Achromatic Lenses Usually Have Two, Sometimes More Elements
Usually Made From Flint and Crown Glass
adjusted for chromatic aberration regarding two selected wavelengths. The elements are designed to control the effects of both chromatic and
spherical aberration. Normally achromatic lenses are adjusted to allow two differing wavelengths of the color spectrum (characteristically red and blue) to come into focus in an identical plane.
The achromatic doublet being he most common type, which is made up of two independent lenses created from glass having different dispersion amounts. One element is usually a concave lens created from flint glass having a somewhat high dispersion, whereas the other, a convex, element is created from crown glass, having lower dispersion. These lens elements are then mounted side by side, typically bonded together, and formed in a way allowing the chromatic aberration from one of the elements to be compensated by the element of the other.
The most frequent type (shown in the above illustration), the positive strength of the crown element is not exactly equal to the negative strength of the flint element. But jointly they fashion a faint positive lens while bringing two dissimilar wavelengths of light into mutual focus. Sometimes negative doublets, where the negative-power element is dominate, are made as well.
The exact first achromatic double creation date is not precisely known, nor the persons name first accomplishing that invention. Theoretical feasibility of this system were deliberated following Newton's statement during the 18th century that such correction was not possible. An English barrister by the name of Chester Moore Hall. is often given credit for the creation of an achromatic refracting lens, sometime in 1733 There were some concepts given a demonstration with lenses made from glass and water, although there in no knowledge of the first useable lenses being created by George Bass beneath the guidance of Chester Hall. until early during the 18th century John Dollond was awarded the very first patent allowing an achromatic doublet about 1758 consequential to his independent experimental and theoretical work.
A triple achromat element exists, that allows reduced secondary color imperfection, was
patented by Dollond's son Peter in 1763. 1/12/2011