Abstract: A diffuser tip assembly is disclosed for generation of uniform cylindrical illumination from a fiber delivered source. Light propagating into the diffuser tip is initially mixed by a spatial overlap of reflections within a waveguide, reducing the sensitivity of illumination uniformity to the modal structure of fiber delivered light. The waveguide output propagates at least two passes through a reflective cavity having transmissive, light-diffusive walls, enabling highly uniform output. The diffuser tip can be configured to use low-absorbing materials for high power applications. In addition, the method of using visible light as an indicator for diffuser output is described. The combination of uniformity, low heat generation, and a visual indicator are intended to promote safety in a phototherapy procedure.