English: Variantor lenses are used to simulate red-green color blindness (protanopia) in color normal (trichromatic) subjects. Their transmittance is plotted on top of the normalized spectral sensitivities of the standard cone opsins. The lens' filter allows wavelengths of light to pass that either do not significantly excite the L- and M-opsins, (short wavelength pass band <490nm) or that excite them equally (long wavelength pass band ~ 560nm). The latter appears to be misaligned, but this is an artefact of the normalization. In most observers, the green cones are more sensitive than red cones, so 570nm is closer to the point of equal excitation, rather than the intersection of the normalized curves (at ~555nm). Transmittance taken from Fig 1 of [Alvaro et al 2022].