Converting image class from single to double loose some data?

I have hdr matrix created through makehdr command. This matrix is mxnx3 of class single.

I want to convert the sRGB color space to XYZ color space to get the luminance information.

hdr=makehdr(file); cform = makecform('srgb2xyz');

a= applycform(hdr,cform); % with hdr L=a(:,:,2); Error using applycform Expected input number 1, IN, to be one of these types: double, uint8, uint16 Instead its type was single.

a1= applycform(hdri,cform); % with hdri=double(hdr) L1=a1(:,:,2); gave me values of the ,matrix L1=1 which is not expected.

I need a help please.