# Source code for colour.constants.cie

#!/usr/bin/env python
# -*- coding: utf-8 -*-

"""
CIE Constants
=============

Defines *CIE* constants.
"""

from __future__ import division, unicode_literals

__author__ = 'Colour Developers'
__maintainer__ = 'Colour Developers'
__status__ = 'Production'

__all__ = ['CIE_E',
'CIE_K',
'K_M',
'KP_M']

CIE_E = 216 / 24389
"""
*CIE* :math:\epsilon constant.

CIE_E : numeric

Notes
-----
-   The original *CIE* value for :math:\epsilon is :math:\epsilon=0.008856,
Lindbloom (2003) has shown that this value is causing a discontinuity
at the junction point of the two functions grafted together to create the
*Lightness* :math:L^* function.

That discontinuity can be avoided by using the rational representation as
follows: :math:\epsilon=216\ /\ 24389.

References
----------
..   Lindbloom, B. (2003). A Continuity Study of the CIE L* Function.
Retrieved February 24, 2014, from
http://brucelindbloom.com/LContinuity.html

"""

CIE_K = 24389 / 27
"""
*CIE* :math:\kappa constant.

CIE_K : numeric

Notes
-----
-   The original *CIE* value for :math:\kappa is :math:\kappa=903.3,
Lindbloom (2003) has shown that this value is causing a discontinuity
at the junction point of the two functions grafted together to create the
*Lightness* :math:L^* function. _

That discontinuity can be avoided by using the rational representation as
follows: :math:k=24389\ /\ 27.
"""

K_M = 683
"""
Rounded maximum photopic luminous efficiency :math:K_m value in
:math:lm\cdot W^{-1}.

K_M : numeric

Notes
-----
-   To be adequate for all practical applications the :math:K_m value has
been rounded from the original 683.002 value. _

References
----------
..   Wyszecki, G., & Stiles, W. S. (2000). Standard Photometric Observers.
In Color Science: Concepts and Methods, Quantitative Data and Formulae
(pp. 256–259,395). Wiley. ISBN:978-0471399186

"""

KP_M = 1700
"""
Rounded maximum scotopic luminous efficiency :math:K^{\prime}_m value in
:math:lm\cdot W^{-1}.

KP_M : numeric

Notes
-----
-   To be adequate for all practical applications the :math:K^{\prime}_m
value has been rounded from the original 1700.06 value. _
"""