Abstract: The catheter tip fits into a cavity in the boot and is held gently (in the longitudinal direction) by a detent formed within the cavity. A calibration substance faces the tip in a mechanically and optically standardized calibration relationship, to reflect light from within the catheter back into the catheter. The calibration substance is held in constant, precise contact with the tip--but passively, not by springs or other longitudinally forcible devices but by close fit between the tip and the precision-molded internal surfaces of the cavity. In the lateral direction the boot may tightly grip the tip, at a point where the optic fibers are protected against such force. To provide a reflection standard for calibration, the calibration substance is of standardized character and quality: it is preferably a homogeneous suspension of reflecting particles in translucent or transparent polymer.