Abstract: The invention relates to an assembly and method for assessing a radiation-sensitive web parameter, such as the thickness of a polymeric film, by passing a web inwardly along a first path and withdrawing the web along a substantially parallel second path between a radiation source (25) and detector (26) mounted on the respective limbs (12, 13) of a stirrup cradle (10), monitoring radiation transmitted through both thicknesses of the web, and, preferably, traversing the cradle across the web paths. The invention enables a constant spatial relationship to be maintained between the source and detector, thereby improving the accuracy of the measurement technique, and is particularly suitable for assessment of the thickness of polymeric packaging films.