Achromat

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Achromatic two-lens lens: two colors (here red and blue) have the same focal length, but with green light it is smaller

A lens is used as achromatic (from ancient Greek ἀχρώματος achrōmatos "no color") or as an achromat designated when the change of the focal distance disappears with wavelength for a wavelength.

Because of the dispersion of the optical materials, each individual lens shows what is known as chromatic aberration . The refractive index increases continuously from red to blue and therefore the focal length of the lens decreases. By combining a positive and a negative lens made of glasses with different steepness of the refractive index curve, see Abbe number , a reversal in the course of the focal length with the wavelength can be achieved, and the longitudinal chromatic aberration is corrected. If the two lenses are thin and have a small distance from one another, the lateral chromatic aberration, which depends on the image height and causes disturbing color fringes at the edges of the observed objects, in particular at the edge of the image field , can be largely corrected.

Invention in the middle of the 18th century

The first achromatic doublet lenses were designed by the English amateur optician Chester Moor Hall around 1733 . Hall wanted to keep his work secret and hired two different companies to manufacture the crown glass and flint glass lenses ( Edward Scarlett and James Mann). However, they both subcontracted actual manufacture to the same optician, George Bass . He realized that both lenses were for the same customer. After joining the two lenses, he recognized their achromatic properties. However, Hall himself did not recognize the width of his invention and remained largely unknown.

In the late 1750s, Bass mentioned Hall's invention to the French-born Englishman John Dollond , who recognized its potential and was able to reproduce its manufacturing principle. Dollond filed a patent which was granted to him in 1758. Around 1760 he built the first good telescopes equipped with achromatic lenses . The lenses were transparently connected ( cemented ) to one another at their contact surface , which means that disturbing reflections remain minimal. In the past, Canada balsam was used for this . The apertures (lens diameter) were initially limited to 2 to 3 inches , but represented a huge step forward compared to single-lens lenses, which had to be stopped down to get a sharp image.

With the help of the assembled achromatic microscopes available to him from 1832 ( Plössl Company , Vienna), Jan Evangelista Purkyně did his pioneering work in the field of anatomical tissue theory .

By 1875, openings of 25 inches were achieved (e.g. the large refractor of the Vienna University Observatory ) , and 40 inches by the turn of the century ( Yerkes Observatory ). But with such one-meter lenses, the deflection of the lenses reaches its limits, so that one went back to smaller lenses or to mirror telescopes .

Further developments

The achromatic corrects the primary spectrum , which is the term used to describe the uncorrected color error between the two design wavelengths. A measure of the remaining color error is called the secondary spectrum .

Various variants were derived from the original design of the Hall / Dollondachromat. For example, Joseph von Fraunhofer did not join the two lenses directly, but instead left a narrow air gap between them (“Fraunhoferdoublet” or “Fraunhofer achromat”). The air gap can be changed in width to a certain extent and allows v. a. to give the opposing lens surfaces a different radius. An achromat, in which both lenses are separated from each other by a small distance in the tube, is also called a dialyte . The degrees of freedom obtained can be used to correct the dependence of the spherical aberration on the wavelength .

For smaller telescopes with the "slow" aperture ratios of approx. F / 15 that were common in the past , there are hardly any disturbing color fringes during observation. In the case of large achromatic refractors, which in principle would have to achieve significantly higher resolutions with low image blurring due to air turbulence , the secondary spectrum again emerges in a highly disturbing manner at higher magnifications. The reason is that the absolute residual color error in arc seconds for a certain aperture ratio (depending on the glass data) is constant and, in contrast to the diffraction-limited resolution, does not decrease with increasing lens diameter.

Another variant of the achromatic doublet was marketed by CA Steinheil & Söhne as a "stone healing lens ". The arrangement of the lenses is slightly different from the standard achromatic lens. The first element on the field side is a negative flint glass lens , which is followed by a positive crown glass lens . As with the Fraunhofer achromat, a narrow air gap separates the two lenses. The inner radii of the lenses are more curved than with the Fraunhofer lens. To avoid zone errors, an area is retouched by hand. This means that the position of the lenses in relation to one another must not be changed.

In photography , achromatic lenses have been used as lenses from the very beginning (official date of invention: 1839). The optically better Petzval lens was developed for portrait photography around 1850 . From around 1870, two slightly meniscus-shaped achromats were combined to form the aplanat . Aplanates were also used for projectors in the past and are still used today for high-quality magnifying glasses . The next important invention followed, the even sharper, 3-lens Cooke triplet . Today's camera lenses often contain 5 to 10 lenses.

So-called apochromats , which typically consist of three lenses , have been developed for more extensive requirements, especially in microscopy .

Web links

Wiktionary: Achromat  - explanations of meanings, word origins, synonyms, translations

Single receipts

  1. ^ Wilhelm Pape , Max Sengebusch (arrangement): Concise dictionary of the Greek language. 3rd edition, 6th impression, Vieweg & Sohn, Braunschweig 1914. 1914, accessed on November 30, 2016 .
  2. ^ Maurice Daumas: Scientific Instruments of the Seventeenth and Eighteenth Centuries and Their Makers. Portman Books, London 1989, ISBN 0-7134-0727-1 .
  3. ↑ a b Fred Watson: Stargazer. The life and times of the telescope . Allen & Unwin, 2007, ISBN 978-1-74175-383-7 , pp. 140-55.
  4. ↑ Eugene Hecht: Optics. Oldenbourg Wissenschaftsverlag, Munich 2009, 5th edition, Chapter 6.3.2, p. 438 ff.
  5. ↑ Fred Hoyle : Astronomy. A history of man's investigation of the universe. Rathbone Books, London 1962, LC 62-14108.
  6. ↑ Sphaera — Peter Dollond answers Jesse Ramsden . Retrieved July 31, 2009. A review of the events of the invention of the achromatic doublet with emphasis on the roles of Hall, Bass, John Dollond and others.
  7. ↑ Terje Dokland, Mary Mah-Lee Ng: Techniques in microscopy for biomedical applications 2006, ISBN 981-256-434-9 , p. 23 (accessed February 23, 2012).
  8. ^ Walter cherry : Jan Evangelista Purkyně 1787–1868. A contribution to the 200th anniversary of his birthday. Akademie-Verlag, Berlin 1989 (= session reports of the Academy of Sciences of the GDR. Year 1988, No. 5 / N), ISBN 3-055-00520-1 , pp. 20-23.
  9. ↑ JE Purkinje: About a large Plössel microscope built for the local university. In: Overview d. Work and changes in the schles. Society f. Patriotic culture in 1832. Breslau 1833, pp. 39–42.
  10. ^ Karl Schwarzschild : Investigations into the geometric optics. III. About the astrophotographic objectives. = Investigations in Geometrical Optics. III. On Astrophotographic Objectives. In: Karl Schwarzschild: Collected works. = Collected Works. Volume 3. Edited by HH Voigt . Springer, Berlin et al. 1992, ISBN 0-387-52457-6 , pp. 156–207, here p. 169.
  11. ^ Rudolf Kingslake : Lens design fundamentals. Academic Press, New York NY et al., ISBN 0-12-408650-0 , p. 87.
  12. ^ Association of the Kuffner Observatory: The Great Refractor of the Kuffner Observatory , accessed on June 5, 2013.