[en] With carbon as an example, the possible accuracy of the thickness measurement of light transparent foils is demonstrated. With a quartz prism-spectrophotometer, the transmission characteristics of carbon foils in the energy range of 200 to 2500 nm was investigated. The transmittance as a function of carbon foil thickness was measured at different wavelengths. With these data the absorption coefficients and the reflectivities of carbon foils were calculated. Knowing these optical constants for one wavelength, it is easy to calculate the absolute thickness of a carbon foil by measuring only its transmittance for this monochromatic light. With this method carbon foils were investigated grown on different parting agents to see if there might be an epitaxial effect giving different crystalline clusters in the almost amorphous carbon foils