achromatism

n. state or quality of being without color

In optics, chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light (the dispersion of the lens). The term "purple fringing" is commonly used in photography, although not all purple fringing can be attributed to chromatic aberration.Longitudinal and lateral chromatic aberration of a lens is seen as "fringes" of color around the image, because each color in the optical spectrum cannot be focused at a single common point on the optical axis.Since the focal length f of a lens is dependent on the refractive index n, different wavelengths of light will be focused on different positions. Chromatic aberration can be both longitudinal, in that different wavelengths are focused at a different distance from the lens; and transverse or lateral, in that different wavelengths are focused at different positions in the focal plane (because the magnification of the lens also varies with wavelength).
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A·chro·ma·tism n. 1. The quality of being achromatic. 2. The correction of chromatic aberration by combining lenses of different refractive indexes [more]Achromatism - Community and Resources

Noun
1. the visual property of being without color
(synonym) colorlessness, colourlessness
(hypernym) visual property
(hyponym) achromia

(n.)
The state or quality of being achromatic; as, the achromatism of a lens; achromaticity.