An apparatus for radiometric measurement of thin fluid films includes a housing (1) enclosing means of radiation, a radiation guide, a reflector (4) and one or more detectors (5). The means of radiation is normally an IR radiator (2). The radiation guide is either a radiation tube (3) or a radiation cone (7). The reflector (4) is of the type non-imagining optics. If there is more than one detector (5) normally at least one detector is a reference detector.