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The finger is placed on the sensor cell. The sensor cell is composed of an adjacent-plates capacitor connected to a capacitance measurement circuit. The entire surface of the imager is covered by a thin layer of nonconductive oxide, labeled Si oxide. The distance between the ridge and Si oxide is labeled valley. When the finger is placed on the oxide surface, it perturbs the field lines of the individual sensor cells to varying degrees, depending on the distance between the ridges and valleys of the finger’s surface from the sensor cells.