Abstract: Interfering internal beams can be used to generate an internal reference interferogram. This interferogram can be used to compensate for changes in FTIR instrument performance in response to variable environmental conditions or other instrument variations. Acquisition of such internal interferograms can be done during, after, or prior to acquisition of actual sample data.

Abstract: A spectrometer (100) includes a light source (102) providing output light (106) to the bundled input ends (108) of multiple light pipes (110). The light pipes (110) branch into sets (118) between their input ends (108) and output ends (114), with each set (118) illuminating a sample detector (126) (via a sample chamber (122)) for measuring light scattered or emitted by a sample, or a reference detector (128) for obtaining a reference/datum measurement of the supplied light, so that comparison of measurements from the sample detector (126) and the reference detector (128) allows compensation of the sample detector measurements for drift. Efficient and accurate measurement is further assured by arraying the multiple light pipes (110) in each set (118) about the input bundle (116) so that each set receives at least substantially the same amount of light from the light source (102).