If you want to calculate the CCT of the NPL white light you must first convolve it with the CIE color matching functions to get X Y Z. You can then plug those into any number of CCT calculators (brucelindbloom.com for example).
Code:
/* SPECTRUM_TO_XYZ
Calculate the CIE X, Y, and Z coordinates corresponding to
a light source with spectral distribution given by the
function SPEC_INTENS, which is called with a series of
wavelengths between 380 and 780 nm (the argument is
expressed in meters), which returns emittance at that
wavelength in arbitrary units. The chromaticity
coordinates of the spectrum are returned in the x, y, and z
arguments which respect the identity:
x + y + z = 1.
*/
void spectrum_to_xyz(double (*spec_intens)(double wavelength),
double *x, double *y, double *z)
{
int i;
double lambda, X = 0, Y = 0, Z = 0, XYZ;
/* CIE colour matching functions xBar, yBar, and zBar for
wavelengths from 380 through 780 nanometers, every 5
nanometers. For a wavelength lambda in this range:
cie_colour_match[(lambda - 380) / 5][0] = xBar
cie_colour_match[(lambda - 380) / 5][1] = yBar
cie_colour_match[(lambda - 380) / 5][2] = zBar
To save memory, this table can be declared as floats
rather than doubles; (IEEE) float has enough
significant bits to represent the values. It's declared
as a double here to avoid warnings about "conversion
between floating-point types" from certain persnickety
compilers. */
static double cie_colour_match[81][3] = {
{0.0014,0.0000,0.0065}, {0.0022,0.0001,0.0105}, {0.0042,0.0001,0.0201},
{0.0076,0.0002,0.0362}, {0.0143,0.0004,0.0679}, {0.0232,0.0006,0.1102},
{0.0435,0.0012,0.2074}, {0.0776,0.0022,0.3713}, {0.1344,0.0040,0.6456},
{0.2148,0.0073,1.0391}, {0.2839,0.0116,1.3856}, {0.3285,0.0168,1.6230},
{0.3483,0.0230,1.7471}, {0.3481,0.0298,1.7826}, {0.3362,0.0380,1.7721},
{0.3187,0.0480,1.7441}, {0.2908,0.0600,1.6692}, {0.2511,0.0739,1.5281},
{0.1954,0.0910,1.2876}, {0.1421,0.1126,1.0419}, {0.0956,0.1390,0.8130},
{0.0580,0.1693,0.6162}, {0.0320,0.2080,0.4652}, {0.0147,0.2586,0.3533},
{0.0049,0.3230,0.2720}, {0.0024,0.4073,0.2123}, {0.0093,0.5030,0.1582},
{0.0291,0.6082,0.1117}, {0.0633,0.7100,0.0782}, {0.1096,0.7932,0.0573},
{0.1655,0.8620,0.0422}, {0.2257,0.9149,0.0298}, {0.2904,0.9540,0.0203},
{0.3597,0.9803,0.0134}, {0.4334,0.9950,0.0087}, {0.5121,1.0000,0.0057},
{0.5945,0.9950,0.0039}, {0.6784,0.9786,0.0027}, {0.7621,0.9520,0.0021},
{0.8425,0.9154,0.0018}, {0.9163,0.8700,0.0017}, {0.9786,0.8163,0.0014},
{1.0263,0.7570,0.0011}, {1.0567,0.6949,0.0010}, {1.0622,0.6310,0.0008},
{1.0456,0.5668,0.0006}, {1.0026,0.5030,0.0003}, {0.9384,0.4412,0.0002},
{0.8544,0.3810,0.0002}, {0.7514,0.3210,0.0001}, {0.6424,0.2650,0.0000},
{0.5419,0.2170,0.0000}, {0.4479,0.1750,0.0000}, {0.3608,0.1382,0.0000},
{0.2835,0.1070,0.0000}, {0.2187,0.0816,0.0000}, {0.1649,0.0610,0.0000},
{0.1212,0.0446,0.0000}, {0.0874,0.0320,0.0000}, {0.0636,0.0232,0.0000},
{0.0468,0.0170,0.0000}, {0.0329,0.0119,0.0000}, {0.0227,0.0082,0.0000},
{0.0158,0.0057,0.0000}, {0.0114,0.0041,0.0000}, {0.0081,0.0029,0.0000},
{0.0058,0.0021,0.0000}, {0.0041,0.0015,0.0000}, {0.0029,0.0010,0.0000},
{0.0020,0.0007,0.0000}, {0.0014,0.0005,0.0000}, {0.0010,0.0004,0.0000},
{0.0007,0.0002,0.0000}, {0.0005,0.0002,0.0000}, {0.0003,0.0001,0.0000},
{0.0002,0.0001,0.0000}, {0.0002,0.0001,0.0000}, {0.0001,0.0000,0.0000},
{0.0001,0.0000,0.0000}, {0.0001,0.0000,0.0000}, {0.0000,0.0000,0.0000}
};
for (i = 0, lambda = 380; lambda < 780.1; i++, lambda += 5) {
double Me;
Me = (*spec_intens)(lambda);
X += Me * cie_colour_match[i][0];
Y += Me * cie_colour_match[i][1];
Z += Me * cie_colour_match[i][2];
}
XYZ = (X + Y + Z);
*x = X / XYZ;
*y = Y / XYZ;
*z = Z / XYZ;
}
