Collimated transmission

When measuring the collimated transmission, the unscattered portion of the light is detected from a collimated illuminated sample. The extinction coefficient can be determined directly from this measured quantity according to the Lambert-Beer law. The extinction coefficient is the sum of the absorption and scatteringcoefficient. The measuring apparatus must be designed in such a way that light scattered in the sample in the forward direction is not detected. This can be achieved by a large distance between the sample and detector or a Fourier

optic in front of the detector. With a standard photometer, which does not take this into account, incorrect values are measured for scattering samples, i.e. not the extinction coefficient. The samples used must have a small optical thickness so that the unscattered portion can be measured. By measuring polystyrene spheres of known diameter, the measurement could be successfully verified with Mie theory calculations.