68
Apr-09
Avantes
www.avantes.com
In this mode the Toshiba detector will generate automatically an additional prescan for
every request from the PC, the first scan contains non-linear data and will be rejected, the
2
nd
scan contains linear data and will be showed on the screen and/or saved. This prescan
mode is default and should be used in most applications, like with averaging (only one
prescan is generated for a nr of averages), with the use of an AvaLight-XE (one or more
flashes per scan) and with multichannel spectrometers. The advantage of this mode is a very
stable and linear spectrum. The disadvantage of this mode is that a minor (<5%) image of the
previous scan (ghostspectrum) is included in the signal.
This mode cannot be used for fast external trigger and accurate timing, since the start of
the scan is always delayed with the integration time (min. 3.7 ms).
2.
The Clear-Buffer mode.
In this mode the Toshiba detector buffer will be cleared, before a scan is taken. This clear-
buffer mode should be used when timing is important, like with fast external triggering. The
advantage of this mode is that a scan will start at the time of an external trigger, the
disadvantage of this mode is that after clearing the buffer, the dector will have a minor
threshold, in which small signals (<500 counts) will not appear and with different integration
times the detector is not linear.
Setup Menu: Wavelength Calibration Coefficients
After clicking this option, a dialog is shown in which the wavelength calibration coefficients can be
changed manually and start/stoppixels can be set to speed up datatransfer.
Background
The wavelength that corresponds to a pixel number (pixnr) in the detector in the spectrometer can
be calculated by the following equation:
= Int X1*pixnr + X2*pixnr
2
+ X3*pixnr
3
+ X4*pixnr
4
in which Intercept and X1 to X4 correspond to Intercept and First to Fourth Coefficient in the figure
below.
For example, if we want to calculate the wavelength at pixel number 1000, using the numbers in the
figure at the right, the wavelength becomes:
=
384,054 + 0,136492*1000 +
-6,71259E-6*1E6 +
-5,66234E-10*1E9
=
513,267 nm.
The „Restore Factory Settings‟ button
restores for all spectrometer channels the
original wavelength calibration coefficients
that were saved to the EEPROM during
factory calibration.
The “Process data only when in following
wavelength range” option can be used to
transfer only a limited number of pixels
from the spectrometer to the PC. This can significantly speed up the transfer time (e.g. for the
AvaSpec-2048 from 30 ms at full wavelength down to 14 ms for a small selection of 10 pixels). A
second advantage is data reduction, because only the spectral data will be saved at the pixels for
which the wavelength is in the specified wavelength range.
