A main digital-to-analog converter receives at least one input and generates an adjusted input. The foot-pocket formation defines a foot-receiving cavity in which a foot can be received. Further, line ends of positive poles at the output side of the smoothing circuits are connected to the input side of rectifiers that make only current in the forward direction pass through, and the output side line ends of the rectifiers are connected to a combination line. The arrangement is such that, whatever the direction from which the radiation is incident upon the active region, its path through the device before arriving at the active region will have similar characteristics to the path the radiation takes upon leaving the active region.