For single slabs of uniform material, such as bulk semiconductors, we derive closed-form expressions for absorption and reflection coefficients, and , respectively, in terms of measured reflectance and transmittance, and . The formula for can replace the several commonly used approximations for as a function of and in particular does not require , where is the thickness. Thus, it can be applied to weak impurity absorptions, such as Fe absorption in Fe-doped GaN. Finally, the real () and imaginary () parts of the index of refraction () can be obtained from and and agree well with and results obtained from other experiments. For multilayer structures, “effective” values of , , , and are obtained, but they can often be assigned to a particular layer. This technique has been successfully applied to many bulk and layered structures.