The importance of radiometer angular response for ultraviolet phototherapy dosimetry

The influence of the angular response of radiometer probes on measurements of irradiance in ultraviolet phototherapy has been studied. Irradiance measurements were made using nine ultraviolet (UV) radiometers employed by phototherapy centres in Scotland and Northern Ireland, and compared with measurements made using two spectroradiometers. The light sources used were UVB TL01 fluorescent lamps, arranged in different geometries. Irradiances within TL01 whole body treatment cabinets were assessed based on a comparison with one of the spectroradiometers. The results show variations of 50% in cabinet irradiance measurements made by different radiometers, even when they were calibrated using the same source geometry and spectroradiometer. Differences in radiometer probe design and construction lead to an under- or over-response at angles of incidence greater than zero. Angular responses of different probes were assessed using banks of fluorescent lamps. The differences found are large enough to account for the variations in measurements of cabinet irradiance. The variations in irradiance measurements are significant in terms of planning and monitoring patient exposure during TL01 phototherapy. Accurate dosimetry can only be achieved if radiometer probes have a good cosine response and recommendations are made for better calibration techniques.