Spectral Descriptor Generator

In some cases you might want to create a regular raster of spectral descriptors covering the entire spectrum or you might want to create descriptors which are located at the center of peaks in the spectrum. Both scenarios result in a lot of work, thus Epina ImageLab provides a tool to create such arrays of spectral descriptors automatically (click the "Generate Descriptors" button in the Spectral Descriptor Editor).

This tool offers two modes of operation which can be switched by selecting the appropriate tab in the mode selector:

  • "Regular Raster" lets you generate an equidistant raster of descriptors of the same type.
  • "Peak Finder" searches for peaks in the spectrum and generates spectral descriptors at the peak positions.

You have to specify the type of the descriptor, the spectral range which should be covered and the descriptor parameters. Whenever you change any of the parameters the new list of descriptors is displayed in the top left panel of the tool window. At the bottom left you see a preview of the descriptor positions. The descriptors shown in the preview list can be added to the existing descriptor list by clicking the "Apply" button.

Common Parameters

All wavelength-related parameters of the tool are layer index numbers. The "Spacing" parameter determines the spacing between the descriptors, the "Distance" parameter defines the distance to the second peak (i.e. for peak ratios) or the upper border for areas. The "#Nb" parameter controls the number of neighbors used for averaging the baseline reference points.

Regular Raster Mode

In the regular raster mode the SPDC generator creates equidistant descriptors within the specified range of layers. The distance of the individual descriptors is controlled by the parameter "Spacing", which is defined in number of layers. The corresponding wavelength difference is indicated in square brackets right to the spacing parameter. Further, the descriptor raster can be shifted along the wavelength axis by specifying a non-zero offset.

Peak Finder Mode

In the peak finder mode Epina ImageLab scans the selected spectral range for peaks and centers the generated descriptors on these peaks. The peak finding algorithm is based on the correlation to a Gaussian template peak, whose width can be controlled by the parameter "Peak Width". The parameter "Sensitivity" specifies the required minimum correlation to the template peak, and the parameter "Minimum Amplitude" defines the minimum expected height of the peak in the spectrum. Further you can choose whether to restrict the peak search to positive or negative peaks only by ticking off the respective check boxes.

Hint: the optimum setting of the three search parameters depends on the type of spectrum. You have to play with them to find optimal settings.


The descriptor generating tool supports the following types of descriptors (see the list of available descriptors for a detailed description of the descriptors):
  • Peak Intensity: single intensity values without any baseline correction (PRW)
  • Peak Ratio: ratio of two peaks, the layer of the first peak is given by the generated raster, the second is "Distance" layers above (RRW)
  • Area (Raw Data): the sum of all raw intensities in a particular area of the spectrum, the lower border is specified by the layer resulting from the raster, the upper border is defined by the "Distance" parameter (ARW)
  • Area (Baseline Subtracted): the sum of all baseline-corrected intensities in a particular area of the spectrum. The lower border is specified by the layer resulting from the raster, the upper border is defined by the "Distance" parameter. The baseline is calculated such that it runs through the reference points at both ends of the area. The reference points are averaged by "#Nb" layers around the borders. (ABL)
  • Triangular Correlation: the correlation to a symmetric triangular peak. The central line is determined by the layer resulting from the raster, the lower and upper borders of the triangle are controlled by the "Distance" parameter. (TC)
  • Centroid: The relative position of the centroid of a particular spectral region. The lower border is specified by the layer resulting from the raster, the upper border is controlled by the "Distance" parameter(CEN)


How To: Please follow these steps to define an array of spectral descriptors:
  1. Click the "Generate Descriptors" button in the Spectral Descriptor Editor.
  2. Select your preferred type of spectral descriptor.
  3. For multi-sensor data, select the spectral type to be used
  4. Add the descriptor to the list of descriptors by clicking the button ("Add Descriptor")
  5. If necessary fine tune the range of the selected spectrum
  6. Set the parameters ("Spacing", "Distance", "#Nb.") as required
  7. Click "Apply" to generate and add the spectral descriptors to the descriptor list