The use of diode array spectroradiometers for dosimetry in phototherapy

An evaluation of two diode array radiometers, an UV spectroradiometer, Type SC-MP-A, from 4D Controls (Redruth, UK) and an USB2000-UV-VIS spectrometer from Ocean Optics (Duiven, NL), was carried out at the Photobiology Unit, University of Dundee. Three parameters of the instruments' performance were investigated, having been identified as the most likely sources of error in phototherapy dosimetry: (1) calibration, (2) stray light rejection, (3) angular response. An assessment was then made of the reliability of this type of instrument for dosimetry in clinical practice by measurement of a selection of phototherapy sources, in direct comparison with calibrated radiometers. Both instruments were found to have significant stray light levels (SC: 13% and USB: 39%). The use of stray light compensation and a high output calibration source improves accuracy to within acceptable limits. Angular responses were satisfactory: f2 values (+/- 60 degrees) of 5.9% and 7.8% for SC and USB, respectively. The SC spectroradiometer is supplied as a calibrated instrument. Using the supplied calibration resulted in errors in measuring phototherapy sources of up to 44% in UVA. Alternative calibration reduced the error in measuring UVA and UVB sources to within 12%. The USB spectrometer was found to have insufficient responsivity in both UVB and UVA to provide reproducible measurements of most phototherapy sources.