colour.complementary_wavelength(xy, xy_n, cmfs=XYZ_ColourMatchingFunctions(name='CIE 1931 2 Degree Standard Observer', ...))[source]

Returns the complementary wavelength \(\lambda_c\) for given colour stimulus \(xy\) and the related \(xy_wl\) first and \(xy_{cw}\) second intersection coordinates with the spectral locus.

In the eventuality where the \(xy_wl\) first intersection coordinates are on the line of purples, the dominant wavelength will be computed in lieu.

The dominant wavelength is indicated by a negative sign and the \(xy_{cw}\) second intersection coordinates which are set by default to the same value than \(xy_wl\) first intersection coordinates will be set to the dominant wavelength intersection coordinates with the spectral locus.

  • xy (array_like) – Colour stimulus xy chromaticity coordinates.
  • xy_n (array_like) – Achromatic stimulus xy chromaticity coordinates.
  • cmfs (XYZ_ColourMatchingFunctions, optional) – Standard observer colour matching functions.

Complementary wavelength, first intersection point xy chromaticity coordinates, second intersection point xy chromaticity coordinates.

Return type:



[CIET14804b], [Erdb]


Complementary wavelength computation:

>>> from pprint import pprint
>>> xy = np.array([0.37605506, 0.24452225])
>>> xy_n = np.array([0.31270000, 0.32900000])
>>> cmfs = CMFS['CIE 1931 2 Degree Standard Observer']
>>> pprint(complementary_wavelength(xy, xy_n, cmfs))  # doctest: +ELLIPSIS
 array([ 0.0104096...,  0.7320745...]),
 array([ 0.0104096...,  0.7320745...]))

Dominant wavelength is returned if the first intersection is located on the line of purples:

>>> xy = np.array([0.54369557, 0.32107944])
>>> pprint(complementary_wavelength(xy, xy_n, cmfs))  # doctest: +ELLIPSIS
 array([ 0.0364795 ,  0.3384712...]),
 array([ 0.0364795 ,  0.3384712...]))