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ciexy2cct

The ciexy2cct function calculates the Correlated Colour Temperature (CCT) T_{\mathrm{cp}} from given CIE 1931 chromaticity coordinates x and y, also known as normalized tristimumuls values. Any errors in the data set or in results generated with the Lighting Toolbox are not in the liability of the CIE nor me, see licence.

See also: cieuv2cc

Usage:

Tcp = ciexy2cct(x,y,'method')

Where:

ParameterDescription
TcpIs a scalar or vector containing the resulting Correlated Colour Temperature(s) (CCT) in K.
x and yAre the input scalars or vectors containing the CIE 1931 chromaticity coordinates x and y.
'method'
(optional)
Specifies the determination method:
‘Robertson’: (default) Robertson’s calculation algorithm, formerly the only recommended algorithm by the CIE. This method is fast and quite accurate.
‘exact’: shortest distance method as described in CIE 15:2018, very accurate but comparably slow. Results may vary with different implementation methods.

Both methods return NaN if the colour coordinates lie outside the meaningful interval of \Delta uv < 0.05 from the planckian locus in CIE 1960 chromaticity diagram.

Examples

Correlated Colour Temperature T_{\mathrm{cp}} for x = 0.3145 and y = 0.2567 using ‘Robertson’ method:

x = 0.3145;
y = 0.2567;
Tcp = ciexy2cct(x,y)

Result:

Tcp = 7511

Correlated Colour Temperature T_{\mathrm{cp}} for x = 0.3145 and y = 0.2567 using ‘exact’ method:

x = 0.3145;
y = 0.2567;
Tcp = ciexy2cct(x,y,'exact')

Result:

Tcp = 7516.7

Correlated Colour Temperature T_{\mathrm{cp}} for several chromaticity coordinates using ‘Robertson’ method:

x = linspace(0.3,0.4,5)
y = linspace(0.3,0.35,5)
Tcp = ciexy2cct(x,y,'Robertson')

Result:

x =

   0.3000   0.3250   0.3500   0.3750   0.4000

y = 
  
   0.3000   0.3125   0.3250   0.3375   0.3500

Tcp =

   7738   5906   4671   3825   3246

References

CIE 15:2018: Colorimetry, 4th Edition. Commission International de l’Éclairage (CIE), Vienna Austria, 2018, ISBN: 978-3-902842-13-8 , (DOI: 10.25039/TR.015.2018).

A. R. Robertson: Computation of Correlated Color Temperature and Distribution Temperature. In: Journal of the Optical Society of America, vol. 58, no. 11, pp. 1528-1535, 1968, (DOI: 10.1364/JOSA.58.001528).

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