CIELAB
A device-independent colour model with one lightness axis and two opponent colour axes, designed so equal numerical steps look equally different.
Formula / rule
ΔE = √(ΔL*² + Δa*² + Δb*²)
Measured in
L*a*b*
In depth
Its perceptual uniformity is what makes it the reference for colour difference measurement: the ΔE figure quoted in monitor and printer reviews is a distance in this space, with values under about 2 being invisible to most viewers. It also separates lightness completely from colour, which is useful for editing tone without touching hue.
Examples in practice
A monitor calibrated to an average ΔE below 1 is considered visually indistinguishable from its target.