Monochromacy

The rarest and most severe form of color vision deficiency, in which an individual has complete or near-complete absence of color perception, seeing the world only in shades of gray. Monochromacy occurs when two or all three types of cone cells in the retina are absent or nonfunctional, with an incidence rate of approximately 0.003% in the general population. People with monochromacy (also called achromatopsia) typically also experience reduced visual acuity, light sensitivity (photophobia), and nystagmus (involuntary eye movements). In digital accessibility, monochromacy represents the extreme case for color-dependent information — any meaning conveyed solely through color differences is completely inaccessible to monochromatic viewers, reinforcing the WCAG principle that color must not be the only visual means of conveying information.