Thermal Contamination of BOMEM FTIR Line Shapes

Abstract

In the standard configuration of the high-resolution BOMEM FTIR model 3.002, the infrared source is focused onto an entrance iris whose aperture can be varied from 0.5 to 10 mm diameter. In the mid-infrared the beam from the sample compartment is normally focused onto a 1 mm diameter MCT detector with an elliptical mirror so that an entrance aperture of about 5 mm fills the detector. However, in order to achieve the full resolution of the instrument in the 10 μm region, it is necessary to adjust the iris for about a 1 mm diameter entrance aperture. Under these conditions the detector sees a combination of the hot source and the warm annulus of the iris around the small entrance aperture. Because the hot source is focused onto the iris, the iris is warmer than room temperature. If this warm iris contributes a significant fraction of the radiation seen by the detector, the full resolution expected from the small entrance aperture will not be achieved.