color
Perceptual uniformity
also called perceptually uniform (community), even steps (community), uniform color space
The property of a colour space where equal numeric distances look like equal visual differences, which is what makes a generated ramp step evenly.
A colour space is perceptually uniform when the arithmetic agrees with the
eye: move the same amount in the numbers, see the same amount of change. HSL is not, and
the failure is not subtle. Its L is an average of the largest and smallest sRGB channel,
which is a fact about the encoding rather than about vision, so hsl(60 90% 50%) is a
blazing yellow and hsl(250 85% 50%) is a deep blue while both claim to be halfway. Run a
ramp between fixed endpoints in each and measure: at a yellow hue the top three HSL steps
change less than two units of CIE L* each and the bottom two change more than twenty, so
the ramp bunches at the light end and falls off a cliff at the dark one.
This is why token scales moved to OKLCH. A palette generated by stepping HSL lightness produces steps that do not read as equal, which shows up as a 100 and a 200 that are indistinguishable next to a 700 and an 800 that are miles apart, and it shows up differently at every hue, so the blue ramp and the yellow ramp in the same system misbehave in different ways. Stepping OKLCH lightness instead gives a ladder whose measured rungs are the same height at every hue, which is what lets one set of numbers serve a whole palette.
Uniformity is a claim about differences, and that is what makes it useful for more than ramps. Interpolation in a uniform space passes through the colours a person would draw between the endpoints instead of veering through grey or through a hue nobody asked for. Gamut mapping walks chroma down in OKLCH precisely because lightness and hue stay put while it does. Contrast scoring wants the same property for a different reason: relative luminance is a physical quantity, not a perceptual one, which is part of why APCA rebuilt the calculation around how light and dark text actually behave.
Two honest limits. OKLCH is uniform enough for interface work, not perfect: its blues drift in hue at high chroma, and no space built on a 1970s dataset is uniform for everyone. And uniformity is about lightness and hue, not about contrast. Two steps that read as equally far apart can still fail a contrast requirement, so a ramp built in OKLCH still needs its contrast ratios checked rather than assumed.
Which word?
| If you want | say |
|---|---|
| explaining why an HSL ramp looks uneven and an OKLCH one does not | perceptual uniformity |
| explaining why two colours of equal saturation contrast differently | relative luminance |
| comparing colourfulness across colours of different lightness | chroma |
| writing colour so that equal numeric steps look equal | oklch |