A cylindrical light diffusing device comprising: a fiber with a non-circular fiber core, and a diffuser light blocking device comprising an end cap member having a length, a diameter, and a pocketing feature, wherein: a. the non-circular fiber core provides a "top hat" core irradiance distribution and has a latitudinal cross-sectional shape of a regular polygon; b. the fiber includes a light diffusing section having a diffusing proximal end surface, a diffusing distal end surface (312, 801), and internal scattering features distributed within the non-circular fiber core of the light diffusing section along the central axis of the fiber core, wherein the light diffusion section provides the "top hat" diffusing irradiance distribution, thereby limiting the variation of longitudinally, radially emitted irradiance from the light diffusing section to be within +/-15% of the average optical irradiance I 0; c. the fiber further includes a distal portion having an exterior shape and the distal portion includes the diffusing distal end surface (312, 801); d the pocketing feature of the diffuser light blocking device includes a side wall and an end reflective surface; e the shape of the pocketing feature corresponds to the exterior shape of the distal portion of the fiber; f. the pocketing feature is configured to engage the distal portion of the fiber; g. an overlapping section of the side wall of the pocketing feature is configured to surround a side wall of the distal portion and prevent at least 95% of light output from the distal portion from escaping out of the side wall of the distal portion; h. the end reflective surface is configured to block any forward propagating light output from the distal end surface (312, 801) and return at least 80% of light coming out of the distal end surface (312, 801) back towards the distal portion; i. the end cap member is thermally conductive, allowing heat generated by absorption of the light output from the distal portion to be dispersed throughout the end cap member; j. the length and the diameter of the end cap member provide an exterior surface area that is at least 1,000% of the surface area of the distal end surface (312, 801); and k. the diffuser light blocking device is configured to reduce creation of a diffuser irradiance hot spot. a. the non-circular fiber core provides a "top hat" core irradiance distribution and has a latitudinal cross-sectional shape of a regular polygon; b. the fiber includes a light diffusing section having a diffusing proximal end surface, a diffusing distal end surface (312, 801), and internal scattering features distributed within the non-circular fiber core of the light diffusing section along the central axis of the fiber core, wherein the light diffusion section provides the "top hat" diffusing irradiance distribution, thereby limiting the variation of longitudinally, radially emitted irradiance from the light diffusing section to be within +/-15% of the average optical irradiance I 0; c. the fiber further includes a distal portion having an exterior shape and the distal portion includes the diffusing distal end surface (312, 801); d the pocketing feature of the diffuser light blocking device includes a side wall and an end reflective surface; e the shape of the pocketing feature corresponds to the exterior shape of the distal portion of the fiber; f. the pocketing feature is configured to engage the distal portion of the fiber; g. an overlapping section of the side wall of the pocketing feature is configured to surround a side wall of the distal portion and prevent at least 95% of light output from the distal portion from escaping out of the side wall of the distal portion; h. the end reflective surface is configured to block any forward propagating light output from the distal end surface (312, 801) and return at least 80% of light coming out of the distal end surface (312, 801) back towards the distal portion; i. the end cap member is thermally conductive, allowing heat generated by absorption of the light output from the distal portion to be dispersed throughout the end cap member; j. the length and the diameter of the end cap member provide an exterior surface area that is at least 1,000% of the surface area of the distal end surface (312, 801); and k. the diffuser light blocking device is configured to reduce creation of a diffuser irradiance hot spot.