Frequency And The Optical Axis - ABB MB3000 Series Installation And User Manual

Appendix E FTIR Frequency scale calibration

Frequency and the optical axis

The optical axis of an interferometer is defined as the direction of light propagation
centered on the circular fringe pattern. For a flat mirror type interferometer it
corresponds to the perpendicular to the flat mirrors. For a cube corner type
interferometer it is the line joining the apex of one cube corner with the apex of the
image of the second cube corner as seen reflected by the beamsplitter. The optical
path difference is greatest along the optical axis. As light beams are angularly
inclined to the optical axis, the optical path difference for the same mirror
displacement diminishes as the cosine of the angle.
A pinhole mask placed centered in the sample beam permits selecting an IR beam
parallel to the optical axis. For this configuration, the measured frequency of the
water vapor lines is highest: The frequencies for the two water vapor lines are
1918.xxx and 7300.10 cm
or right or up or down will cause a decrease in measured frequency. A shift of the
pinhole by 2.4 mm from the center corresponds to 1 degree angular shift of the IR
AA003700-01 rev. G. 2.0
-1
. Moving the pinhole away from the center sideways left
Frequency and the optical axis
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