The human visual system is a remarkable biological sensor that adapts to extreme lighting conditions. This adaptation is governed by two distinct systems: scotopic vision (rods) for low-light levels and photopic vision (cones) for bright-light conditions. The shift between these two systems is known as the Purkinje shift (or Purkinje effect), named after the 19th-century Czech anatomist Jan Evangelista Purkyně, who noticed that in dim twilight, red flowers appeared black while blue flowers seemed brighter. This phenomenon is rooted in the different spectral sensitivities of rods and cones.

Under photopic conditions (bright daylight), vision relies on cone photoreceptors. Cones are most sensitive to the middle of the visible spectrum, peaking around 555 nm (yellow-green). The photopic luminous efficiency function V(λ) describes how bright a light of a given wavelength appears to a light-adapted human eye. For example, a yellow-green light will seem much brighter than a red or blue light of equal physical power.

In contrast, under scotopic conditions (dim starlight or moonlight), vision relies on rod photoreceptors, which are highly sensitive to light but do not perceive color. Rods are most sensitive to shorter wavelengths, peaking around 507 nm (blue-green). The scotopic luminous efficiency function V’(λ) shows a significant shift toward the blue end of the spectrum. This means that in very dim light, a blue-green object will appear brighter than a red object of the same physical intensity, even though the red object may appear brighter at high light levels.

The Purkinje shift is the transition in perceived relative brightness of different colors as illumination decreases from photopic to scotopic levels. During twilight, when both rods and cones are active (mesopic vision), the sensitivity peak gradually migrates from 555 nm toward 507 nm. This causes red objects to darken faster than blue objects. For visual scientists, the Purkinje shift is crucial for designing displays, lighting, and night-vision devices. It also helps explain why astronomers prefer red flashlights—red light preserves scotopic adaptation better than blue or white light. Understanding luminous efficiency functions V(λ) and V’(λ) is fundamental to photometry and colorimetry, ensuring that measurements of light account for how the human eye actually perceives brightness across different lighting conditions.