Helmholtz–Kohlrausch effect

The Helmholtz–Kohlrausch effect (after Hermann von Helmholtz and V. A. Kohlrausch[1]) is a perceptual phenomenon wherein the intense saturation of spectral hue is perceived as part of the color's luminance.

It appears in both self-luminous and surface colors, although it is most pronounced in spectral lights.

An exception to this is when the human observer is red-green colorblind, they cannot distinguish the differences between the lightness of the colors.

The Helmholtz–Kohlrausch effect works best in darker environments where there are not any other outside factors influencing the colors.

[4] Helmholtz–Kohlrausch effect has been described in mathematical models by Fairchild and Pirrotta 1991, Nayatani 1997, and most recently High, Green, and Nussbamm 2023.

A comparison of runway LED lamps and filtered and unfiltered incandescent lights all at the same luminance shows that in order to accomplish the same brightness, the white reference incandescent lamp needs to have twice the luminance of the red LED lamp, therefore suggesting that the LED lights do appear to have a greater brightness than the traditional incandescent lights.

LEDs in the dashboard and instrument lighting are designed for use in mesopic luminance.

In studies, it has been found that red LEDs appear brighter than green LEDs under these conditions, which means that a driver would be able to see red light more intensely and would thus be more alerting than green lights when driving at night.

Each color on top has approximately the same lightness level and yet they do not appear equally bright. The yellow sample (second from the left) appears to be much dimmer than the magenta (right-most) one. However, when the top image is converted to grayscale, we have the image on the bottom--a single shade of gray.