A multipart cooled piston for a combustion engine has a piston upper part of steel comprising a combustion bowl and a ring wall with ring belt, and a piston lower part comprising a piston skirt, pin bosses for receiving the piston pin connecting the piston to the connecting rod, and pin boss supports connected to the piston skirt. Then, in a second step, if the temporary motion vector does not satisfy a reliability criterion, the first step is repeated in relation to a following image, on the basis of the same reference data. The tissue material of the patient is disposed between the upper and lower housing elements such that the light energy emitted by the source passes through the tissue material to the detector.