The present invention provides a cylindrical light diffusing device (300) comprising a non-circular core fiber (302) having (i) a fiber core (350) that provides a "top hat" core irradiance distribution； optionally (ii) light blocking means (314) that prevent frontal light emisson from distal end of the non-circular core fiber (302)； and (iii) a light diffusing section (308) having a diffusing proximal end (310), a diffusing distal end (312), and internal scattering features (362) distributed within the fiber core (350) of the light diffusing section (308) along central axis (364) of the fiber core (350), wherein the light diffusion section (308) emits irradiance in a radially symmetric longitudinally "top hat" diffusing irradiance distribution. The disclosure also provides a device (600) comprising a non-circular core fiber section (604) and a lens (606), wherein the non-circular core section has a "top hat" core irradiance distribution and the device provides a "top hat" spectral irradiance distribution at a targeted location. The disclosure also provides a devie (700) comprising an optical fiber (702) and a collimation lens assembly (704) that includes a variable aperture (714) that blocks portions of light outcoupled from the optical fiber thereby allowing only a central portion of the light to exit from the variable aperture resulting in a flat irradiance distribution.
