When light passes through an optical system (lenses made of glass), so-called “color errors” can occur. In this phenomenon, for example, white light is refracted into its components, which also explains the play of colors on glass edges when exposed to sunlight. A similar phenomenon is known from water surfaces, and rainbows are also basically based on this principle.
An apochromat (Greek for free of color, colorless) is an optical system, e.g. a lens, in which these color errors are largely corrected.
Since such color errors should be avoided when digitizing, our systems work with a chromatic-corrected lens that compensates for this phenomenon. Otherwise, colored fragments would show around the edges of black letters, for example, and the scan would not be identical with the original.