The attenuation in the film can be ignored because the thickness of the film is small relative to the length of the transmission wave path (x 2 ≪ x 1 and x 3 ) and the attenuation coefficients of gases are typically much larger than solids. Thus, the contributions of the film to attenuation can be neglected (exp (-αx 2 ) ≈ 1 ≫ exp (-αx 1 ) and exp (-αx 3 )), and the expression for P obs simplifies to Equation .