Performance of isotropic light dosimetry probes based on scattering bulbs in turbid media

In a previous paper the calibration of an isotropic light detector in clear media was described and validated. However, in most applications the detector is used to measure light distribution in turbid (scattering) media, that is, in tissues or tissue equivalent optical phantoms. Despite its small diameter (typically 0.8 mm), inserting the detector in a turbid medium may perturb the light distribution and change the fluence rate at the point of measurement. In the present paper we estimate the error in the fluence rate measured by a detector in turbid media after calibration in a clear medium (air), using an optical phantom and detector bulbs of different optical properties. The experimental results are compared with calculations using the diffusion approximation to the transport equation in a spherical geometry. From measurements in optical phantoms and the results of the calculations it appears that introduction of the detector into a water-based turbid medium with refractive index, absorption- and scattering coefficients different from those of the detector bulb may require corrections to the detector response of up to 10-15%, in order to obtain the true fluence rate in that medium. The diffusion model is used to explore the detector response in a number of tissues of interest in photodynamic therapy, using tissue optical properties from the literature. Based on these model calculations it is estimated that in real tissues the fluence rate measured by the detector is up to 3% below the true value.