The refractive index and extinction coefficient, "n" and "κ", cannot be measured directly. They must be determined indirectly from measurable quantities that depend on them, such as reflectance, "R", or transmittance, "T", or ellipsometric parameters, "ψ" and "δ". The determination of "n" and "κ" from such measured quantities will involve developing a theoretical expression for "R" or "T", or "ψ" and "δ" in terms of a valid physical model for "n" and "κ". By fitting the theoretical model to the measured "R" or "T", or "ψ" and "δ" using regression analysis, "n" and "κ" can be deduced.