a process is provided for detecting small particles on wafer surfaces by irradiating the wafer surface with two light beams having a small difference in wavelength , collecting the light scattered by the wafer in at least one direction , separating the collected light into two component beams having the wavelengths of the irradiating beams , and comparing the intensities of the two component beams . the intensities of the component beams are transduced to digital signals which are fed to a comparator . an apparatus is also provided which comprises a stage for supporting a wafer , laser source and optics for generating two laser beams having different wavelengths , superimposing them and scanning the wafer with them , a sensor for sensing the light scattered by the wafer and separating it into two components having the said wavelengths , an a / d converter for generating digital signals corresponding to said components and a comparator for analyzing whether said signals indicate the presence of small particles .