It's only a difference of coordinates.
CIELAB uses cartesian coordinates, that is, xyz-type coordinates. LCh is only a conversion from cartesian coordinates to polar coordinates, but the underlying color model is the same.
The 1931 CIE "standard observer" is a device independent "virtual eye" that is based on studies of human perception of color. The related color space is the XYZ space, which uses cartesian coordinates and is essentially linear in regards to light in the real world.
Human perception of light is not linear as light exists in the real world. In an effort to create a model that was perceptually uniform, meaning linear to perception as opposed to light, in 1976 the CIE released CIELAB
CIELAB is defined as cartesian coordinates in a euclidian space, i.e. xyz coordinates. However, as there are other color spaces referred to as XYZ and xyY, LAB uses
L* for perceptual lightness (100) to darkness(0), a± for red/green opponent colors and b± for yellow/blue opponent colors.
L* is perceptual lightness, from 0 (black) to full white (100).
- It is a perception and not a measure of light (i.e. it is not luminance, it is the perception of luminance).
a* values are redder, negative
a* values are greener
b* values are yellower, negative
b* values are bluer
LCh is only a conversion from the
L*a*b* cartesian coordinates to polar coordinates, wherein the
L* values remains exactly the same, but the
a*b* coordinates are converted to a hue and a colorfulness (chroma) correlate.
- hue is a value from 0° to 360° (361° = 1°)
- Chroma is a value from 0 (achromatic grey) to over 133 (for sRGB blue).
It should be noted that CIELAB is not truly perceptually uniform, but is substantially more uniform than RGB or HSB/HSV/HSL etc.