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Daylights, 4000K to 25000K
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In our Ultimate Coloring Database, we present the CIE references S0, S1, and S2 for calculating Daylight spectra of any color temperature. Until now, the exact way to do the calculation has been a mystery to me. No longer: Danny Rich of Datacolor has contributed the following.
This is the process as documented in CIE Publication 15.2:
For correlated color temperature (T) between
4000K and 7000K:
x = -4.6070*(10^9/T^3) + 2.9678*(10^6/T^2) + 0.09911*(10^3/T)
+ 0.244063
or for correlated color temperature (T) between
7000K and 25000K:
x = -2.0064*(10^9/T^3) + 1.9018*(10^6/T^2) + 0.24748*(10^3/T)
+ 0.237040
y = -3.000*(x^2) + 2.870*x - 0.275
M1 = (-1.3515 - 1.7703x + 5.9114y)/(0.0241+ 0.2562x - 0.7341y)
M2=(0.0300 - 31.4424x + 30.0717y)/(0.0241 + 0.2562x - 0.7341y)
Thus the only unknown is T, the correlated color temperature which is supplied by you. Then, for each wavelength, the CIE's S components are used in:
S(wl) = S0(wl) + M1*S1(wl)+M2*S2(wl)
In this example, assume we want the distribution of D5000 (D50).
x = -4.6070*0.008 + 2.9678*0.04 +
0.09911*0.2 + 0.244063 = 0.34574
y = -3.0 *( 0.34575* 0.34574) + 2.870 * 0.34574 - 0.275 = 0.35867
M1 = 0.156680 / -0.150621 = -1.040476
M2 = -0.055079 / -0.150585 = 0.365767
I shall leave the wavelength (wl) by wavelength calculations to the reader.
Danny Rich, DannyRich@aol.com
Datacolor International: Home Page at
http://www.datacolor.com/index.html
If the above is too much bother, Dave Porter, dporter@voicenet.com, has calculated 62 daylights from 4000 to 25000K at 5nm increments from 300 through 830nms. They are posted at the links below: