i want to convert srgb to CIElab color space without using srg2lab and through x,y,z values. so what will be reference white here???

RGB(1, 1, :) = [255 255 255];

rRGB=RGB(:,:,1);

gRGB=RGB(:,:,2); bRGB=RGB(:,:,3);

M = [0.4124564 0.3575761 0.1804375

0.2126729 0.7151522 0.0721750

0.0193339 0.1191920 0.9503041] ;

X1 = rRGB * 0.4124564 + gRGB * 0.3575761 + bRGB* 0.1804375;

Y1 = rRGB * 0.2126729 + gRGB * 0.7151522 + bRGB* 0.0721750 ;

Z1= rRGB * 0.0193339 + gRGB * 0.1191920 + bRGB* 0.9503041 ;

X= (X1./RGB(2))*100;

Y = (Y1./RGB(2))*100;

Z = (Z1./RGB(2))*100;

 yn=116 *(Y/Yn);

cn= 500*(((X/Xn)^1/3)-((Y/Yn)^1/3));

bn= 200*(((Y/Yn)^1/3)-(Z/Zn)^1/3); so what will be this xn,yn,zn or reference whites?????