Lab color space

Hi,

I need to understand the formula you use to transform colorMacbeth chart into La*b* color space.
I had an old code that doesn't match with the one in Imatest.
This old code is a reference for us, but we wish to use Imatest instead.
The formula we used is from : http://www.tsi.enst.fr/tsi/enseignement/ressources/mti/RVB_ou_LAB/html/colorspace.html

First we convert our RGB data into XYZ:
The range of the RGB and XYZ is [0:1].
The matrix is:
X = 0.618 * red + 0.177 * green + 0.205 * blue
Y = 0.299 * red + 0.587 * green + 0.114 * blue
Z = 0.056 * green + 0.944 * blue

Second we transform from XYZ into La*b* (using illuminant D65 (95.04,100,108. Cool

)
ty = Y / 100
     if(ty > 0.008856)
   L = 116 * pow(ty, 1/3) - 16
     else
     L= 903.3 * ty

tx = X / 95.04
tz = Z / 108.8

     if(tx > 0.008856)
   fx = pow(tx, 1/3)
else
      fx = 7.7787 * tx + 16/116
if(ty > 0.008856)
      fy = pow(ty, 1/3)
else
        fy = 7.7787 * ty + 16/116
if(tz > 0.008856)
      fz = pow(tz, 1/3)
     else
     fz = 7.7787 * tz + 16/116

a = 500 * (fx - fy)
b = 200 * (fy - fz)

Do you see why our code and Imatest code is different ?
Thanks for your help ;-)

Cheers,
Nicolas.