Abstract

Measurements of the light energy fluence rate in turbid media (e.g., biological tissues) require an isotropic probe, which is a small bulb of highly light scattering material mounted on an optical fiber. For an isotropic response the bulb must be nearly spherical and the photon mean free path in the bulb must be small compared with its diameter. Using an epoxy mixed with a highly light scattering powder we occasionally obtained probes of good quality. Recently, bulbs with a diameter of not more than 0.8 mm have been reproducibly turned on a lathe from a hard white plastic. The probe can be calibrated in a parallel light beam in air. In a medium with refractive index n larger than one, the sensitivity of the probe decreases because more scattered light can escape from the probe. A calibration factor as a function of nis shown. The calibration for measurements in diffuse light has been checked in both a plane and spherical geometry. The optical constants of Intralipid were measured, and with these the energy fluence rate in optical phantoms of Intralipid with and without the absorber was calculated and measured, and good agreement was found.

© 1988 Optical Society of America