A system for measuring optical detector linearity according to the present invention employs a laser source that illuminates an integrating sphere. The sphere randomizes the laser signal phase and produces a uniform intensity over the sphere output. A collimator expands the sphere output for entry into an interferometer, where the incident optical energy is amplitude modulated in a sinusoidal fashion by a linear mechanical mirror movement. Optical band filters eliminate significant harmonic content being present on a pre-detected optical signal. Sampling of the detected signal energy is performed synchronous to the mechanical mirror position to assure sinusoidal response. The sampled signals are processed to separately determine signal harmonic components attributed to detector non-linearity and multiple laser reflections within the system. The system utilizes at least two measurements at two different laser intensities. An optional third measurement of background radiance may be applied to the first two measurements to enhance accuracy.