The presented optical multimode online probe uses telecentric illumination with a high performance micro LED (493 lm) and objectspace-telecentric objective to analyze particles online and in situ. The relative measurement errors are between < 2 - 5% (transmitted light) and below 12% (incident light, with regard to the particle diameter). The distance transform approach for transmitted-light images is extended for the analysis of diffuse incident-light images by an adaptive pre-segmentation approach based on the random forest classifier algorithm. The error with regard to the pixel number is typically between 1 to 6 %, while the evaluation of particle diameters gives deviations up to 12.9 %.