Normalisation
When comparing multiple spectra with different intensity scales, such as those collected on different instruments or under different conditions, it may be helpful to normalise the intensity scale to compare spectral features. For example, consider the plots below, which show un-normalised (left) and normalised (right) plots of a cathodoluminescence spectrum of apatite ( Ca5(PO4)3.(OH,F,Cl) ) and photoluminescence spectra of four xenotime (YPO4) specimens singly-doped with rare earth elements. Due to a shorter acquisition time being used for the CL spectrum, the CL spectrum is not visible in the un-normalised plot of the CL and PL spectra (below left) as it is too close to the x-axis. However, in the normalised plot (below right), the spectra are now displayed on an comparable intensity scale, from which it can be seen that the photoluminescence lines for Dy, Eu, Nd and Ho line up with the observed emission bands in the apatite spectrum.
The ‘Y-axis→Normalise...’ menu option.
To enable normalisation, select the ‘Y-axis→Normalise...’ menu option, as below.
The ‘Y-axis→Normalise...’ menu option.
The ‘Y-axis→Normalise...’ menu option opens the ‘Normalisation’ window (shown below), which you may use to control the normalisation settings.
Using the ‘Normalisation’ window.
The default normalisation settings will scale all spectra so that their maximum output intensity is 100, with no offset applied (i.e. zero input intensity will still be zero when normalised). It is possible to change the input intensity values that are respectively converted to 0 and 100 in the normalised spectrum using the ‘100 at’ and ‘0 at’ drop-down list controls in the ‘Normalisation’ window.
The options for the ‘100 at’ control point are:
| Maximum | The maximum intensity values in the input spectra will be scaled to 100 in the normalised spectra. |
|---|---|
| Cursor (live) | The intensity values in the input spectra at the current inspection marker position will be scaled to 100 in the normalised spectra. If the marker is moved, the normalisation will be updated to the new intensity values. |
| Cursor (snapshot) | The intensity values in the input spectra at the current inspection marker position will be scaled to 100 in the normalised spectra. If the marker is moved, the normalisation will not be updated to the new intensity values; instead, the position of the marker will be stored as a ‘Fixed position’ control point (see next). |
| Fixed position | The intensity values in the input spectra corresponding to a user-defined x-axis position will be scaled to 100 in the normalised spectra. |
The options for the ‘0 at’ control point are:
| Zero | Input intensity values at zero will remain at zero in the normalised spectra (no offset applied). |
|---|---|
| Minimum | The minimum intensity values in the input spectra will be offset to zero in the normalised spectra. |
| Cursor (live) | The intensity values in the input spectra at the current inspection marker position will be offset to zero in the normalised spectra. If the marker is moved, the normalisation will be updated to the new intensity values. |
| Cursor (snapshot) | The intensity values in the input spectra at the current inspection marker position will be offset to zero in the normalised spectra. If the marker is moved, the normalisation will not be updated to the new intensity values; instead, the position of the marker will be stored as a ‘Fixed position’ control point (see next). |
| Fixed position | The intensity values in the input spectra corresponding to a user-defined x-axis position will be offset to 0 in the normalised spectra. |
To clear normalisation, press the ‘Reset’ button in the ‘Normalisation’ window, or un-check the ‘Y-axis→Normalise...’ menu option (as below).
The ‘Y-axis→Normalise...’ menu option.
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