An insight into the contributions of self-shielding and lamp reflectors to patient exposure in phototherapy units

Calibration of phototherapy equipment can prove to be difficult. One problem is that the self-shielding produced by a patient reduces the irradiance relative to that determined when the cabinet is empty. A model has been developed to determine the factor to apply to the irradiance measured with the cabinet empty to give the irradiance with the patient present, i.e. the self-shielding correction factor. The model assumes that the cabinet consists of a number of discrete infinite line sources backed by perfect mirrors. The patient is treated as a barrier that prevents some of these sources being seen by the detector in the mirror it faces. The model was tested using a Waldmann 8001 K unit and three UV meters for UVA and UVB sources. The measurements suggested some modifications to the model--for UVA multiple reflections were important and for UVB the reflectors were only 30% efficient. The correction factors obtained were 0.87 for UVA and 0.96 for UVB.