Protanopia & Protanomaly: The Physics of L-Cone Spectral Shift & Optical Notch Filters

Trichromatic human color vision is mediated by three cone photoreceptors: Short (S, peak ~420 nm), Medium (M, peak ~530 nm), and Long (L, peak ~560 nm). The L-cone photopigment is encoded by the OPN1LW gene on the X chromosome (Xq28).

In protanomaly, gene conversion or hybrid exon rearrangement replaces critical amino acids (such as alanine for serine at position 180), shifting the L-cone peak absorption sensitivity 15 to 25 nanometers shorter—directly toward the M-cone. As the L-cone and M-cone absorption curves overlap almost completely:

$$\Delta \lambda_{\text{peak}} = \lambda_{L} - \lambda_{M} < 10\ \text{nm}$$

The retinal neural circuitry (bipolar and parvocellular ganglion cells) can no longer compute the differential signal ($L - M$) required for red-green opponent processing. In severe protanopia, functional L-cones are entirely missing, extinguishing the entire red end of the spectrum and causing brake lights to appear dim brown or black.

Spectacle lenses engineered for color deficiency (such as multi-cavity dielectric notch filters) cannot magically create new cones or cure genetic mutations. Instead, they exploit spectral notch filtration:

By depositing alternating quarter-wavelength thin-film dielectric stacks (such as titanium dioxide $\text{TiO}_2$ and silicon dioxide $\text{SiO}_2$), the lens creates a sharp, narrow transmission notch centered precisely at 580 to 590 nm—the zone of maximal spectral overlap between L and M cones. Light at 585 nm (which stimulates both cones equally and produces muddy ambiguous yellow) is selectively rejected.

This notch cuts the confounding overlap, allowing pure 535 nm (green) and pure 620 nm (red) photons to stimulate their respective remaining photoreceptors independently, thereby increasing perceived saturation and expanding chromatic contrast!

When dispensing chromatic notch filter lenses:

• Anomalous Trichromats vs. True Dichromats: Notch lenses provide substantial contrast enhancement for protanomalous patients who still retain functional (though shifted) L-cones. For true protanopes with zero L-cones, the brain lacks the neural wiring to perceive true red, though the lenses improve object edge discrimination.
• Photopic Luminance Reduction: Because notch filters absorb 25% to 40% of ambient light, they perform optimally under bright daylight or high-CRI photopic conditions (>1000 lux) and are strictly contraindicated for nighttime driving.
• Broadband Anti-Reflective Backside Coatings: Back-surface reflections from behind the wearer introduce unfiltered stray photons that dilute the notch contrast; vacuum AR multi-coatings (such as ELLASUV Magenta HMC) are essential.