochromatic lens

Important for Machine Vision

In practice, you will soon notice whether your lens is corrected in this way or not. Illuminate with red light and focus your image. Now change the light and use blue light. If the image has become blurred and you have had to refocus, your lens has not been specially corrected. The shift in focus is even more extreme when using IR illumination. Special IR corrected lenses are available for these applications.

The longitudinal chromatic aberration can be compensated for somewhat by stopping down the lens, but unfortunately not the lateral chromatic aberration. Please stop down a bit!
This error is also visible in the image with a monochrome camera system, but not as a colour band, but as a grey band. If the use of white light is not absolutely necessary, coloured monochromatic LED light can be used in this case. A chromatic aberration can then no longer occur.
Buy a proper lens that has been designed to correct as many errors as possible.

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