Patent ID: 8434938

Claim:
An apparatus for monitoring a temperature and/or a temperature-dependent parameter of an optical element arranged in a beam path of a CO 2 laser beam having a wavelength λL in an infrared wavelength range, the apparatus comprising: the optical element arranged in the beam path of the CO 2 laser beam, wherein the optical element is at least partially transparent to light at the wavelength λL of the CO 2 laser beam to allow the CO 2 laser beam to at least partially transmit through the optical element; a measuring light source configured to produce measuring radiation at a measuring wavelength λM and direct the measuring radiation to the optical element, wherein the measuring wavelength λM is different from the wavelength λL or is in a measuring wavelength range of λM±Δλ that does not coincide with the laser beam wavelength λL; a detector arranged to detect a portion of the measuring radiation that has passed through a passage region of the optical element; and an evaluating device connected to the detector to monitor the temperature and/or the temperature-dependent parameter of the optical element, wherein the evaluating device is configured to determine the temperature and/or the temperature-dependent parameter based on an intensity of the measuring radiation detected by the detector in the passage region.