Abstract: An optical probe structure for illuminating and collecting returned light from a region of tissue. The optical probe is configured with illumination and collection windows to controllably collect light from the region of tissue in vivo. The probe is designed to have a defined spacing between the fiber or fibers connected to the light source, and the fiber or fibers connected to the optical detector, so that light reaching the detector has passed through a localized region of tissue of a small and well-defined size or dimension. The probe can assay or detect tissue cell distribution, a tissue constituent such as hydrogen ions (pH), lactate, or an endogenous tissue component such as hemoglobin, myoglobin, or lipids in plaque. Measurements may indicate conditions such as metabolic state, stress, shock or ischemia in different tissues.