protan

Protan refers to the red-sensitive photopigment of the human long-wavelength cone (L-cone), and to disorders associated with its function. In normal human vision, three cone pigments enable color perception: S (blue), M (green), and L (red). The protan pigment absorbs best in the long-wavelength part of the spectrum, with peak sensitivity around 560 nm. This pigment is encoded by the OPN1LW gene on the X chromosome; genetic variation, including differences in exon sequence and copy number of L-cone opsin genes, affects spectral sensitivity and can influence color perception.

In color vision deficiency, protan denotes red-vision anomalies arising from impaired or absent L-cone pigment. Protanopia is the complete loss of L-cone function, producing severe red-green color discrimination difficulties and a shift toward dichromatic vision. Protanomaly is a milder condition in which the L-cone pigment is present but its spectral sensitivity is altered, reducing the ability to distinguish reds from greens. Both protan deficiencies are inherited in an X-linked pattern and are more common in males.

In contrast, deutan refers to green-cone (M-cone) deficiencies. People with protan conditions may have difficulty naming red hues and distinguishing red from certain greens, though other colors may appear normal. Diagnostic evaluation uses standardized color vision tests, and genetic testing can confirm L-cone gene variations.