Describing an optical system corrected for chromatic aberration, producing clear images without color distortion.
From apochromat + -ic (adjectival suffix). Modern optical terminology.
The jump from achromatic (correcting two wavelengths) to apochromatic (correcting three or more) in lens design was a watershed moment—it's why modern scientific images are so crisp and true-to-color, a victory of physics over physics.
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