Operation
13
Please note that the absorbance is sometimes defined as the base 10 logarithm and
sometimes as the natural (base e) logarithm. In Waves, you can choose in the Options
window from the File menu which logarithm to use.
Measurement
In order to measure absorption, reflection or transmission you need to take a reference
spectrum of the light source first. If your measurement setup is not in complete darkness,
it is also generally recommended to take a background spectrum before the
measurement.
To take a background spectrum, turn off the light source, set an appropriate exposure
time and click the ‘‘Take Background Spectrum’’ button [ ] on the Exposure Toolbar. In
order to minimize noise, you may also want to use temporal averaging. When you’ve
taken a background spectrum, you can select the ‘‘Use Background Spectrum’’ button [
]. If you take a spectrum afterwards, the background spectrum is automatically
subtracted from the measured spectrum, eliminating unwanted background signals.
To take a reference spectrum for transmission or absorption spectroscopy, turn on the
light source again, remove the sample, set an appropriate exposure time and temporal
averaging and click the ‘‘Take Reference Spectrum’’ button [ ]. After selecting the ‘‘Use
Reference Spectrum’’ button [ ], you can take spectra of your sample for measuring
absorption or transmission.
To take a reference spectrum for reflection spectroscopy, you need a white reference
sample to calibrate the setup for 100 % reflection. However, all real-world reference
samples have a reflection of less than 100 % and an uneven spectral distribution.
Therefore, for accurate measurements you also need to supply a reference spectrum for
your reference sample. To take a reference spectrum, place the white reference sample
where you’ll later place the sample to be measured. Then set an appropriate exposure
time and temporal averaging and click the ‘‘Take Reference Spectrum’’ button [ ]. In the
following dialog window, you can also specify a data file that includes a calibrated
spectrum of your reference sample. After selecting the ‘‘Use Reference Spectrum’’ button
[ ], you can measure reflectivity spectra.
The y axis of the spectrum now shows the relative intensity of the measured spectrum
compared to the reference spectrum. In the ‘‘Options’’ dialog from the File menu you can
choose different units for displaying the spectra:
o
Ratio
shows the ratio of the measured spectrum to the reference spectrum as in
the formula for
(λ)
above.
o
Percentage
shows the same value
(λ)
, but written as a percentage (i.e. multiplied
by 100).
2
This unit is usually used for transmittance.
o
Absorbance
shows the spectral absorbance
(λ)
according to the formula above.
You can choose between natural and base 10 logarithm.
2
Please note that „percent‘‘ means nothing else than ‘‘divided by 100’’.
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