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achromatic lens(ak-rŏ-mat -ik) (achromat) A two-element lens – a doublet – that greatly reduces chromatic aberration in an optical system. The components, one converging and the other diverging in action, are of different types of glass (i.e. they have different refractive indices); the combination focuses two selected colors, say red and blue, at a common image plane with a small spread in focal length for other colors. The difference in optical power (reciprocal of focal length) for the two colors in one element must cancel that in the other element. By a suitable choice of glasses and surface curvatures, the doublet can be aplanatic as well as achromatic, so that three major aberrations are minimized (see aplanatic system).
Residual color effects in an achromat can be further reduced by using a compound lens of three or more elements – an apochromatic lens; each element has an appropriate shape and dispersive power so that three or more colors can be focused in the same image plane.