Operation
15
may not be as accurate as later measurements. Therefore, for precise measurements we
recommend not to use the auto-exposure feature and to set a fixed exposure time
instead.
Wavelengths
The mapping of the individual pixels of the spectrum to wavelengths is done using a
third-order polynomial. The four parameters for this function are measured during
manufacturing and stored in the device. Usually, this wavelength calibration does not
change. Therefore, in most cases you don’t need to recalibrate the wavelengths.
However, if the ambient temperature has changed substantially, if the device has been
used for a very long time or if you just would like to make sure that the wavelengths are
really correct, you can recalibrate the wavelength if you have a suitable light source with
several sharp atomic emission lines. These wavelength calibration sources are usually
low-pressure gas discharge lamps filled with Krypton, Neon, Mercury, Argon or Xenon
gas.
To recalibrate, take a spectrum of this light source first. Make sure that the relevant peaks
are automatically detected. If not, adjust the ‘‘Peak Finder Parameters’’ in the Tools Menu.
Then choose ‘‘Calibrate Wavelengths’’ from the Tools menu. In the ‘‘Reference Spectrum’’
combo box select the type of light source used. If you don’t find your light source in the
list, you need to supply your own calibration lines. In this case, please take a look at the
‘‘
CalibrationLines xxx.dat
’’ files in the installation folder on your hard drive and
create a new similar file containing the spectrum lines of your light source.
After you have selected your light source, click on ‘‘Start Fit’’. The software tries to match
the peaks in the measured spectrum and the given spectrum lines for your light source
automatically. Not all of the given spectrum lines have to be present in the measured
spectrum, but all measured peaks must be known spectrum lines. If a match has been
found, the average deviation is displayed. If the match seems to be successful, it is
displayed in green, otherwise in red. You can also graphically display the deviations by
clicking on ‘‘Show Deviations’’. If the measured peaks have successfully been identified,
you can set the four new wavelength calibration parameters by clicking on the ‘‘Save’’
button and confirm by clicking ‘‘OK’’. The new wavelength calibration is then saved to the
device.
Dark Spectra
Inside the Qstick spectrometer, the spectra are detected with a CCD line sensor. Like all
image sensors, this sensor detects a small signal even if there is no light entering the
spectrometer. This signal depends on the individual pixel, the exposure time and the
temperature. In order to obtain a precise measurement with a small noise level, this ‘‘Dark
Spectrum’’ must be taken while no light enters the spectrometer and then subtracted
from the actual measurement.
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