A system and method for controlled illumination in a bioanalysis or other system where excitation of fluorescent molecules is desirable. In an embodiment, an illumination system is described which can provide excitation light at a controlled intensity to provide quantitative results. In an embodiment, a solid state light engine is described which includes a plurality of color channels each providing light output suitable for exciting a fluorescent molecule, a light to frequency converter which receives a portion of the light output, a counter which maintains a count of a signal from the light to frequency converter, and a light intensity circuit, responsive to the counter, which adjusts the color channels to control the intensity of the light output.