Exact formulas describing reflectance and transmittance R and T of the system absorbing film%substrate%back ambient are examined. Basing on the consideration that, unlike R and T, the quantity T%(l-R) does not oscillate with wavelength, the entity of terms entering the full equation are evaluated, and some approximations are made. As a result, a non-oscillating analytical equation is obtained, that can be inverted and therefore utilized to compute the film absorption coefficient with an accuracy that is better than 3 %for α>103cm-1. The effect of further approximations, like fixing the refractive index of the film and the substrate, is also evaluated. A comparison with a normally used, simplified formula, as well as an example of application, are also reported.