Cosmic ray removal

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Cosmic rays and Raman

If you run long duration Raman measurements you will probably see some sharp spikes in your spectra which are not related to the sample. These are generated by cosmic rays hitting your detector, producing spurious features, which are typically narrower than Raman bands. You can use the Cosmic Ray Remover (CRR) options in Renishaw's WiRE™ software to remove these features from your spectra quickly and easily.

Origin of cosmic rays

Primary cosmic rays are high energy particles from outer space. They produce a shower of particles (secondary cosmic rays) when they hit the atmosphere. These random and unavoidable events generate spurious spikes in the Raman spectra.

Avoid cosmic rays

inVia's design is stigmatic—the spectrum forms a narrow band of concentrated light on the detector. This not only improves signal-to-noise ratios, but also makes inVia less prone to the effects of cosmic rays.

Watch our 2-minute video on WiRE software's automated cosmic ray removal (CRR) tool. Save time when processing Raman image data, and get to the right results faster.

Benefits of removing cosmic ray features from spectral data

Your data processing and analysis will be easier
Your data will be easier to interpret
Spectral band shape can be maintained
When analysing data using unsupervised chemometric methods, such as principle component analysis (PCA), you can be confident you are analysing solely sample-related data (Raman/photoluminescence)
Your images will be artefact-free and therefore more representative and attractive

Removing cosmic rays during data collection

You can eliminate cosmic rays with WiRE's median filtering option. This acquires two additional accumulations for each desired spectrum. WiRE then, for each Raman frequency, takes the median of the three values. This automatically rejects cosmic rays of all shapes and sizes as they will always be an extreme value, rather than the median. This approach also reduces noise.

Superficially you might expect this approach takes three times longer to collect the data. However, if you reduce the acquisition time and increase the number of accumulations, then the increase in total experiment time is relatively small.

Removing cosmic rays after data collection

A small number of obvious cosmic ray features can be removed using a simple zap function. This replaces the ray region with a single line. Where we have many spectra within a data file this method becomes impractically slow.

For rapid results on files containing many cosmic ray features, Renishaw's Cosmic Ray Remover processing tool provides a combination of suitable methods. These are applied either fully automatically or with your guidance.

CRR screenshot

CRR screenshot

Cosmic ray removal in action. CRR has identified 14 candidate cosmic ray features on this long duration map of crossing ink lines on a questioned document. The operator has selected one of the candidates. The pane at the bottom shows the spectrum with the cosmic ray and the spectrum from the nearest neighbours that matches it best. The operator can then remove one, many, or all of the candidate cosmic ray features.
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Many aspects of Renishaw's innovative technology are covered by patents.