Abstract: A radiometrically stable, spectrally tunable, solid-state source combines the radiometric outputs of individually controlled, narrow bandwidth, solid-state sources (e.g., LEDs) with different spectral distributions in an integrating sphere so as to approximate any desired spectral distribution. By using a sufficient number of independent solid-state source channels, the source can be tuned to approximate the spectral distribution of any desired source distribution. A stable reference spectroradiometer, integrated into the solid-state light source, measures the spectral radiance or irradiance and is used to adjust the output of the individual channels of the individually controlled sources.