Definition: Lens type in which light of two wavelengths are brought to common focus. * An achromat combines a positive lens with a negative elements whose dispersions are in opposite directions; these tend to cancel out chromatic aberrations with a peak correction for two wavelengths so light of these wavelengths are focused at the same point on the lens axis. * Other wavelengths are left uncorrected as secondary spectrum.

Related Terms: chromatic aberration

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