Photopic vision

Photopic vision is human vision under bright-light conditions, mediated mainly by retinal cone cells. It supports color discrimination and high spatial acuity.

Cone cell: A retinal photoreceptor specialized for vision in brighter conditions and for color discrimination. Cone cells provide most of the photoreceptor input during photopic vision.
Scotopic vision: Vision under very low light, mediated mainly by retinal rod cells. Scotopic vision relies on rods rather than the cones that dominate photopic vision.
Luminous efficacy: The amount of visible luminous flux produced per unit of radiant power. Its standard photopic weighting reflects human brightness sensitivity across wavelengths.
Retina: The light-sensitive neural tissue lining the back of the eye. Photopic vision begins in the retina, where cones transduce light.
Color vision deficiency: A reduced ability to distinguish some colors, often caused by altered or missing cone photopigments. Differences in cone photopigments change photopic color discrimination.
Phototransduction: The conversion of light into electrical signals by photoreceptor cells. Phototransduction turns absorbed light into the cone signals underlying photopic vision.
Mesopic vision: Vision at intermediate light levels, where rods and cones both contribute. Mesopic conditions blend cone-mediated and rod-mediated responses.
Candela: The SI base unit of luminous intensity, defined using monochromatic radiation and photopic sensitivity. The candela’s definition embeds the standardized sensitivity of bright-light vision.
Fovea centralis: A small central retinal region with the highest density of cone photoreceptors. Its cone-rich structure supports the sharpest photopic vision.
Tetrachromacy: Color vision based on four functionally distinct classes of cone photoreceptors. It raises questions about how cone variation might expand color discrimination beyond typical trichromacy.

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