CIELUV

CIELUV is based on CIEUVW and is another attempt to define an encoding with uniformity in the perceptibility of color differences.[4] The non-linear relations for L*, u*, and v* are given below:[4]

${\displaystyle {\begin{aligned}L^{*}&={\begin{cases}\left({\frac {29}{3}}\right)^{3}Y/Y_{n},&Y/Y_{n}\leq \left({\frac {6}{29}}\right)^{3}\\116\left(Y/Y_{n}\right)^{1/3}-16,&Y/Y_{n}>\left({\frac {6}{29}}\right)^{3}\end{cases}}\\u^{*}&=13L^{*}\cdot (u^{\prime }-u_{n}^{\prime })\\v^{*}&=13L^{*}\cdot (v^{\prime }-v_{n}^{\prime })\end{aligned}}}$

The quantities u′_n and v′_n are the (u′, v′) chromaticity coordinates of a "specified white object" – which may be termed the white point – and Y_n is its luminance. In reflection mode, this is often (but not always) taken as the (u′, v′) of the perfect reflecting diffuser under that illuminant. (For example, for the 2° observer and standard illuminant C, u′_n = 0.2009, v′_n = 0.4610.) Equations for u′ and v′ are given below:[5][6]

${\displaystyle {\begin{aligned}u^{\prime }&={\frac {4X}{X+15Y+3Z}}&={\frac {4x}{-2x+12y+3}}\\v^{\prime }&={\frac {9Y}{X+15Y+3Z}}&={\frac {9y}{-2x+12y+3}}\end{aligned}}}$

(u′, v′) chromaticity diagram, also known as the CIE 1976 UCS (uniform chromaticity scale) diagram.

The transformation from (u′, v′) to (x, y) is:[6]

${\displaystyle {\begin{aligned}x&={\frac {9u^{\prime }}{6u^{\prime }-16v^{\prime }+12}}\\y&={\frac {4v^{\prime }}{6u^{\prime }-16v^{\prime }+12}}\end{aligned}}}$

The transformation from CIELUV to XYZ is performed as follows:[6]

${\displaystyle {\begin{aligned}u^{\prime }&={\frac {u^{*}}{13L^{*}}}+u_{n}^{\prime }\\v^{\prime }&={\frac {v^{*}}{13L^{*}}}+v_{n}^{\prime }\\Y&={\begin{cases}Y_{n}\cdot L^{*}\cdot \left({\frac {3}{29}}\right)^{3},&L^{*}\leq 8\\Y_{n}\cdot \left({\frac {L^{*}+16}{116}}\right)^{3},&L^{*}>8\end{cases}}\\X&=Y\cdot {\frac {9u^{\prime }}{4v^{\prime }}}\\Z&=Y\cdot {\frac {12-3u^{\prime }-20v^{\prime }}{4v^{\prime }}}\\\end{aligned}}}$