PerkinElmer Frontier Optica FT-IR, User Manual

Page: 52 / 94

52 . Frontier Optica FT-IR User's Guide
Optimizing Your Instrument Settings
The Frontier Optica FT-IR instrument has been designed to have high ordinate accuracy for
use in applications such as optical component measurements. This section contains some
guidelines to help you to get the best performance out of your instrument.
J-stop
The J-stop is a variable aperture that has been partially blanked off to prevent back
reflections propagating through the interferometer. It defines the size of the beam at the
sample and on the detector. If the beam is deviated by the sample, there is a chance that
the beam, or part of the beam, could miss the detector. If the diameter of the beam is small
in relation to the detector there is more tolerance for this deviation. The diameter of the
beam at the sample position will be about 1.2 times the diameter of the J-stop.
For high ordinate accuracy we recommend that you use a 6 mm or smaller J-Stop when
making transmittance measurements. However, the smaller the diameter of the J-stop, the
lower the level of signal reaching the detector and, therefore, the lower the signal-to-noise
ratio.
For a given J-stop Image Size , the selected resolution is only achievable in the spectrum up
to a wavenumber value known as the J-Stop Wavenumber (Figure 18). Increasing the
J-stop size beyond the default value will decrease the J-stop wavenumber, and thus the
wavelength range over which a particular spectral resolution can be achieved.
Figure 18 Setup Instrument BeamPath tab in Spectrum software
NOTE: If you change the resolution in Spectrum software, the J-Stop Image Size will be set
to the default size for that resolution. However, for resolutions ≥ 2 cm
−1
we
recommended that you reset the J-Stop Image Size to 6 mm.