The most common choices for detectors for visible Raman spectrometers are CCD (charge-coupled device) and EM-CCD (electron multiplied charge-coupled device) cameras. While there is more to a Raman spectrometer than just the camera, a basic understanding of the differences between these technologies can help in choosing the right instrument for a specific application. Both options can be used for most applications; however, there are some differences that make one or the other the optimal choice.
In photometry, the intensity of light is ascribed to the sensitivity of the human eye. With regards to EM-CCD and CCD cameras, sensitivity relies on converting impinging photons into an electrical charge and quantifying that charge in the presence of the noise associated with the measurement. The signal-to-noise ratio (S/N) is a measure of how well the signal can be distinguished from many different sources of noise. This can be used as an important metric when evaluating camera sensitivity. In order to achieve a high signal-to-noise ratio, it is preferred to have a camera with high quantum efficiency (QE) so that the signal can be maximized while the various sources of noise are kept to a minimum. The ability of the camera to convert impinging photons to a measurable electronic signal is referred to as quantum efficiency. A high QE is particularly essential with samples exhibiting weak Raman scattering or when using measuring conditions that considerably reduce the number of photons reaching the detector. The camera (detector) generates a signal, which is the product of the number of photons impinging on the camera and the quantum efficiency.
It is beyond the scope of this article to discuss the various technologies involved to improve the quantum efficiencies of EM-CCDs and CCDs and hence will not be covered here. For this article, it is enough to note that CCD and EM-CCD cameras can be either back or front illuminated and that back illuminated (back thinned) cameras usually have higher QEs than front illuminated cameras.
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This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.
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