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Measuring the brightness of colors anew

JLU research team reveals the limits of the international unit of light, the candela: a simple RGB rule predicts how bright colors appear to people

Nr. 118 • 20. August 2026 
(deutsche Version)

Two scatter plots compare test rankings with luminance and wMaxRGB. Luminance shows high dispersion and an unexplained variance of 34.12%, whereas wMaxRGB correlates much more closely with the test ranking, with an unexplained variance of just 4.73%
Prediction of perceived brightness by the established photometric measure (left) and by the new rule (right). Each point represents one color. The more closely the points follow a single curve, the better the prediction. Graphic: Gegenfurtner
The candela, the international unit of light in use for almost a century, forms the basis of photometry. According to a new study, measurements derived from it systematically misjudge the brightness of colored light sources and depart markedly from human perception. An international team led by the perception researcher Prof. Dr. Karl Gegenfurtner at Justus Liebig University Giessen (JLU) has found a surprisingly simple rule for how bright colored surfaces actually appear: perceived brightness can be predicted almost completely by taking only the highest of the three weighted color components red, green, and blue. The rule accounts for more than 95 percent of the judgments observed and outperforms all established photometric models.

“The candela is the only SI unit that rests not on physics alone but on human perception,” says Prof. Gegenfurtner. That the quantities derived from it diverge so clearly from perception in the case of colors has consequences for the labeling of lamps, the calibration of displays, and the planning of lighting in offices, on streets, and in homes. For the observers in the study, now published in the journal Proceedings of the National Academy of Sciences (PNAS), saturated colors appeared considerably brighter, and blue light contributed more strongly to perceived brightness than photometric measurement predicts.

Gegenfurtner traces the discrepancies to the assumption underlying the standard, which was established in the 1920s: that the brightness of a color mixture equals the sum of its parts. Measurement by classical brightness matching is also slow, difficult, and unreliable across observers. Working within the European Research Council project Color 3.0, the Giessen team took a different route and simply asked observers to sort colored patches from bright to dark.

The method proved remarkably robust. Observers ranked 144 colors, and their orderings were nearly identical when repeated more than six months later. They remained just as stable when the same people repeated the task at home on their own uncalibrated monitors, and when a further 486 participants completed it online. For the first time, the brightness of different colors could be measured on a large scale.

With these data the team tested a wide range of candidate models, including luminance, radiance, and established color appearance models such as CAM16. “None of these models captured the pattern. What worked in the end was strikingly simple,” explains Gegenfurtner. “You take the red, green, and blue components of a color, weight them, and keep only the largest value. That predicts the brightness judgments almost perfectly.”

The same signature appeared in the brain. In EEG recordings, responses to rapidly flickering lights followed luminance exactly, as classical photometry would lead one to expect. Under slow modulation, closer to steady viewing, they followed the new rule instead. In a real room lit by spectrally tunable LEDs, participants consistently chose the illumination that the new model rates as brighter, a clear indication that the rule describes everyday seeing.

The findings come at a time of rapid technological change. Modern LEDs make it possible to alter the spectral composition of light freely while holding luminance constant, so two installations can be rated as equally bright and still look visibly different. The study therefore provides not only a reliable method for measuring the brightness of mixed colors but also a simple and previously unrecognized computational basis for the perception of light in everyday scenes. “These insights could substantially influence how lighting in buildings is planned, how displays are calibrated, and how energy-efficient light sources are developed,” Gegenfurtner emphasizes.

Publication

S. Guan, J. Chen, R. Ennis, M. Toscani, M. Valsecchi, A. van Doorn, J. Koenderink, & K.R. Gegenfurtner, Perceptual and neural constraints on photometric measures of heterochromatic brightness, Proc. Natl. Acad. Sci. U.S.A. 123 (34) e2610398123, https://doi.org/10.1073/pnas.2610398123 (2026).

Funding

European Research Council, ERC Advanced Grant Color 3.0 (884116); German Research Foundation, SFB/TRR 135 (project number 222641018); DFG Cluster of Excellence EXC 3066/1 “The Adaptive Mind” (533717223).

Contact

Prof. Karl R. Gegenfurtner, General Psychology
E-mail: gegenfurtner

 

 

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