A catheter enables real-time light measurements, for example, without limitation, diffuse reflectance, fluorescence and etc., from biological materials, such as tissue (including blood) and while performing RF ablation. The catheter tip design isolates illumination and collection paths such that light exits the catheter tip and travels through the tissue of interest (e.g., cardiac tissue or blood) before returning to the catheter tip. Such a design advantageously avoids saturation of the optical detector and ensures diffusion of the illumination light within the medium of interest. The catheter has a catheter body and a tip electrode. The tip electrode has an exterior shell, an inner layer of diffuse material and a hollow cavity and wherein the inner layer is configured to transmit light outside the tip electrode to a tissue via a set of illumination openings in the shell wall and the hollow cavity is configured to receive light from the tissue via a set of collection openings in the shell wall and the inner layer. An inner surface of the inner layer has a reflective coating to isolate light injected into the inner layer from light collected in the hollow cavity. There are a first optical waveguide extending between the catheter body and the tip electrode to inject light into the inner layer and illuminate the tissue and a second optical waveguide extending between the catheter body and the tip electrode to collect the recaptured light in the hollow cavity.
