Abstract: A method and system for calibrating focal plane arrays wherein a reference source is periodically disposed between the scene and the detectors so that response measurements can be made at three or more different controlled flux levels. In a first embodiment, a passive infrared reference is used with multiple discrete reflectivity levels. By varying the reflectivity of passive sources during construction, the detectors see some portion of the self-emission of the source and some portion of the detector cryostat temperature. This information is used to obtain accurate calibration. A second embodiment uses continuously varying passive infrared references. Third and fourth embodiments replace the passive discrete and passive continuous infrared sources with active sources. Signal processing is provided to interpolate between the reference measurements.