The present invention provides a frontal light diffusing device (600) for delivering a predetermined optical irradiance value to a targeted treatment area with a "top hat" profile, comprising: a non-circular core fiber section (604) having a proximal end, a distal end, an optical axis and a non-circular fiber core； and a lens component (606) having a lens or a collimation lens assembly (704)； wherein: the lens component or collimation lens assembly is physically attached to the distal end of the non-circular core fiber section； the non-circular fiber core has a latitudinal, cross sectional shape of a regular polygon. When the frontal light diffusing device is in light communication with a light source the lens or collimation lens assembly receives light rays exiting the distal end of the non-circular core fiber section and projects the light rays onto the targeted treatment area located at a predetermined standoff distance from the lens resulting in the targeted treatment area receiving a targeted "top hat" spatial irradiance distribution with less than +/- 20% of the predetermined optical irradiance value.
