SEVERAL X-RAY TECHNIQUES HAVE BEEN DEVELOPED FOR THE INSPECTION OF PARCELS AND LUGGAGE, INCLUDING DIRECT FLUOROSCOPY, LOW-LEVEL FLUOROSCOPY, PULSED X-RAY FLUOROSCOPY, FLYING SPOT X-RAY FLUOROSCOPY, SCANNED SENSOR FLUOROSCOPY, MULTIPLE ENERGY SYSTEMS, AND COMPUTERIZED TOMOGRAPHIC SCANNING. THE U.S. POSTAL SERVICE HAS BEEN A LEADER IN THE DEVELOPMENT OF VERY LOW-LEVEL X-RAY SYSTEMS, SYSTEMS OPERATING FAR BEYOND THE RADIATION LEVEL WHERE KNOWN DAMAGE OCCURS TO PACKAGE CONTENTS. THE INSPECTION OF PACKAGES FOR CONTRABAND REQUIRES THAT A PERSON BE ABLE TO SEE, AT A MINIMUM, THE DETAIL AND GRAY SCALE OR CONTRAST NECESSARY TO DEFINE THE SMALLEST ITEM OF INTEREST. IN THE CASE OF BOMBS, THIS GENERALLY MEANS BEING ABLE TO RESOLVE FUSE OR DETONATOR WIRES AGAINST BACKGROUNDS OF VARIABLE CONTRAST DUE TO THE LARGE RANGE OF X-RAY ATTENUATION POSSESSED BY DIFFERENT MATERIALS. AN X-RAY IMAGE IS FORMED BY AN OBJECT ATTENUATING RADIATION FROM A SOURCE THAT WOULD NORMALLY CAUSE A SURFACE SENSITIVE TO RADIATION TO EMIT LIGHT OR REACT IN A DETECTABLE MANNER. EXPOSURE LEVELS REQUIRED FOR FILM SAFE OPERATION USUALLY PRODUCE AN IMAGE ON CONVENTIONAL FLUOROSCOPIC SCREENS TOO DIM TO BE INTERPRETED BY THE NAKED EYE. TO VIEW AN IMAGE FORMED BY THE VARIED ABSORPTION OF X-RAYS BY DIFFERENT DENSITIES OF MATERIALS AND COMPONENTS COMPRISING AN OBJECT UNDER INSPECTION, THE X-RAY PATTERN MUST BE CONVERTED INTO VISIBILE LIGHT OR EQUIVALENT ELECTRICAL SIGNALS BY AN APPROPRIATE IMAGE CONVERTER. METHODS MOST COMMONLY USED FOR THIS PURPOSE INVOLVE EITHER FLUORESCENT SCREENS OR CRYSTAL SCINTILLATION DETECTORS. RECOMMENDATIONS FOR AN APPROPRIATE X-RAY SYSTEM FOR THE U.S. POSTAL SERVICE DEPEND PRIMARILY ON OPERATIONAL FACTORS ASSOCIATED WITH THE PARTICULAR INSPECTION SITUATION. HIGH-LEVEL DIRECT FLUOROSCOPY IS NOT RECOMMENDED FOR MAIL WITH UNKNOWN CONTENTS, EXCEPT UNDER EMERGENCY SITUATIONS. LOW-LEVEL REAL-TIME FLUOROSCOPY IS LIKELY TO HAVE THE LEAST COST PER INSPECTED ITEM OF ALL X-RAY SYSTEMS. FOR MORE STRINGENT OR DIFFICULT IDENTIFICATION REQUIREMENTS, SUCH AS POSSIBLE BOMBS IN RADIOS, MOTIONLESS IMAGING IS PROBABLY REQUIRED TO PROVIDE SUFFICIENT CONTRAST AND DETAIL. ILLUSTRATIONS AND REFERENCES ARE INCLUDED. (DEP)