Fraunhofer lens

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Lenses that consist of two individual lenses separated by an air gap are called Fraunhofer lenses after their inventor Joseph von Fraunhofer . As a rule, Fraunhofer lenses are designed in such a way that they eliminate the chromatic aberration for two wavelengths. The Fraunhofer lens is therefore an achromatic lens .

CA Steinheil & Söhne later produced a differently structured achromatic doublet with an air gap.

prehistory

Until the middle of the 18th century, telescopes exhibited disruptive color errors due to dispersion of the glass used, the so-called chromatic aberration . In the middle of the 18th century, the first two-lens objectives were developed that allowed relatively pure color observations ( achromatic doublets ).

The immediate inventor of the achromatic doublet was the Englishman Chester Moor Hall . He combined a converging lens made of crown glass with a diverging lens made of flint glass . The lenses are cemented , i.e. H. the touching 'inner' surfaces of the two lenses have oppositely equal radii. The microscopic gap is filled with Canada balsam . Achromatic doublets were first noticeably distributed by John Dollond (1706–1761). An important further development step was then carried out by Fraunhofer by separating the two lenses and introducing an air gap.

nature

Like the original doublet by Hall and Dollond, the Fraunhofer lens is based on the use of different lens materials that differ in their refractive index . This enables at least two colors ('red' and 'blue') to be united in a common focal point . This means that the ( chromatic aberration or longitudinal chromatic aberration) that arises due to the different dispersion for different wavelengths can be reduced.

The Fraunhofer lens consists of a converging lens made of crown glass (low refractive index) and a diverging lens made of flint glass (high refractive index). In contrast to the original chrome, however, the two lenses are not cemented . Instead, there is a narrow air gap between them. This further development allows an improved correction of the aberrations, since more degrees of freedom are available in the optical construction due to the distance between the lenses and the relative independence of all lens radii.

If we designate the radii of the optically refractive surfaces starting with R 1 to R 4 starting from the object , a Fraunhofer lens is usually constructed as follows:

  • R 1 > R 2
  • R 2 is similar to but not the same as R 3
  • R 4 > R 3

literature

  1. ^ Krosigk, Esther von, Collected writings from Joseph von Fraunhofer, Vdm Verlag Dr. Müller, 2007, ISBN 978-3-8364-2275-8

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