Calibrated infrared and range imaging sensors are used to produce a true-metric three-dimensional surface model of any body region within the fields of view of both sensors. The retaining ring has holding lugs whose circumcircle can be changed in diameter D1 or D2 by a resilient reversible changeover to selectively hold the runner plates together or release the holding. The layer is applied as a liquid to either the plate or the membrane and preferably is polymerized prior to assembly of the stack. The housing comprises two housing parts joined with a seal. In this arrangement, preheating is performed with a converter frequency, raised by comparison with continuous operation, and with the aid of a preheating transformer TR2. The emitter of the second bi-polar transistor may also be electrically connected to the base of the first bi-polar transistor.