Doublets and Triplets
An achromatic doublet is an optical system S of two lenses L 1 and L 2 with different refractive indices and Abbe numbers (see (3.106)) that are separated by a very thin layer of air. In the Fraunhofer doublet, the first lens is positive and the second lens is negative, whereas the opposite is true in the Steinheil doublet. Moreover, the positive lens has a lower refractive index than the negative lens, but its Abbe number is much greater than that of the negative lens. The Steinheil design is used when it offers a better correction than the Fraunhofer or when the positive element exhibits poor resistance to weathering.
KeywordsFocal Length Blue Light Green Light Spherical Aberration Lower Refractive Index
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