CIELCh

CIELCH is a cylindrical representation of the CIELAB and CIELUV color spaces, designed to describe color using lightness, chroma, and hue. It provides an intuitive way to think about color: L* denotes lightness, C* denotes chroma (color intensity), and h° denotes hue angle around a color wheel. There are two common variants: CIELCHab, derived from CIELAB, and CIELCHuv (often referred to as CIELCHuv), derived from CIELUV.

In the CIELAB-based form, chroma is C* = sqrt(a*^2 + b*^2) and hue is h° = atan2(b*, a*) in degrees, typically mapped to 0–360. L* remains the same as in CIELAB. The inverse conversion is a* = C* cos(h°), b* = C* sin(h°), with L* unchanged. The CIELCHuv variant uses the CIELUV space, where C* = sqrt(u*^2 + v*^2) and h° = atan2(v*, u*), with L* likewise carried over from the L* in LAB or Luv representations. In both cases, C* measures how saturated the color is, while h° places it on a circular hue wheel.

Advantages of CIELCH include a more perceptually uniform separation of lightness, chroma, and hue in many regions of color space, facilitating tasks such as color selection, palette design, and color correction. Hue becomes continuous around the wheel, though hue is undefined when chroma is zero (achromatic colors). Conversions between CIELCH and CIELAB or CIELUV are straightforward, enabling practical use across imaging, printing, and digital design workflows.