Equal colour contrast between equally wide segments of one colour gradient as seen through the human eye.
The perception of a certain colour, or colour gradient, by the optical cortex depends not only on the optical properties of the object, but also on the optical properties of both light source and object background, and the eye’s light receptors (rod and cone cells for short, medium and long wavelengths).
A constant incremental colour and lightness contrast along a colour map is a proxy for its perceptual uniformity. (a) While a certain incremental data variation is either under- or strongly overrepresented with unscientific colour maps like jet (a.k.a. rainbow) depending on the colour map segment, it is instead (b) evenly represented all along a colour bar when using a scientific colour map like batlow, due to its uniform colour and lightness contrast.
Perceptual order is given when individual colours of a colour map can be sequentially ordered effortlessly without consulting the colour bar. While (c) a sequential ordering is not intuitively possible for unscientific colour maps like jet (a.k.a. rainbow), it is (d) possible to sequentially order individual colours of a scientifically derived colour map like batlow, thanks to its constant lightness gradient.
Available perceptually-uniform colour maps versus the non-uniform rainbow (i.e., jet; bottom row) as seen with either of the three common forms of human colour-vision deficiency (deuteranopia, protanopia, and tritanopia), and for grey-scale (representing total colour-blindness or simple black-and-white prints). Rainbow, the most-widely used colour map, fails to reproduce a meaningful smooth gradient, yet the Scientific colour maps, like batlow, are all universally readable.