The transmittances of coated paper and base paper were measured in a narrow wavelength range over which the absorption of the coating was high and the base paper's absorption was low. The effect on the accuracy of the large spatial variations of the reflectance of the paper was eliminated by the application of a highly reflective, partially transparent plate. The plate was placed between the radiation source and the paper. Because of the high reflectance of the plate, most of the radiation that was transmitted by the plate was pumped through the paper. Based on a model of diffuse transmittance of weakly absorbing coated media, it was shown that there was a nearly linear correlation between coating weight and relative transmittance. After experimental confirmation, the results were used to develop an apparatus for the measurement of coating weights during the paper coating process. Two measuring units were installed along the paper pass for measuring the transmittance of the base paper and of the coated paper, respectively. The units were interfaced with a computer that was programmed to calculate the coat weight and display the result on monitors.