Optical properties of a slightly absorbing film for oblique incidence

Approximate equations are derived for calculating the transmittance and reflectance of a slightly absorbing film when radiation is incident at an arbitrary angle. These formulas are compared with those derived from wave optics. Examination of the real and the imaginary parts of the complex phase change and the complex angle of refraction shows the simple equations to be consistent with the wave-optics formulation under the assumption that the imaginary part of the refractive index of the film is much smaller than its real counterpart.