Purkinje Effect

The Purkinje effect, also called the Purkinje phenomenon or Purkinje shift, is the tendency for the eye's peak sensitivity to light to shift toward the blue end of the visible spectrum as illumination drops, part of the eye's broader process of dark adaptation; as a practical consequence, red hues look relatively darker compared to blues and greens as light levels fall. The shift happens because vision passes through mesopic, or twilight, lighting conditions in which rod photoreceptors take over sensitivity from cone cells: rods peak in sensitivity near 507 nanometers, a blue-green wavelength, while cones peak near 555 nanometers, a green wavelength, and the pooled signal from both receptor types produces the perceived shift toward blue. The Czech anatomist Jan Evangelista Purkyne discovered the effect in 1819 during walks at dawn through Bohemian fields, noticing that geranium flowers appeared vividly red in full sunlight but very dark at daybreak, and the effect has since been documented across many animal species beyond humans. Because rods are largely insensitive to long-wavelength red light, red lighting is used in settings such as submarines, aircraft cockpits and research laboratories to let people read instruments while preserving their night vision.