FIELD: medicine.SUBSTANCE: group of inventions relates to medicine and medical equipment, namely to system and method of determining level of flowing medium in surgical cassette by means of projecting light from linear source of light on the cassette wall. System contains linear matrix of light sources, including first light source and second light source, located on vertical axis with respect to first light source. First light source projects first light beam on the first wall of cassette along first way of spreading. First way of spreading is characterised by such an angle of falling on the surface of the first cassette wall that, at least, larger part of first light beam is refracted, if border of separation of cassette material/first flowing medium crosses first way of spreading and passes through the second cassette wall. Second source of light projects second light beam on the first wall of cassette along second way of spreading. Second way of spreading is characterised by such an angle of falling on the surface of the first cassette wall that, at least, larger part of second light beam is internally reflected on the second wall of cassette, if border of separation of cassette material/second flowing medium crosses second way of spreading. System also includes linear matrix of sensors, which contains first section, located for reception of first light beam, and second section, located for reception of second light beam, and formation of outlet signal for indication whether each of first and second sections is lit. By means of claimed system realised is method, which lies in the following: multitude of light beams are irradiated on wall of surgical cassette on parallel vertically spaced ways of spreading, value of illumination of different vertically located sections of linear matrix of sensors of respective sensors, located for reception of multitude of light beams is determined, and level of flowing medium is determined by illumination of different vertically
