HLC is an easy to understand representation of the CIELAB colour space. The CIELAB colour model was defined in 1976 by the Commission Internationale de l’Eclairage. It is a mathematical colour model in which the spectral properties of an object and the perceptual properties of our visual system are offset against each other.
CIELAB ( or: Lab-colours) has become generally accepted until today, wherever exact colour calculation is required, be it in colour measurement, formulation or in the common operating systems: Colour management usually works via CIELAB.
Instead of the rectangular coordinates for L, a and b the CIELAB polar coordinates HLC can be used.
Here, the vertical L-axis is identical.
The horizontal position of a colour location is determined here by the angle H to the positive a-axis (counter-clockwise and in the unit degree) and the radius C, the distance to the L-axis.
HLC, also LCh or LCH(ab), stands for
• Hue (colour tone [angle])
• Lightness (brightness)
• Chroma (saturation)
and is much easier to comprehend than the abstract a and b coordinates, for which hardly anyone can imagine the corresponding colour.
This text is taken from the freieFarbe e.V. CIELAB HLC Colour Atlas, which you can download here for free.