PerkinElmer Frontier Optica FT-IR, Руководство пользователя

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Measuring Optical Components . 53
B-stop
The B-stop is a circular, variable aperture that defines the area of the beam passing through
the interferometer. It also controls the divergence of the beam at the sample position (by
restricting the angle of the cone of radiation).
The beam angle is proportional to the B-stop diameter. For high ordinate accuracy when
measuring transmittance we recommend that you use a 6 mm or smaller B-Stop Image
Size . A 6 mm B-stop image size produces a beam in the sample area with a cone half-angle
of 4° (70 milliradians).
Reducing the B-stop size decreases the optical beam distortions at the detector, increasing
the ordinate accuracy. Reducing the B-stop size may also improve the wavelength accuracy
of multi-layer optical filter measurements by decreasing the divergence of the beam.
However, the smaller the diameter of the B-stop, the smaller the amount of signal reaching
the detector and, therefore, the lower the signal-to-noise ratio.
Resolution
FT-IR measurements in wavenumbers (cm
− 1
) have constant resolution over the entire range.
If wavenumbers are converted to wavelength (for the Frontier Optica FT-IR, measured in
microns or nanometers) the resolution varies as a function of wavelength.
The relationship between wavenumber resolution, wavenumber, wavelength resolution and
wavelength is given by:
Δ Δ
λ
λ
υ
υ
=
This relationship is demonstrated in the following table.
Wavelength Resolution (nm)
corresponding to 16 cm
−1
Resolution (nm)
corresponding to 4 cm
−1
2500 nm (2.5 μm , 4000 cm
−1
)
10 2.5
5000 nm (5.0 μm , 2000 cm
−1
)
40 10
10 000 nm (10 μm , 1000 cm
−1
)
160 40
20 000 nm (20 μm , 500 cm
−1
)
640 160
NOTE: If the resolution is changed in Spectrum, the software automatically resets the
J-stop Image Size to the default for that resolution.