A structure for testing a luminescent film includes a Lambertian light source, an integrating sphere having an input port, and a measuring device. The Lambertian light source includes a mixing chamber having an input port and an output port, and a light emitter coupled to the input port. During testing the luminescent film is positioned between the output port of the mixing chamber and the input port of the integrating sphere. The measuring device is optically coupled to the integrating sphere.