Qstick User Manual
16
For your Qstick spectrometer, this dark spectrum has already been taken, is stored in the
device and automatically subtracted from the measured spectrum each time you take a
new spectrum. Because the dark spectrum depends on the exposure time in a non-linear
way, there are actually multiple dark spectra stored in the device for a certain number of
exposure times. The dark spectrum that is needed for a chosen exposure time is then
obtained by interpolating between these dark spectra.
In case you find that the spectra you get are not zero on the average, even if no light
enters the spectrometer, then you probably need to recalibrate the dark spectra. You can
also check the dark spectrum calibration using the ‘‘Calibrate Dark Spectrum’’ window
(see below).
To recalibrate the dark spectra, simply close the spectrometer entrance, make sure that
no light enters the device and choose ‘‘Calibrate Dark Spectrum’’ from the Tools menu.
Then click the ‘‘Take’’ button and go for a coffee. After a few minutes the software has
taken dark spectra for a series of exposure times. You can check the new dark spectra by
clicking on the ‘‘Test Button’’. While the test is running, you can in the lower diagram the
spectra taken without (red) and with (green) dark spectrum correction. In the upper
diagram you can see the noise level, also without (red) and with (green) dark spectrum
correction for a series of exposure times. You can see that the noise level of the raw data
(red) increases significantly with increasing exposure time. With dark spectrum correction
(green) however, the noise level should remain pretty much at the same level for
exposure times less than 1 second and increase slower for exposure times larger than 1
second. If you find this to be the case, close the window by clicking on the ‘‘OK’’ button
and the new dark spectrum calibration is saved into the device. Otherwise you may
calibrate again or click ‘‘Cancel’’.
Spectral Sensitivity
Like in all spectrometers, the sensitivity of the Qstick spectrometer is not uniform across
the spectrum, but depends on the wavelength. This is called ‘‘Spectral Sensitivity’’ and
can be corrected by dividing each value of the spectrum by the corresponding sensitivity.
This sensitivity calibration is also stored in the device. However, it is not always used
when taking spectra, because if the sensitivity is corrected, the noise level is unevenly
distributed across the spectrum. Instead, you can choose to use the spectral sensitivity
correction by selecting the ‘‘lightmeter’’ button in the Exposure Toolbar.
If you would like to recalibrate the spectral sensitivity, you need a light source with a
continuous spectrum (like a tungsten halogen lamp) and a calibrated spectrum for this
lamp.
To recalibrate, take a spectrum of your light source with spectral sensitivity correction
turned off. Make sure the spectrum contains as little noise as possible by using a strong
light source, a suitable exposure time and use Temporal Averaging. Then choose
‘‘Calibrate Spectral Sensitivity’’ from the Tools menu. Click on the ‘‘Load reference data’’
button to load the calibrated reference spectrum (in other words: a spectrum file that
shows how your light source really looks like). Based on the measured spectrum and this
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