Recent developments render untenable a proposed method of distinguishing between intrinsic and extrinsic infrared absorption on the basis of the proposed temperature dependence. However, when the proper temperature dependence of multiphonon absorption is accounted for and the possibility of other intrinsic processes is taken into account, thetemperature and frequencydependence of the absorption of both the best available and intentionally imperfected crystals should be useful in studying extrinsic processes.