An energy discriminating x ray detector has been developed for dual energy, scan projection digital radiography. The detector is comprised of a pair of x ray intensifying screen/linear photodiode arrays, aligned one behind the other. Energy discrimination is achieved by employing a low atomic number phosphor in the front screen and a high atomic number phosphor in the back screen. The x ray response, modulation transfer function, and defective quantum efficiency of the detector are reported along with the experimental methodology utilized for the measurements. Also presented is an analysis which indicates that in a typical patient's lung field, the detector can resolve the projected density (g/cm2 ) of a 3 mm thick, 1 cm2 area of bone to better than 1.5%. © 1987, American Association of Physicists in Medicine. All rights reserved.