The dispersion state of composite materials is known to primarily govern their macroscopic properties. With nanoscopic fillers tiny fluctuations in the interactions among particles may even become the prominent parameter. In this article, the dispersion of carbon nanotubes within a polymer was studied by means of UV-visible spectroscopy and transmitting light microscopy. With thin films, it was found that all the measured absorbances obey a parallel model between the dispersed and the aggregated phases. A method could thus be proposed and validated to gain micrographies of the optical densities within the samples. The Beer-Lambert law was applied to the description of this solid/solid structure, leading to an extinction coefficient for carbon nanotubes comparable to that proposed in solutions. In conclusion, it is shown that one can obtain valuable information from the dispersed phase in optical micrographies, especially the effective filler concentration and a dispersion index in agreement with the literature. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1778-1786, 2013