A device for limiting a measurement volume in an optical measurement system, e.g. for the measurement of one or more particles in the measurement volume with • an optical imaging system with an object plane in the measurement volume and an image plane, • an optoelectronic surface sensor with surface segments (12, 13) comprising at least one focus segment and at least one edge segment, which is arranged in the image plane such that the imaging system images the measurement volume above all onto at least one focus segment and a surrounding area of the measurement volume above all onto at least one edge segment and • an electronic evaluation apparatus or electronic data processing apparatus connected electrically with the surface segments (12, 13), characterized in that • the evaluation apparatus or data processing apparatus is designed to evaluate the electronic signals delivered by the at least one focus segment and the at least one edge segment and to detect whether they originate from a light source arranged in the measurement volume and to vary the measurement volume using hardware and/or software in the following manners: • through the free allocation of the individual surface segments (12, 13) to the at least one focus segment and to the at least one edge segment and/or • through freely selectable and adjustable aplification a.) of the individual signals of at least one focus segment, b.) of the total signal of all focus segments, c) of the individual signals of at least one edge segment and d.) of the total signal of all edge segments, • as well as through freely selectable combinations of amplified individual signals or through freely selectable and adjustable combinations of both total signals and their ratio with respect to each other. • an optical imaging system with an object plane in the measurement volume and an image plane, • an optoelectronic surface sensor with surface segments (12, 13) comprising at least one focus segment and at least one edge segment, which is arranged in the image plane such that the imaging system images the measurement volume above all onto at least one focus segment and a surrounding area of the measurement volume above all onto at least one edge segment and • an electronic evaluation apparatus or electronic data processing apparatus connected electrically with the surface segments (12, 13), characterized in that • the evaluation apparatus or data processing apparatus is designed to evaluate the electronic signals delivered by the at least one focus segment and the at least one edge segment and to detect whether they originate from a light source arranged in the measurement volume and to vary the measurement volume using hardware and/or software in the following manners: • through the free allocation of the individual surface segments (12, 13) to the at least one focus segment and to the at least one edge segment and/or • through freely selectable and adjustable aplification a.) of the individual signals of at least one focus segment, b.) of the total signal of all focus segments, c) of the individual signals of at least one edge segment and d.) of the total signal of all edge segments, a.) of the individual signals of at least one focus segment, b.) of the total signal of all focus segments, c) of the individual signals of at least one edge segment and d.) of the total signal of all edge segments, • as well as through freely selectable combinations of amplified individual signals or through freely selectable and adjustable combinations of both total signals and their ratio with respect to each other.