Luminous Efficiency Function

A function that describes how the human visual system converts light intensity at different wavelengths into perceived brightness. The standard LEF used in WCAG contrast ratio calculations is based on a "standard observer" with typical colour vision. However, people with CVD — particularly protanopes and achromats — have abnormal LEFs, meaning reds appear dimmer to them than the standard function predicts. This discrepancy means WCAG-compliant contrast ratios may overestimate the contrast experienced by CVD users, a gap that current accessibility standards do not address.