TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Claimed Method: The opening in the casing defines a first region of the sensor which is exposed to radiation over a pre-determined period of time and then the sensor and casing are moved relative to each other so that this first region is closed and a second exposure region is opened. This sequence is repeated a given number of times.Preferred Device: The sensor comprises a round bio-film which is sensitive to UV radiation and made up of different concentric zones. The casing is formed by upper and lower covers which can be interchanged depending on the type of radiation measurement being carried out, which can be an exposure or calibration process. The upper cover is an exposure cover (28) containing an off-center round window (27) or a calibration cover containing a C-shaped window, each window allowing UV radiation to reach distinct regions of the bio-film. The covers include a means for preventing the lateral spread of radiation inside the casing which could otherwise result in unwanted regions of the bio-film being exposed. The bio-film and lower cover (29) are stationary and the upper film is movable relative to both of them.The electromechanical system comprises a step motor (15) which actuates gear-wheels (17, 19), which in turn move the upper cover. The casing, electromechanical system and electronic programming system are all incorporated into a cylindrical housing with its top end sealed by the upper cover. A means is provided for securing the device into position on a suitable surface so that it can withstand poor weather conditions. The housing comprises upper and lower sections which are releasably connected to each other and includes an intermediate cover inside for separating the two sections and defining separate upper and lower compartments. The casing, electromechanical system and electronic programming system are housed inside the upper compartment (4) and the lower compartment contains the batteries for powering the device.Preferred Method : More than one concentric sensor region is used and previously exposed regions are blocked out.