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Protanopia is a rare form of color vision deficiency caused by the complete absence of red retinal photoreceptors that occurs in 1% of the human males.
Lacking the long-wavelength sensitive retinal cones, those with this condition are unable to distinguish between colors in the green-yellow-red section of the spectrum.
They have a neutral point at a wavelength of 492 nm—that is, they cannot discriminate light of this wavelength from white.
For the protanope, the brightness of red, orange, and yellow is much reduced compared to normal. This 'dimming' can be so pronounced that reds may be confused with black or dark gray, and red traffic lights may appear to be extinguished. They may learn to distinguish reds from yellows and from greens primarily on the basis of their apparent brightness or lightness, not on any perceptible hue difference. Violet, lavender, and purple are indistinguishable from various shades of blue because their reddish components are so dimmed as to be invisible. E.g. Pink flowers, reflecting both red light and blue light, may appear just blue to the protanope. Very few people have been found who have one normal eye and one protanopic eye. These unilateral dichromats report that with only their protanopic eye open, they see wavelengths below the neutral point as blue and those above it as yellow.