Bezold–Brücke shift

File:CIE1931xy blank.svg, and violets) shift towards blue, colors with a dominant wavelength above ~500 nm (reds, oranges, and greens) shift towards yellow.]]

File:Bezold–Brücke hue shift examples.png

The Bezold–Brücke shift or luminance-on-hue effect{{cite journal |last1=Pridmore |first1=Ralph W. |last2=Melgosa |first2=Manuel |title=All Effects of Psychophysical Variables on Color Attributes: A Classification System |journal=PLOS ONE |year=2015 |volume=10 |issue=4 |pages=e0119024 |doi=10.1371/journal.pone.0119024 |pmid=25859845 |pmc=4393130 |doi-access=free|bibcode=2015PLoSO..1019024P }} is a change in hue perception as the luminance (light intensity) of a color changes. As intensity increases, the apparent hue of stimuli of a constant spectral distribution shifts towards blue, if its dominant wavelength is below around 500 nm; or yellow, if its dominant wavelength is above 500 nm. As intensity is decreased, apparent hue shifts towards red or green.{{cite book |last=Fairchild |first=Mark |title=Color Appearance Models |edition=3rd |year=2013 |publisher=Wiley |chapter=Bezold-Brücke hue shift (hue changes with luminance) |at=§ 6.3, {{pgs|120–121}} |isbn=9781119967033 }}

The effect was noted in 1866 by physiologist Ernst Wilhelm von Brücke, and experimental investigations by physicist and meteorologist Wilhelm von Bezold were published in 1873.{{cite book |last1=Shamey |first1=Renzo |last2=Kuehni |first2=Rolf G. |year=2020 |title=Pioneers of Color Science |at=Ch. 44, {{pgs|213–216}} |chapter=von Bezold, Johann Friedrich Wilhelm; 1837–1917 |isbn=978-3-319-30809-8 |doi=10.1007/978-3-319-30811-1_44 }} It was re-investigated more thoroughly by Donald McL. Purdy in 1931.{{cite journal |last=Purdy |first=Donald McL. |title=Spectral Hue as a Function of Intensity |journal=The American Journal of Psychology |volume=43 |number=4 |year=1931 |pages=541–559 |doi=10.2307/1415157 |jstor=1415157 }}

Stimuli of certain wavelengths ("invariant hues") retain their apparent hue despite changes in luminance; these have similar but not quite the same wavelengths as the unique hues red, yellow, blue, and green.{{cite journal |last=Vos |first=J. J. |year=1986 |title=Are Unique and Invariant Hues Coupled? |journal=Vision Research |volume=26 |number=2 |pages=337–342 |doi=10.1016/0042-6989(86)90031-3 |pmid=3716226 }}{{cite journal |last1=Ayama |first1=Miyoshi |last2=Nakatsue |first2=Takehiro |last3=Kaiser |first3=Peter K.|title=Constant hue loci of unique and binary balanced hues at 10, 100, and 1000 Td |journal= Journal of the Optical Society of America A|year=1987 |volume=4 |issue=6 |pages=1136–1144 |doi=10.1364/JOSAA.4.001136|pmid=3598757 |bibcode=1987JOSAA...4.1136A }}

A similar hue shift, the Abney effect, occurs when a visual stimulus is mixed with white light. Both the Abney effect and the Bezold–Brücke shift apply not only to colors in isolation, but also to surface colors: for example, due to the Bezold–Brücke shift, the highlights and shadows of an object can appear to have different hues.{{cite journal |last1=Pridmore |first1=Ralph W. |title=Bezold–Brucke effect exists in related and unrelated colors and resembles the Abney effect |journal=Color Research and Application |year=2004 |volume=29 |issue=3 |pages=241–246 |doi=10.1002/col.20011}}

The shift in the hue is also accompanied by the changes in the perceived saturation. As the brightness of the color stimuli increases, their color strength also increases to a maximum point and then decreases again; in such a way that it is still wavelength specific. This can, to an extent, be considered as an inverse of the Helmholtz–Kohlrausch effect.{{fact|date=February 2024}} In the case of the Helmholtz–Kohlrausch effect, the partially desaturated stimulus is seen to be brighter than fully saturated or achromatic stimulus.