Abstract: Method and apparatus for establishing a valid sample volume in a beam of radiation, wherein a sample stream is introduced substantially transversely to the radiation beam, an optical system is configured to produce images on first and second image planes, the two planes preferably being perpendicular to one another and established by means of a beam splitter, first and second photosensors being positioned in the image planes, the first photosensor having greater optical and an electronic gain and a masked portion, and the second photosensor being either unmasked or having masked portions corresponding generally to the unmasked portions of the first photosensor, the first photosensor being connected to comparator means and the second photosensor also being connected to the comparator means which accepts data only when the signal from second photosensor is of a greater magnitude than the signal from the first photosensor, whereby images on the photosensors from points outside the selected sample volume will be