The present invention provides a diffuser light blocking device comprising an end cap member (820) having a pocketing feature (821) that has a side wall (822) and an end reflective surface (810); the pocketing feature's shape corresponds to exterior shape of distal portion (830) of a diffuser (800) having a distal end surface (801); the pocketing feature engages the distal portion; an overlapping section (815) of the pocketing feature's side wall surrounds the distal portion's side wall (802) and prevents at least 95% of the light output from the distal portion from escaping out of the distal portion's side wall; the end reflective surface blocks any forward propagating light output from the distal end surface and returns at least 80% of light coming out of the distal end surface back towards the diffuser; the end cap member is thermally conductive; the end cap member's length (831) and diameter (832) provide an exterior surface area that is at least 1,000% of the surface area of the distal end surface; and the device reduces generation of diffuser irradiance hot spots.