What is the shift called that occurs as the illumination increases, causing longer wavelengths to appear brighter?

The phenomenon that describes the shift in perceived brightness of longer wavelengths as illumination increases is known as the Purkinje shift. This effect occurs due to the differences in sensitivity between the rod and cone photoreceptors in the human eye. Under low-light conditions, rods are more active and sensitive to shorter wavelengths, which is why blue colors tend to appear brighter. As illumination increases, the cones become more dominant and are more responsive to the longer wavelengths (reds and yellows), leading to the perception that these colors appear brighter.

This shift highlights the transition from scotopic (rod-driven) vision to photopic (cone-driven) vision as ambient light increases. Recognizing the Purkinje shift is essential for understanding color perception across varying lighting conditions, particularly in contexts like visual acuity and color discrimination in different environments.