In various embodiments, an illumination apparatus includes a substantially planar waveguide, a plurality of light sources embedded within the in-coupling region of the waveguide, and a film disposed over the out-coupling region of the waveguide. The film has a thickness less than approximately 100 μm and comprises or consists essentially of a photoluminescent material. The photoluminescent material converts a portion of light emitted by at least one of the light sources to a different wavelength. A surface of the out-coupling region has an average reflectivity of greater than approximately 90%. Light emitted by the light sources mixes with light converted by the photoluminescent material to form substantially white light, and the conversion efficiency of the illumination apparatus is greater than approximately 70%.