What Is the Purkinje Shift and How Does It Work?

The Purkinje shift describes the change in the human eye’s sensitivity to color as light levels decrease from daylight to twilight. This phenomenon is why some colors appear more vibrant while others seem to fade as dusk settles. As illumination lessens, our perception of these colors alters, a change that happens before color vision disappears completely.

The Eye’s Light Receptors

The retina, a layer of tissue at the back of the eye, contains two types of photoreceptor cells that detect light: cones and rods. The retina has approximately 4.5 million cone cells responsible for color vision in bright light, called photopic vision. Cones allow us to perceive the fine details and rich colors of the world.

The retina also houses about 90 million rod cells. These photoreceptors are much more sensitive to light than cones and are responsible for vision in low-light conditions, known as scotopic vision. Rods do not distinguish between different colors, perceiving the world in shades of gray. Their primary function is to detect shapes and movement when light is scarce.

The Mechanism of the Shift

The Purkinje shift occurs during mesopic vision, the transitional period when both rods and cones are active. This transition involves a handover from cone-dominated photopic vision to rod-dominated scotopic vision. The phenomenon is caused by the different spectral sensitivities of these photoreceptors. Cones are most sensitive to light in the yellowish-green part of the spectrum, with a peak sensitivity around 555 nanometers.

Rods have a peak sensitivity to light of a shorter wavelength, in the bluish-green range at about 500 nanometers. As light fades and rods become the primary photoreceptors, our eyes become more sensitive to blues and greens. Conversely, rods are not very sensitive to long-wavelength light, such as red. As darkness falls, red objects appear to lose their vibrancy and fade toward black much more quickly than other colors.

Real-World Examples and Observations

The effects of the Purkinje shift are observable in many everyday situations. A classic illustration is viewing a garden at dusk. Red flowers, like geraniums or roses, will appear to darken significantly, while the surrounding green leaves and any adjacent blue flowers will appear comparatively bright. This happens because the red petals reflect long-wavelength light that rods cannot easily see, while the leaves and blue flowers reflect shorter-wavelength light.

This same principle can be observed while driving at twilight. A red car will seem much darker and harder to distinguish than a blue or green vehicle of similar brightness. Similarly, if you watch a colorful painting while slowly dimming the lights, you will notice the reds and yellows fade first, while the blues and greens remain visible for longer in shades of gray.