White Points and Chromatic Adaptation: von Kries, Bradford, and CAT02
Carry a white shirt from noon daylight into a candle-lit room and it still looks white - your eye quietly rescales itself to the new light. Color management has to do the same trick in math: when a D65 screen image meets a D50 print, the numbers must be adapted so white stays white. This is the interactive guide to white points and the chromatic adaptation transforms - von Kries, Bradford, CAT02 - that keep color honest across illuminants.
What a white point is
Every color space and every illuminant has a white point: the chromaticity it treats as neutral, the anchor from which all other colors are measured. sRGB and most screens use D65 (roughly 6500 K daylight); print and the ICC connection space use D50 (5000 K); a tungsten bulb is close to Illuminant A (2856 K). Each white sits at a different point on the chromaticity diagram - D65 cool and bluish, A warm and orange.
The problem is that the same object sends different light to the eye under each illuminant, yet we want it to keep its identity - and in particular, we want the illuminant's own white to read as plain white. To convert color values measured under one white point so they make sense under another, we need a chromatic adaptation transform (CAT).
Adapting a scene between illuminants
Here is the everyday face of chromatic adaptation: white balance. A photo shot under a warm bulb comes out orange; correcting it is a chromatic adaptation transform from the bulb's white point back to a neutral D65. Choose the light the scene was shot under and watch the transform pull the cast back to neutral.
Pull the cast back to neutral
The left panel is the scene as captured under the chosen illuminant (its white point gives the whole image a cast). The right applies a Bradford chromatic adaptation transform from that illuminant to D65, restoring neutral whites - exactly what "white balance" does, done with the real transform.
The von Kries model: scaling cones
Nearly every CAT follows the von Kries idea: adaptation is just an independent gain on each cone channel. Convert the color into a cone-like space (L, M, S), then scale each channel by the ratio of the destination white's cone response to the source white's. White becomes white because the white's own L, M, S are scaled exactly to the target white's. See the scaling factors at work.
One gain per cone channel
Pick a color and source/destination white points. The bars show the per-channel gain applied to L, M, and S - the ratio of the two whites' cone responses. Apply those gains to the color's L, M, S and convert back, and you have the adapted color. The same gains turn the source white into the destination white exactly.
The standard illuminant white points
The CIE defines standard illuminants, each a fixed white point. The most common cluster along the daylight locus (the D-series), with the warm tungsten A off to the orange side. Their positions on the chromaticity diagram are what every adaptation transform is moving between. Explore them.
Where the standard whites live
The standard illuminant white points plotted on the CIE xy chromaticity plane, near the central white region. Hover or tap a point: warm A sits low and to the right (orange), the daylight D-series climbs toward cooler blue-white as the temperature rises, and E (equal energy) sits dead centre. The swatch shows that white's approximate color.
von Kries, Bradford, and CAT02
The transforms differ only in which cone-like space they scale in - the matrix that maps XYZ to the channels being scaled. "XYZ scaling" scales raw XYZ (crude). Von Kries uses physiological cone fundamentals. Bradford uses a sharpened, optimized matrix and is the workhorse of ICC color management. CAT02 (from CIECAM02) is a refined modern version. Compare how well each keeps a color plausible across a big white-point jump.
Same jump, three transforms
Adapt a color from D65 to a warm white (A) three ways. None leaves the cast. XYZ scaling over-corrects and skews hues. Bradford keeps the color natural while neutralizing the white. The reference swatch is the source; compare how each method lands.
Where adaptation happens
Pitfalls and gotchas
Test your understanding
Six questions on white points, von Kries, Bradford, and where adaptation is used. Instant feedback, no scores recorded - a wrong answer comes with a short explanation.
Quick check
Continue your journey
Adaptation sits between the eye's constancy, the master XYZ space, and the profiles that move color around.
Color Constancy, Adaptation, and Context
The perceptual phenomenon these transforms model in math.
Physics · KelvinColor Temperature and White Balance
Where these white points come from, and white balance in practice.
Colorimetry · XYZCIE XYZ Explained
The space the adaptation matrices operate in.
Colorimetry · 41Color Appearance Models: CIECAM
Where CAT02 lives - adaptation as the first step of appearance.
Digital · ICCICC Profiles and How Color Management Works
The D50 connection space and the Bradford adaptation it relies on.
Colorimetry · ChromaChromaticity Diagrams Without the Intimidation
The plane the white points and daylight locus are plotted on.