Correlated color temperature of a light (spectral power distribution)

Calculate the correlated color temperature of a spectral power distribution. The temperature refers to which blackbody (planckian) radiator is most like the light.

The correlated color temperature is typically used characterize an illuminant. It measures the yellow-blue appearance of the light. Low temperatures (3500K) are yellowish and high temperatures (7000K)are bluish. A typical mix of blue sky and the sun is 5500 or 6500K.

The CCT calculation was developed by Wyszecki and Judd in the 1960s. It relies on the old (u,v) format, not the (u',v') format. See notes in the function xyz2uv .

The function spd2cct names a scene illuminant by its color temperature. This function is used when the user does not assign a name.

See also: spd2cct, xyz2uv, blackbody

Copyright Imageval Consulting, LLC 2013

Contents

ieInit

Plot the light's spectral energy

wave = 400:5:720;
spd = blackbody(wave,3500);

vcNewGraphWin;
plot(wave,spd);
grid on; xlabel('Wavelength (nm)'); ylabel('Energy (watts/sr/nm/m^2)')

Calculate the correlated color temperature (spd2cct)

cTemp = 3500;
d = blackbody(wave, cTemp);
fprintf('Estimated CCT %.1f and actual %.1f\n',spd2cct(wave,d),cTemp)

cTemp = 6500;
d = blackbody(wave, cTemp);
fprintf('Estimated CCT %.1f and actual %.1f\n',spd2cct(wave,d),cTemp)

cTemp = 8500;
d = blackbody(wave, cTemp);
fprintf('Estimated CCT %.1f and actual %.1f\n',spd2cct(wave,d),cTemp)

Estimated CCT 3493.1 and actual 3500.0
Estimated CCT 6497.0 and actual 6500.0
Estimated CCT 8518.7 and actual 8500.0

Calculate and plot several lights at once

cTemps = 4500:1000:8500;
spd = blackbody(wave, cTemps);

vcNewGraphWin;
plot(wave,spd);
grid on; xlabel('Wavelength (nm)'); ylabel('Energy (watts/sr/nm/m^2)')

Print out the list

for ii=1:length(cTemps)
    fprintf('Estimated CCT %.1f and actual CCT %.1f\n', ...
        spd2cct(wave,spd(:,ii)),cTemps(ii));
end

Estimated CCT 4496.0 and actual CCT 4500.0
Estimated CCT 5495.3 and actual CCT 5500.0
Estimated CCT 6497.0 and actual CCT 6500.0
Estimated CCT 7507.9 and actual CCT 7500.0
Estimated CCT 8518.7 and actual CCT 8500.0

Published with MATLAB® R2024a