Spectroscopic ellipsometry is a useful technique for studying surfaces and thin films. It has been used to determine the real part and imaginary part of indices and thicknesses of thin layer synchronically. However, photometric ellipsometer can only measure cos Δ, cannot determine directly the value of Δ at each wavelength. We apply the Kramers-Kronig transformation relation to complex reflection ratio and derive Ψ-Δ relation. As an example, we approach a single layer film on a substrate.