What is a CCD detector?

A CCD (Charge Coupled Device) is a silicon based multichannel array detector of UV, visible and near-infra light.  They are used for Raman spectroscopy because they are extremely sensitive to light (and thus suitable for analysis of the inherently weak Raman signal), and allow multichannel operation (which means that the entire Raman spectrum can be detected in a single acquisition).  CCDs are widely used, not least as the sensors in digital cameras, but versions for scientific spectroscopy are of a considerably higher grade to give the best possible sensitivity, uniformity and noise characteristics.

CCD detectors are typically one dimensional (linear) or two dimensional (area) arrays of thousands or millions of individual detector elements (also known as pixels).  Each element interacts with light to build up a charge – the brighter the light, and/or the longer the interaction, the more charge is registered.  At the end of the measurement read out electronics pull the charge from the elements, at which point each individual charge reading is measured.

In a typical Raman spectrometer, the Raman scattered light is dispersed using the diffraction grating, and this dispersed light is then projected onto the long axis of the CCD array.  The first element will detect light from the low cm-1 edge of the spectrum, the second element will detect light from the next spectral position, and so on...the last element will detect light from the high cm-1 edge of the spectrum.

CCDs require some degree of cooling to make them suitable for high grade spectroscopy.  Typically this is done using either peltier cooling (suitable for temperatures down to -90oC), and liquid nitrogen cryogenic cooling.  Most Raman systems use peltier cooled detectors, but for certain specialised applications liquid nitrogen cooled detectors still have advantages.

CCD Detector
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