A formalism for calculating the reflectance and transmittance of optical media consisting of discrete elements is described, in the case where the reflected and transmitted beams from each element are incoherently related to those from the other elements. The elements may be absorbing and may have unequal reflectances from each side. The formalism of Kubelka and Munk [P. Kubelka and F. Munk, Z. Technichse Physik 12 (1931) 593] originally developed for scattering media, is shown to give equivalent results. The discrete formalism is applied to media consisting of stacked slides, and to combinations of interference filters. The filter example has particular relevance to the problem of eye sight protection against lasers. Results indicate that the high reflectance associated with many narrow band filters can lead to a substantial increase in transmission when more than one filter is used in combination. Experimental measurements are used to illustrate and verify the theory.