A new fiber optic detector to measure optical radiant energy fluence rates in tissue has been developed by incorporating a fluorescent dye into the rounded tip of a polymethylmethacrylate cylindrical extension on a quartz optical fiber. Model probes were built to test the concept. Nile Blue A free base was used as the fluorescent dye with the absorption maximum blue-shifted to 495 nm, giving an emission maximum at 580 nm. Fluorescence (>570 nm) was monitored using a photodiode. The response was linear with modest fluence rates in the 1 mW/cm2 to 700 mW/cm2 range. The angular response of the probe was investigated using a 3 mm diameter scaled-up model. The influence of dye concentration, geometrical shape and distance between fluorescence tip and fiber were evaluated. A 3 mm long rounded tip, separated by 7 mm from the distal end of the fiber, was isotropic in angular response to within ± 10% between 0° and 150° of the forward direction.