Improving Modulation Transfer Functions of different materials

Compton Backscatter Imaging (CBI) is a decades old approach to single sided imaging where transmission imaging is not possible; it is often applied in security, infrastructure, and aerospace applications.  Historical applications have focused on its use in identifying specific material signatures or lack of signature within a target medium.  For example, the metal in a CBI image for airport screening would contrast dramatically against a person while narcotics or explosives would contrast brightly when smuggled in a car door.  Alternatively, an air gap or water retention would contrast differently than wooden cross ties on a railway inspection.   Recent research at the University of Florida has focused on the potential to differentiate between multiple materials often used in explosive device construction; specifically, lead, copper, steel, aluminum, and plastic.  Lead serves as a heavy metal signature, copper is used in most circuitry, steel can serve as shrapnel, aluminum is the primary casing for blasting caps, and plastic mimics the composition for a primary explosive.  Differentiating these materials and their respective impacts within a detector could provide greater detail to first responders, assisting their decisionmaking. In order to test previous detector response, researchers used a Bar Pattern approach to characterize the detector’s response in terms of a simple Modulation Transfer Function (MTF) and an Edge Spread Function (ESF).  This is a commonly used approach for characterizing detector response for railway inspection, where a uniform sampling and target medium is expected.  Furthermore, the MTF and ESF approaches used were based on a comparison of CBI MTF approaches versus conventional photography.1 While this approach provided an MTF and an ESF for each of the materials of interest, the MTFs demonstrated inconsistency between materials and measurement frequencies.  To improve upon previous results, researchers for this paper utilized an Edge-Method approach by material to obtain a more uniform MTF for each material.  Doing so will allow for a cleaner characterization of the detector in question, with the potential to help later analysis identify materials of concern within the targeted medium.