A system (10) for calibrating an image detector includes a calibration platform (16) supporting a calibration element (18). The calibration element (18) may be made up of a first portion (20) and a second portion (22), each having a curved surface (20A, 22A) to appropriately alter characteristics of reference light levels (33) generated by a reference source (24). The second portion (22) may have its surface positioned differently to the focal plane array (14) than the first portion (20) for enhanced calibration operation. The reference source (24) generates the reference light levels (33) over a range of temperatures to mimic the image energy generated by the scene (12). The reference source (24) may also include a reference portion element (26) that transmits reference light energy (33) generated by the reference source (24) or reflects scene base energy (34) generated by the scene (12) and allowed to pass through the first portion (20) and the second portion (22). The calibration platform (16) operates to position the first portion (20) and the second portion (22) in front of the focal plane array (14) during calibration operation and remove the first portion (20) and the second portion (22) from in front of the focal plane array (14) during normal image detection.