Patent ID: 8220291

Claim:
A method for measuring a quantity in a furnace, comprising the steps of: providing a device comprising: an arm ( 7 ), a first end of which supports a box: the wall of said box placed facing the second end of the arm is transparent, within which is placed a backreflecting means capable of receiving a light beam originating from the first end of the arm and parallel to said arm and of returning it in the opposite direction parallel to the incident optical path, a transparent screen attached to the second end of the arm so as to face the transparent wall of the box, and a means for cooling the arm and the box; directing a light beam from an emitter through the transparent screen parallel to the arm and through the transparent wall along the incident optical path; allowing the light beam directed through the transparent wall to be reflected by the backreflector and returned through the transparent wall in the opposite direction parallel to the incident optical path; receiving the returned light beam with a receiver; introducing gas close to the surfaces of the transparent wall and of the transparent screen and optionally in the box; and measuring a quantity in the furnace with at least one analyzer based on absorption spectroscopy using the returned light beam received by the receiver.