A system for plant growth lighting comprises a generally planar light guide sheet having an illumination surface, a back surface situated opposite the illumination surface, and at least one pair of opposingly disposed peripheral edge portions. The light guide sheet includes a plurality of structures to scatter and mix light received therein. Light is received within the light guide sheet as emitted from a series of Light Emitting Diode light sources (LEDs) selected to provide a photosynthetically active radiation (PAR) value. The series of LEDs may be located on the opposingly disposed peripheral edge portions, such that they direct light towards the other of the opposingly disposed peripheral edge portions. A reflective sheet is overlaid in optical communication with the back surface of the light guide. Light as received from the LEDs within the light guide is scattered and randomly mixed by the plurality of structures therein. A first amount of the scattered and randomly mixed light is directly propagated generally orthogonally through an illumination surface of the light guide, while another amount is reflected from the reflective sheet on the back surface of the light guide before being re-directed orthogonally through the illumination surface.
