Frequency and the optical axis Appendix E FTIR Frequency scale calibration
66 AA003700-01 rev. G. 2.0
beam and hence the frequency reduces to COS (1 degree)= 0.999848 times the
nominal frequency; a shift of 1.11 cm
-1
at 7300 cm
-1
and 0.29 cm
-1
at 1918 cm
-1.
A plot of available light intensity as a function of off-axis angles linearized along
the frequency axis shows a rectangle when a circular symmetric illumination
distribution occurs. This is the case for the MB3000 where the output J stop limits
the illumination to a circle. The quartz halogen lamp of the MB3600 is not quite
wide enough to provide circular symmetric illumination through the output J stop
and hence the illumination distribution is no longer rectangular. Figure 2 shows the
illumination distribution as a function of frequency for figure 1.
The mean frequency for the distribution in figure 2 is shifted to higher frequency as
a result of the declining intensity at lower frequency and is 7299.73 cm-1. This is
Figure E- 1. different field of view defining elements for the MB3600 with Quartz
halogen source
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