90 . Spectrum Two N User's Guide
AVI Correction
If available for your current instrument and sampling configuration, AVI correction can be
selected on the Setup Instrument Advanced tab in Spectrum (Figure 79).
Figure 81 Setup Instrument Advanced tab in Spectrum
The objectives of Absolute Virtual Instrument (AVI) correction are:
• Consistent performance over time and between instruments
• Traceability for all measurements.
Although FT-IR spectrometers use a reference laser, the wavenumber calibration and
lineshape are affected by differences in beam divergence and uniformity. This is true for all
FT-IR spectrometers. Differences can occur between instruments, when using different
sampling accessories and when components are changed. AVI enables calibration and
lineshape to be maintained.
The Absolute Virtual Instrument is an instrument with theoretical performance, such that the
result of measuring a known sample on such an instrument can be predicted. So, if we
measure with a real instrument and calculate the software transform to match the theoretical
result, we can apply this transform to future measurements.
The Absolute Virtual Instrument is defined by wavenumber calibration, instrument lineshape
and ordinate accuracy.
When AVI is switched on, the software measures the current instrument profile relative to an
absolute standard (a methane cell) and an ideal lineshape function, and applies a correction.
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