The interpretation of measurements of magnetic dichroism in angle-resolved photoemission from the valence bands of 3d ferromagnetic metals is complicated by the simultaneous presence of spin-dependent surface transmission of the spin-polarized photoelectrons into the vacuum. As a result, two separate mechanisms contribute to the change in the photoemission intensity spectra when the sample magnetization is reversed. The magnitude of the surface transmission effect in the "light" 3d metals has been measured using Cu(001) as a sample, and an intensity asymmetry of +or-3% has been found. This is smaller than, but approximately the same size as, the magnetic dichroism measured for iron and cobalt, and demonstrates experimentally that neither mechanism can be generally neglected in favor of the other.