Everything about Achromatic Lens totally explained
An
achromatic lens or
achromat is a
lens that's 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, typically cemented together, and shaped so that the chromatic aberration of one is counterbalanced by the chromatic aberration of the other.
In the most common type (illustrated above), the positive
power of the crown lens element isn't 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.
The exact date of the first achromatic doublet's creation isn't known, nor is the name of the person who first accomplished the task. Theoretical considerations of the feasibility of the system were debated in the 18
th century following
Newton's statement that such a correction was impossible (see
History of the telescope). Credit for the first invention, around 1733, of the achromatic refracting lens is often given to an English
barrister named
Chester Moore Hall. Some of the concepts were demonstrated with lenses made of glass and water, but the first useful lenses were not known to have been made until the early 18
th century by
George Bass under the direction of Hall. The first patent for an achromatic doublet was awarded to
John Dollond around 1758 following his independent theoretical and experimental work.
The triple achromat, which reduced secondary colour defects, was invented in 1763 by Dollond's son
Peter.
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