!!! D . R . A . F . T !!!¶

Lightness¶

Lightness is defined as the brightness of an area judged relative to the brightness of a similarly illuminated area that appears to be white or highly transmitting. [1]

Colour defines the following Lightness computation methods:

In [1]:

import colour

sorted(colour.LIGHTNESS_METHODS.keys())

Out[1]:

['CIE 1976',
 'Fairchild 2010',
 'Fairchild 2011',
 'Glasser 1958',
 'Lstar1976',
 'Wyszecki 1963']

Note: 'Lstar1976' is a convenient aliases for 'CIE 1976'.

Glasser, Mckinney, Reilly and Schnelle (1958) Method¶

Glasser, Mckinney, Reilly and Schnelle (1958) described a visually uniform colour coordinate system close to Adams chromatic-value system but where the quintic-parabola function has been replaced with a cube-root function: the Cube-Root Color Coordinate System.

Lightness $L$ in the Cube-Root Color Coordinate System is calculated as follows: [2]

$$ \begin{equation} L=25.29Y^{1/3}-18.38 \end{equation} $$

where $Y$ defines the luminance in domain [0, 100].

The colour.lightness_Glasser1958 definition is used to compute Lightness $L$:

In [2]:

colour.colorimetry.lightness_Glasser1958(10.08)

Out[2]:

36.250562645752595

Note: Input luminance $Y$ is in domain [0, 100], output Lightness $L$ is in domain [0, 100].

The colour.lightness definition is implemented as a wrapper for various lightness computation methods:

In [3]:

colour.lightness(10.08, method='Glasser 1958')

Out[3]:

36.250562645752595

In [4]:

from colour.plotting import *

In [5]:

colour_style();

In [6]:

# Plotting the "Glasser (1958)" "Lightness" function.
plot_single_lightness_function('Glasser 1958');

Wyszecki (1963) Method¶

Wyszecki (1963) recommended the following cube root function to compute Lightness $W$ as a function of the luminance factor $Y$ within the practically important range of $1.0\%<Y<98\%$: [3]

$$ \begin{equation} W=25Y^{1/3}-17 \end{equation} $$

The colour.lightness_Wyszecki1963 definition is used to compute Lightness $W$:

In [7]:

colour.colorimetry.lightness_Wyszecki1963(10.08)

Out[7]:

37.004114912764535

Note: Input luminance $Y$ is in domain [0, 100], output Lightness $W$ is in domain [0, 100].

Using the colour.lightness wrapper definition:

In [8]:

colour.lightness(10.08, method='Wyszecki 1963')

Out[8]:

37.004114912764535

In [9]:

# Plotting the "Wyszecki (1963)" "Lightness" function.
with colour.utilities.suppress_warnings():
    plot_single_lightness_function('Wyszecki 1963');

CIE 1976 Method¶

The CIE $L^a^b^$* approximately uniform colourspace defined in 1976 computes the *Lightness* $L^*$ quantity as follows: [4]

$$ \begin{equation} L^*=\begin{cases}116\biggl(\cfrac{Y}{Y_n}\biggr)^{1/3}-16 & for\ \cfrac{Y}{Y_n}>\epsilon\\ \kappa*\biggl(\cfrac{Y}{Y_n}\biggr) & for\ \cfrac{Y}{Y_n}<=\epsilon \end{cases} \end{equation} $$

where $Y_n$ is the reference white luminance. with $$ \begin{equation} \begin{aligned} \epsilon&\ =\begin{cases}0.008856 & Actual\ CIE\ Standard\\ 216\ /\ 24389 & Intent\ of\ the\ CIE\ Standard \end{cases}\\ \kappa&\ =\begin{cases}903.3 & Actual\ CIE\ Standard\\ 24389\ /\ 27 & Intent\ of\ the\ CIE\ Standard \end{cases} \end{aligned} \end{equation} $$

The original $\epsilon$ and $\kappa$ constants values have been shown to exhibit discontinuity at the junction point of the two functions grafted together to create the Lightness $L^*$ function. [5]

Colour uses the rational values instead of the decimal values for these constants.

See Also: The CIE $L^*a^*b^*$ Colourspace notebook for in-depth informations about the CIE $L^a^b^$* colourspace.

The colour.lightness_CIE1976 definition is used to compute Lightness $L^*$:

In [10]:

colour.colorimetry.lightness_CIE1976(10.08)

Out[10]:

37.985629097653039

Note: Input luminance $Y$ and $Y_n$ are in domain [0, 100], output Lightness $L^*$ is in domain [0, 100].

Using the colour.lightness wrapper definition:

In [11]:

colour.lightness(10.08)

Out[11]:

37.985629097653039

In [12]:

colour.lightness(10.08, method='CIE 1976', Y_n=95)

Out[12]:

38.916598757092821

In [13]:

colour.lightness(10.08, method='Lstar1976', Y_n=95)

Out[13]:

38.916598757092821

In [14]:

# Plotting the "CIE 1976" "Lightness" function.
plot_single_lightness_function('CIE 1976');

In [15]:

# Plotting multiple "Lightness" functions for comparison.
plot_multi_lightness_functions(['CIE 1976', 'Glasser 1958']);

Fairchild and Wyble (2010) Method¶

In [16]:

colour.colorimetry.lightness_Fairchild2010(10.08 / 100, 1.836)

Out[16]:

24.902290269546651

In [17]:

colour.lightness(10.08 / 100, method='Fairchild 2010', epsilon=1.836)

Out[17]:

0.027048835900068357

In [18]:

# Plotting the "Fairchild and Wyble (2010)" "Lightness" function.
plot_single_lightness_function('Fairchild 2010');

In [19]:

# Plotting multiple "Lightness" functions for comparison.
plot_multi_lightness_functions(['CIE 1976', 'Fairchild 2010']);

Fairchild and Chen (2011) Method¶

In [20]:

colour.colorimetry.lightness_Fairchild2011(10.08 / 100, 0.710)

Out[20]:

26.459509817572265

In [21]:

colour.lightness(10.08 / 100, method='Fairchild 2011', epsilon=0.710)

Out[21]:

1.1405928270854113

In [22]:

# Plotting the "Fairchild and Chen (2011)" "Lightness" function.
plot_single_lightness_function('Fairchild 2011');