Abstract: A portable apparatus for uniformly illuminating a detector that includes an array of charge-coupled devices (CCDs) is provided for facilitating calibration of the respective gains of the CCDs. The illumination apparatus includes a housing, such as a cup-shaped housing, which defines an internal cavity and which opens through a first end. The illumination apparatus also includes a light source, such as a photoluminescent sheet, that is disposed within the internal cavity for uniformly illuminating the plurality of CCDs to thereby permit the CCDs to be calibrated. The illumination apparatus is also adapted to be mounted to the detector, such as by means of a mounting ring, in much the same fashion that a camera lens is attached to a camera. As such, the illumination apparatus can be readily mounted to a detector, such as a CCD camera, such that the array of CCDs can be calibrated in an efficient manner in the field, i.e., without requiring the detector to be transported to a laboratory for calibration.