An optical particle counter of the type first described by Umhauer (1983) was modified for the purpose of in situ measurements at high concentrations to achieve very low coincidence errors. Therefore, the optically defined measuring volume was reduced to 36 x 52 x 35 mu m(3) and additionally a digital signal processing (DSP) as described by Sachweh (1991) was applied to measure the fraction of coincident signals for different aerosols (quartz, Monospheres and glycerin) and number concentrations up to 2 x 10(6) particles cm(-3). Thus, errors from coincidence events in particle size distribution and number concentration could be corrected successfully. The fraction of coincident signals could directly be measured by the DSP. The results were found to be in good agreement with the theoretical model derived by Raasch and Umhauer (1984). From these experiments the upper concentration limit could be assessed to be approximately 10(7) particles cm(-3). A lower 50% size detection limit of 0.2 mu m was also determined for PSL spheres in comparison with a condensation particle counter. Performance comparisons are also reported for a commercial HC15 counter (Polytec) based on the same operating principle but "normal" sensing volume of 110 x 220 x 220 mu m(3).