We describe a method to characterize spatial or temporal changes in the optical properties of turbid media using diffuse reflectance images acquired under broad-beam illumination conditions. We performed experiments on liquid phantoms whose absorption (μa) and reduced scattering (μs′) coefficients were representative of those of biological tissues in the near infrared. We found that the relative diffuse reflectance R depends on μa and μs′ only through the ratio μa/μs′ and that dependence can be well described with an analytical expression previously reported in the literature [S. L. Jacques, Kluwer Academic Dordrecht (1996)]. We have found that this expression for R deviates from experimental values by no more than 8% for various illumination and detection angles within the range 0°–30°. Therefore, this analytical expression for R holds with good approximation even if the investigated medium presents curved or irregular surfaces. Using this expression, it is possible to translate spatial or temporal changes in the relative diffuse reflectance from a turbid medium into quantitative estimates of the corresponding changes of (μa/μs′)1/2. In the case of media with optical properties similar to those of tissue in the near infrared, we found that the changes of μa/μs′ should occur over a volume approximately 2 mm deep and 4 mm × 4 mm wide to apply this expression.
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