Abstract: Apparatus for treating workpieces by directing a beam at the workpieces, with resultant heating of the workpieces, the apparatus including a moving support element for carrying the workpieces past a source of the beam in both a scanning direction and in a control direction generally orthogonal to the scanning direction, an infrared detector mounted to move in the control direction but not in the scanning direction to receive black body radiation from the workpieces as they pass under the detector, and means to correct black body radiation measurements of said workpieces for noise resulting from dark current variation, whereby an indication of the temperature of the individual workpieces can be obtained as workpiece movements and beam treatment occur.