<microscopy> Literally, colour-free. A lens or prism is said to be achromatic when corrected for two colours. The remaining colour seen in an image formed by such a lens is said to be secondary chromatic aberration.
A truly achromatic lens would transmit light without decomposing it spectrally and there would be no colour fringing in the image. A doublet, composed of a positive and negative lens element, can be made achromatic for two colours which much improves the performance for most of the others.
(05 Aug 1998)
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