PerkinElmer Frontier Optica FT-IR, Руководство пользователя

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Measuring Optical Components . 55
Sample Properties
When you measure the transmittance of your sample, you are effectively measuring the
change in energy of the beam as it passes through the sample. The properties of your
sample – for example, the shape, position and material – will affect the path taken by the
beam. Deflections and multiple reflections may decrease the accuracy of your %T value.
Sample wedge
The beam will be deflected by samples with non-parallel faces. The greater the wedge angle
(the angle by which one face deviates from parallel to the other face) the greater the
deflection of the beam. This increases the probability that part, or all, of the beam will miss
the detector.
The angular deviation of the beam is the wedge angle multiplied by (
n − 1), where n is the
refractive index of the sample material. You may find reduced accuracy when, for example,
measuring zinc selenide (
n
≈ 2.4) samples with wedge angles of more than 0.2° or when
measuring germanium (
n
≈ 4) samples with wedge angles of more than 0.1°.
Sample tilt
If a sample is tilted, then the beam will be displaced from its axis. Any multiple reflections
will also be displaced. The detector optics in the Frontier Optica FT-IR are more susceptible
to error when the beam is displaced horizontally (when, for example, the sample is tilted
about a vertical axis) than when displaced vertically. Therefore, we recommend that if a
sample is tilted it should be about the horizontal axis. Figure 19 shows the displacement of
the beam as a sample is tilted.
Figure 19 Sample tilt about a horizontal axis showing the vertical displacement of
the beam
When the sample is tilted the deflection of the beam (and any multiple reflections) will
increase with the thickness of the sample.
You may find reduced accuracy when measuring, for example, a germanium sample (
n
≈ 4)
with a thickness of 1 mm that is tilted by more than 5° about a horizontal axis.