Abstract: An optical system suitable for determining a characteristic as a function of time of at least a part of a liquid volume comprising a plurality of objects. The optical system provides a fast detection of a change in the liquid volume. The optical system comprises—an optical detection assembly comprising at least one image acquisition device configured to acquire images of an image acquisition area; —a sample device comprising at least one sample container suitable for holding a sample of said liquid volume; —a translating arrangement configured to translate said image acquisition area through at least one part of said sample container to perform a scan along a scanning path through said part of said sample container; and—an image analyzing processing system.