Reasons For Residual Transmittance At Highly Absorbing Bands - ABB MB3000 Series Installation And User Manual

Appendix C Spectroscopy Background
Figure C- 4. Expanded view of figure 3 showing transmittance near zero%. The deviation from zero

Reasons for residual transmittance at highly absorbing bands

An FTIR needs a highly linear response detector and high precision electronics to
insure low residual transmittance. At the same time the throughput of an FTIR is
maximized to insure very high signal to noise ratio even at strongly absorbing
bands.
In the ABB MB3000 series FTIRs, the open beam flux of radiation on the detector
often exceeds the linear flux limit for the detector. In that case Photometric linearity
may be unsatisfactory for demanding wide absorbance range measurements. The
open beam linearity is quite satisfactory for a limited absorbance range from 0 to 1
A. For a wider range of absorbance measurements, it is recommended to use the
supplied attenuator grid.
The attenuator grid is supplied and installed on each new MB3000.
For all general purpose measurements it is best to use the attenuator with the
spectrometer. When the spectrometer is used extensively with a sample accessory
that already attenuates the open beam intensity by about a factor of 3, the attenuator
can be removed thus providing the highest sensitivity for the accessory.
AA003700-01 rev. G. 2.0
Reasons for residual transmittance at highly absorbing bands
transmittance for the 6 bands is +/-0.1%T
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