The device is formed in a casing including a support, a spacer body, and a mirror element fixed together. A light-emitting element and a light-receiving element are arranged on a bearing surface of the support and face a reflecting surface of the mirror element. The light-emitting element is configured to generate infrared radiation, and the light-receiving element is configured to receive light radiation reflected by the reflecting surface. The spacer body has an emission opening housing the light-emitting element and a reception opening housing the light-receiving element; the reception opening comprises a radiation-limitation portion configured to enable entry of reflected light radiation having an angle, with respect to a normal to the bearing surface, of less than a preset value.