The CCD (charged-coupled device) camera is the last element in the detection beam path and converts the photons that reach the chip into electrons and then a signal that can be used in the software. The CCD camera is an important component of a Raman microscope and choosing the correct CCD strongly affects the performance of the instrument. The quantum efficiency (QE) is a key element of these cameras. QE is the percentage of detected photons out of the total of incoming photons. The figure below shows the room-temperature QE-curves of three typical cameras.
The following CCDs are commonly used for Raman spectroscopy:
- In the visible range, the back-illuminated CCD camera is the camera of choice. It has a high QE and increases the instrument throughput.
- Front-Illuminated CCDs do have a broader operating range but with a low QE compared to back-illuminated CCDs. They also show a very low dark noise.
- For detecting wavelengths longer than approximately 850-900nm, deep-depletion CCD cameras are the best choice as they have a NIR-optimized coating and the best QE in this range. This type of camera shows a higher dark noise than the ones mentioned previously.
- Back-Illuminated EMCCDs (electron multiplying CCD) are currently the most sensitive spectroscopic detectors on the market with a total amplification of the signal of up to 1000x. Thus ultra-fast Raman imaging can be performed (acquisition of ca. 1300 spectra per second).