As illumination increases, which wavelengths appear brighter?

The perception of brightness in relation to wavelength is primarily influenced by the sensitivity of the human visual system, particularly the photoreceptors in the retina. The peak sensitivity of the human eye is at around green wavelengths, which are considered medium wavelengths. However, as illumination increases, it has been observed that longer wavelengths, such as red and yellow light, can appear brighter than shorter wavelengths like blue or violet light.

This phenomenon is due to a combination of factors, including the spectral sensitivity of the photoreceptors and how different wavelengths are processed in terms of brightness perception. The human visual system exhibits a nonlinear response to different light intensities, and at higher illumination levels, the longer wavelengths tend to stimulate the visual receptors more effectively, leading to an overall perception that they appear brighter.

While shorter wavelengths can provide some brightness perception, especially in low-light conditions, they do not maintain the same level of brightness perception as longer wavelengths under increased illumination. Medium wavelengths, although they contribute to brightness sensation, do not exhibit the same pronounced increase in brightness perception as longer wavelengths do with rising light levels.

In summary, under conditions of increased illumination, longer wavelengths appear brighter due to their enhanced stimulation of the visual system and the characteristics of human brightness perception.