A CCD or Charge Coupled Device is a highly sensitive photon detector. It is divided up into a large number of light-sensitive small areas known as pixels, which can be used to assemble an image of the area of interest.
A CCD is a silicon-based multi-channel array detector of UV, visible and near-infra light. These are used for spectroscopy, since they are extremely sensitive to light. That makes these detectors suitable for analysis of the inherently weak Raman signal. It also allows multi-channel operation, meaning the entire spectrum can be detected in a single acquisition.
CCDs are widely used beyond sensors in digital cameras. Versions that are used 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, referred to as linear, or two dimensional, referred to as area arrays of thousands or millions of individual detector elements. Those elements are 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, readout electronics pull the charge from the elements, and each individual charge reading is measured.
In a typical Raman spectrometer, the Raman scattered light is dispersed using a diffraction grating. This dispersed light is 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. This is typically done using either Peltier cooling, which is suitable for temperatures down to -90oC, and liquid nitrogen cryogenic cooling. Most Raman systems use Peltier cooled detectors, but liquid nitrogen cooled detectors still have advantages for certain specialized applications.
An Electron Multiplying Charge-Coupled Device (EMCCD) is an image sensor. It is able to detect single photon events without an image intensifier, using a unique electron multiplying structure built into the chip.
EMCCD cameras are designed to overcome a fundamental physical constraint to deliver high sensitivity with high speed. Traditional CCD cameras offered high sensitivity, with low readout noises, but at the expense of slow readout. These were often referred to as ‘slow scan’ cameras.
EMCCD overcame this by amplifying the signal. That means that the readout noise is effectively by-passed and is no longer is a limit on sensitivity.
What distinguishes EMCCD technology is the addition of a specialized extended serial register on the CCD chip. It produces multiplication gain through the process of impact ionization in silicon.
When photons are scarce, the signal reaching the imaging device may be weak enough to blend with the background noise. EMCCD technology is designed to reduce the inherent electronic noise of the readout process.
EMCCD cameras overcome most low light imaging. These detectors also support faster frame acquisition rates than their CCD counterparts, making them highly suitable for live imaging. Additionally, EMCCD cameras can offer ultimate sensitivity for the observation of the darkest scenes. It accomplishes this by becoming wide-field real-time photon-counting imaging devices.
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