One hypothesis:
If mathematica's using this set of formulae to convert from XYZ to RGB:
//This is coming from www.easyrgb.com
var_X = X / 100 //X from 0 to 95.047 (Observer = 2°, Illuminant = D65)
var_Y = Y / 100 //Y from 0 to 100.000
var_Z = Z / 100 //Z from 0 to 108.883
var_R = var_X * 3.2406 + var_Y * -1.5372 + var_Z * -0.4986
var_G = var_X * -0.9689 + var_Y * 1.8758 + var_Z * 0.0415
var_B = var_X * 0.0557 + var_Y * -0.2040 + var_Z * 1.0570
if ( var_R > 0.0031308 ) var_R = 1.055 * ( var_R ^ ( 1 / 2.4 ) ) - 0.055
else var_R = 12.92 * var_R
if ( var_G > 0.0031308 ) var_G = 1.055 * ( var_G ^ ( 1 / 2.4 ) ) - 0.055
else var_G = 12.92 * var_G
if ( var_B > 0.0031308 ) var_B = 1.055 * ( var_B ^ ( 1 / 2.4 ) ) - 0.055
else var_B = 12.92 * var_B
R = var_R * 255
G = var_G * 255
B = var_B * 255
AND, if their algorithm tries to normalize the result to have the highest R or G or B
value to be equal to 255
AND, if any RGB value below 0 is simply ignored and set to 0
THEN, 570 nm having the CIE xy coordinates:
x = 0.4441
y = 0.5547
Should give, after following these rules, the XYZ values:
X = 72.77 // this is proportionnal to Y
Y = 90.89 // this is to give the value 255 to the G value
Z = 0.2 // this is proportionnal to Y (and extremely low!)
These values give the following rounded RGB values at gamma = 2.4
R = 230
G = 255
B = - 471
Of course, if you cut the - 471 and replace it with 0, that makes quite a difference,
let's see what CIE L*a*b* basic delta E color difference formulae will say
(the delta E is just the Cartesian distance between 2 points in the CIE L*a*b* colorspace)
We call C the original conversion an C' the 'minus-cut' value
C L*= 96.367
a*= -26.924
b*= 163.337
C' L*= 95.305
a*= -31.712
b*= 92.307
The delta E is:
D = sqrt((96.367-95.305)^2+(-26.924+31.712)^2+(163.337-92.307)^2)
D = -71.199 (!!!!)
Knowing that a delta E superior to 1~2 is supposed to be perceptible, we see that here
the color difference is supposed to be HUGE
One can argue that a color difference formula applied in this context might not be appropriated, and be right thinking so, but it is still a good illustration of what cutting negative values will produce...
Strangely, the software from EasyRGB is giving the following values, which are more yellowish:
R = 250.46
G = 255
B = 0
Couldn't find why...
**
EDIT:
Anyway,
Never forget that fact that it is simply impossible to display any spectral color on a screen!
Spectral colors in RGB will produce Red/Green/Blue values above 255 or below 0, When it's RED = 380 or -600, the difference between what's displayed and what it should be is SIGNIFICANT.
A better way I know to render the color spectra on a computer screen is to compute a mixture of white light + spectral colors, like this guy Nick Spiker did:
The principle is: white light is added until the negative values become positive or 0 and until the RGB values are below or equal to 255
This rendering gives you a visual result which is close to what spectral lights viewed in daylight conditions look like, and it is more exact than anything else I.M.Opinion.
