The characteristics of a defect counting tool, COMNET< based on scattering light measurement, is presented. This prototype supports the development of defect-free EUV blanks. Thanks to new improvements, it becomes possible to detect PSL particles having a diameter as low as 100 nm, with a video CCD camera, on silicon substrates or on EUV blanks. To reach this sensitivity, one of the enhancements consists in a laser irradiation close to 65°. The present configuration and the use of a CCD camera, with a variable exposure time, should lead to the detection of 80 nm PSL particles deposited on silicon substrate. This extrapolation is based on experimental results and on a simple model. To detect 100 nm particle and smaller particles, it is essential to reduce the level of stray light and to increase the signal to noise ratio. In our application, the stray light essentially comes from three sources: the noise induced by the roughness of the sample, the Rayleigh scattering of the atmosphere, and the stray light in the room. The restrictions induced by these phenomena are described in some detail. All the improvements are not only available for the characterization of silicon substrates but also for transparent blank substrates and for EUV mask blanks. The additional noise induced by the tranparent substrate is analyzed. The defects, whatever the compoent sizes and the component shapes can be detected. A cross characterization achieved with a commercial tool on silicon substate is reported. Counting measurements performed on EUV blanks are shown. Furthermore, a more explicit definition of added defects is proposed.