Active millimeter-wave imaging is in widespread use for security screening and other applications. The Pacific Northwest National Laboratory (PNNL) has developed a variety of microwave and millimeter-wave imaging systems and technology, including the cylindrical imaging technology that forms the basis of the L3/Leidos ProVision system. Since 2016, PNNL has been actively participating in a working group that is developing a proposed American National Standards Institute (ANSI) standard (N42.59) that will be used to evaluate and verify performance of active millimeterwave imaging systems used for security screening of humans. The standard is developing image quality tools (IQTs) that will be used to assess a variety of imaging metrics, such as lateral resolution, contrast, and depth resolution. Depth resolution is vital for high-performance microwave and millimeter-wave imaging because it enables precise focusing over a full 3D volume, and allows for differentiation of reflections from multiple surfaces, such as a layer of clothing over the human body. In this paper, depth resolution is analyzed using theoretical simulations and experimental 3D imaging studies. Presented results examine depth resolution using IQTs developed from a thin partially transparent film placed in front of a metallic surface, creating reflections that are laterally aligned and at variable separation. Coherence between these reflections is investigated as it complicates the interpretation of the imaging results.
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