Color.Temperature (Color v0.4.0)

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Correlated colour temperature (CCT) on the Planckian locus.

  • cct/1 — McCamy's 1992 approximation: given any colour (or an {x, y} chromaticity pair), returns the CCT in Kelvin.

  • xy/1 — Kim et al. 2002 piecewise polynomial: given a CCT in Kelvin, returns the {x, y} chromaticity of the corresponding blackbody radiator (accurate to ~1e-3 over 1667–25000 K).

  • xyz/1,2 — same as xy/1 but returns a Color.XYZ struct tagged D65/2° by default, with an optional luminance.

Summary

Functions

cct(color)

Returns the correlated colour temperature of a colour, in Kelvin, using McCamy's 1992 approximation.

daylight(kelvin)

Returns the {x, y} chromaticity on the CIE daylight locus (CIE 15.2) at the given colour temperature. Valid over [4000, 25000] K.

planckian(kelvin)

Returns the {x, y} chromaticity on the Planckian (blackbody) locus at the given temperature, using the Kim et al. 2002 polynomial. Valid over [1667, 25000] K.

xy(kelvin)

Returns the {x, y} chromaticity for a colour temperature in Kelvin.

xyz(kelvin, luminance \\ 1.0)

Returns a Color.XYZ struct at the given colour temperature, with Y scaled to the given luminance (default 1.0).

Functions

cct(color)

@spec cct({number(), number()} | Color.input()) :: float() | {:error, Exception.t()}

Returns the correlated colour temperature of a colour, in Kelvin, using McCamy's 1992 approximation.

Accuracy is about ±2 K from 2856 K (illuminant A) to 6500 K (D65) and degrades smoothly outside that range. The McCamy formula is the classical one; for more accurate results use a Planckian-locus minimum-distance search (not provided here).

Arguments

  • color is any colour accepted by Color.new/1, or an {x, y} chromaticity tuple of floats.

Returns

  • A float — CCT in Kelvin.

Examples

iex> Float.round(Color.Temperature.cct({0.31271, 0.32902}), 0)
6504.0

iex> Float.round(Color.Temperature.cct({0.44757, 0.40745}), 0)
2857.0

iex> Float.round(Color.Temperature.cct("white"), 0)
6503.0

daylight(kelvin)

@spec daylight(number()) :: {float(), float()}

Returns the {x, y} chromaticity on the CIE daylight locus (CIE 15.2) at the given colour temperature. Valid over [4000, 25000] K.

Arguments

  • kelvin is a number in [4000, 25000].

Returns

  • An {x, y} tuple.

planckian(kelvin)

@spec planckian(number()) :: {float(), float()}

Returns the {x, y} chromaticity on the Planckian (blackbody) locus at the given temperature, using the Kim et al. 2002 polynomial. Valid over [1667, 25000] K.

Arguments

  • kelvin is a number in [1667, 25000].

Returns

  • An {x, y} tuple.

xy(kelvin)

@spec xy(number()) :: {float(), float()}

Returns the {x, y} chromaticity for a colour temperature in Kelvin.

For T < 4000 K (incandescent range), returns the chromaticity of the Planckian (blackbody) radiator at that temperature using the Kim et al. 2002 piecewise polynomial.

For T ≥ 4000 K (daylight range), returns the chromaticity on the CIE daylight locus (CIE 15.2), which is what the standard daylight illuminants D50, D55, D65, D75 are built from. xy(6500) returns approximately D65's chromaticity; xy(5000) approximately D50's.

Valid over [1667, 25000] K.

Arguments

  • kelvin is a number in [1667, 25000].

Returns

  • An {x, y} tuple of floats.

Examples

iex> {x, y} = Color.Temperature.xy(6504)
iex> {Float.round(x, 4), Float.round(y, 4)}
{0.3127, 0.3291}

iex> {x, y} = Color.Temperature.xy(2856)
iex> {Float.round(x, 4), Float.round(y, 4)}
{0.4471, 0.4075}

xyz(kelvin, luminance \\ 1.0)

@spec xyz(number(), number()) :: Color.XYZ.t()

Returns a Color.XYZ struct at the given colour temperature, with Y scaled to the given luminance (default 1.0).

Arguments

  • kelvin is a number in [1667, 25000].

  • luminance is the Y value to scale to. Defaults to 1.0.

Returns

Examples

iex> xyz = Color.Temperature.xyz(6504)
iex> {Float.round(xyz.x, 4), Float.round(xyz.y, 4), Float.round(xyz.z, 4)}
{0.9502, 1.0, 1.0883}