Protein microarray “spots” printed by a microdroplet arrayer can be highly heterogeneous and uneven, which can be a source of noise for label-free detection of BNPs. Depending on the spotting solution contents and the surface material, deposited droplets may dry to form spots with small diffraction-limited islands (top left), striations (top center), multi-scale heterogeneities (bottom left), and isolated regions of high immobilization (bottom center, near the top, and right edges of the spot). They may also accumulate salt deposits (top right). Contrast these poor spots with the one at the bottom right, which is very smooth, uniform, and flat. Developing a virus particle counting software that can filter out all of these types of features has been a significant challenge. Shown regions are 200×200 microns.