An immersible bioreactor illumination assembly is provided. An immersible bioreactor illumination assembly includes an optical waveguide with a cylindrical structure that possesses a known refractive index and causes light within the optical waveguide to longitudinally propagate through a length of the optical waveguide at a reflective angle of incidence upon meeting an inside surface of the cylindrical structure. A non-reflective surface is formed on an end of the optical waveguide for the light from an external light source propagating to that end of the optical waveguide. Diffusing structure is formed with an outer surface of the optical waveguide that alters the reflective angle of incidence. A bioreactor medium with a different refractive index than the known refractive index causes the light within the optical waveguide to diffuse at the diffusing structure through the inside surface of the optical waveguide and into the medium at a different angle of incidence.