A facial validation sensor includes an imaging element, a validating unit and a feedback unit. The above display element is composed of simple and easily available members, can be operated with a low voltage, exhibits high display contrast and satisfactorily high reflectance for the white display, and is reduced in the fluctuation of the white reflectance. A quality of seal is determined by measuring after formation of the cavities surface features of the cavities at a level of the one outer surface of the respective foil, and calculating based on the measured surface features an imprint depth of the cavity.