where ΔR/R is the normalized change in reflectance, α (≡1/R×∂R/∂ε 1) and β (≡1/R×∂R/∂ε 2) are the "Seraphin coefficients" which contain filmstack information, and Δε 1 and Δε 2 are the pump induced changes in the complex dielectric function. However, in conventional photo-reflectance analysis, it is not necessary to independently determine the refractive and absorptive components (the first and second terms in ΔR/R, respectively) of the signal. Rather, a fit to the overall signal is performed using the third derivative functional form given by Aspnes. This fit procedure yields the interband transition energies, amplitudes, and widths. However, because the signal depends on the uniformity of the perturbation, the extraction of such parameters must be treated with care.