The integrated irradiance (energy fluence rate) within tissue can exceed the incident irradiance due to backscattered and multiply reflected light near the sample surface. This was studied quantitatively using pulsed photothermal radiometry, which measures blackbody radiation emitted by a sample during and after absorption of an optical pulse. Aqueous gels containing absorbing dye with or without various scattering materials were studied using a fast sensitive IR detector system and 1-μs tunable pulsed dye laser. For nonscattering samples, the temperature transient (T-jump) due to absorption of a laser pulse was consistent with Beer’s law for homogeneous absorbing media. When scattering was present, increases of up to almost an order of magnitude in the T-jump were observed. For a given absorption coefficient, there was a proportional relationship between the increase in the T-jump and the sample’s diffuse reflectance. A model describing the reflectance of diffuse radiation at the sample boundary was derived to explain this result. To test the model, the refractive index was varied with air as the external medium and was also matched to that of BaF2 as the externalmedium. The subsurface fluence is, to a reasonable approximation, given by E ≅ E0(1 + 2bR), where E0 is the incident fluence of an infinitely wide collimated beam, b is a coefficient strongly dependent on only the refractive index, and R is the measured diffuse reflectance of the sample. This study shows that irradiance within tissues can greatly exceed the irradiance of incident collimated light, an effect that should be accounted for in photomedical dosimetry or research.
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