Achromatic lens

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Chromatic aberration of a single lens causes different wavelengths of light to have differing focal lengths.

An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane.

The most common type of achromat is the achromatic doublet, which is composed of two individual lenses made from glasses with different amounts of dispersion. Usually one element is a concave lens made out of flint glass, which has relatively high dispersion, while the other, convex, element is made of crown glass, which has lower dispersion. The lens elements are mounted next to each other, often cemented together, and shaped so that the chromatic aberration of one is counterbalanced by that of the other.

An achromatic doublet brings red and blue light to the same focus.

In the most common type (illustrated at right), the positive power of the crown lens element is not quite equalled by the negative power of the flint lens element. Together they form a weak positive lens that will bring two different wavelengths of light to a common focus. Negative doublets, in which the negative-power element predominates, are also made.

History

Theoretical considerations of the feasibility of correcting chromatic aberration were debated in the 18th century following Newton's statement that such a correction was impossible (see History of the telescope). Credit for the invention of the first achromatic doublet is often given to an English barrister and amateur optician named Chester Moore Hall.[1][2] Hall wished to keep his work on the achromatic lenses a secret and contracted the manufacture of the crown and flint lenses to two different opticians, Edward Scarlett and James Mann.[3][4][5] They in turn sub-contracted the work to the same person, George Bass. He realized the two components were for the same client and, after fitting the two parts together, noted the achromatic properties. Hall failed to appreciate the importance of his invention, and it remained known to only a few opticians.

In the late 1750s, Bass mentioned Hall's lenses to John Dollond, who understood their potential and was able to reproduce their design.[2] Dollond applied for and was granted a patent on the technology in 1758, which led to bitter fights with other opticians over the right to make and sell achromatic doublets.

Dollond's son Peter invented the apochromat, an improvement on the achromat, in 1763.[2]

Types

Several different types of achromat have been devised. They differ in the shape of the included lenses as well as in the optical properties of their glass (most notably in their optical dispersion or Abbe number).

In the following, 'R' denotes the radius of the spheres that define the optically relevant refracting lens surfaces. By convention, R1 denotes the first lens surface counted from the object. A doublet lens has four surfaces with radii R1 to R4.

Littrow doublet

Uses an equiconvex crown glass lens with R1=R2, and a second flint glass lens with R3=-R2. The back of the flint glass lens is flat. A Littrow doublet can produce a ghost image between R2 and R3 because the lens surfaces of the two lenses have the same radii. It may also produce a ghost image between the flat R4 surface and rear of the telescope tube.

Fraunhofer doublet (Fraunhofer objective)

The first lens has positive refractive power, the second negative. R1 is set greater than R2, and R2 is set close to, but not equal to, R3. R4 is usually greater than R3.

Clark doublet

Uses an equiconvex crown with R1=R2, and a flint with R3≃R2 and R4≫R3. R3 is set slightly shorter than R2 to create a focus mismatch between R2 and R3, thereby reducing ghosting between the crown and flint.

Oil-spaced doublet

The use of oil between the crown and flint eliminates the effect of ghosting, particularly where R2=R3. It can also increase light transmission slightly and reduce the impact of errors in R2F and R3.

Steinheil doublet

The Steinheil doublet, devised by Carl August von Steinheil, is a flint-first doublet. In contrast to the Fraunhofer doublet, it has a negative lens first followed by a positive lens. It needs stronger curvature than the Fraunhofer doublet.[6]

See also

References

  1. ^ Daumas, Maurice, Scientific Instruments of the Seventeenth and Eighteenth Centuries and Their Makers, Portman Books, London 1989 ISBN 978-0713407273
  2. ^ a b c Watson, Fred (2007). Stargazer: the life and times of the telescope. Allen & Unwin. pp. 140–55. ISBN 9781741753837.
  3. ^ Fred Hoyle, Astronomy; A history of man's investigation of the universe, Rathbone Books, 1962, LC 62-14108
  4. ^ "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.
  5. ^ Dokland, Terje; Ng, Mary Mah-Lee (2006). Techniques in microscopy for biomedical applications. p. 23. ISBN 9812564349. Retrieved July 31, 2009.
  6. ^ Kidger, M.J. (2002) Fundamental Optical Design. SPIE Press, Bellingham, WA, pp. 174ff