Lasers, Spectrographs, and Detectors - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 17

measuring the entire spectrum simultaneously had been clearly understood,
the early multichannel detectors (typically a photodiode array coupled to an
image intensifier) exhibited poor signal-to-noise compared to photomultiplier
tubes (PMT). The CCD was able to combine the low-light sensitivity of the
PMT with the durability and multiplex advantage of the PDA. As with diode
lasers, the spectroscopy community has benefited from detector technology
developments made in other fields with low-light applications - specifically
astronomy, where the CCD was first applied for stellar imaging, and the mil-
itary - as well as general advances in silicon device manufacturing.
The operation of the CCD has been reviewed at length and in great detail
elsewhere [48, 49], and only the basic operation will be discussed here to
enable discussion of more recent advances. Each element (or pixel) of the
detector array is a photoactive capacitor that will collect and hold charge
based on the number of photons that strike it. Although a wide variety of chip
architectures are possible, the general scheme shown in Fig. 1.7 is the typical
CCD readout method. CCDs operate by shifting the charge accumulated in
each pixel simultaneously to the adjacent pixel elements until they reach the
'bottom' shift register, where they are read out individually. The histogram
representation of a spectrum in Fig. 1.8 highlights the function of the CCD
in generating intensity data; each pixel is a specific resolution element at
a given wavelength, and a variable quantity of photons. A number of key
performance parameters can be inferred from this form of operation, such as
how much noise is associated with each detector element, how much noise
is generated during shifting/reading, the eciency of the array in detecting
individual photons, and how fast the detector can be read out.
Noise associated with the detector elements themselves (dark or thermal
noise) can be reduced by decreasing the temperature, typically from − 20 to

70 ◦ C; although cooling to liquid nitrogen temperatures essentially eliminates
dark noise, it is typically impractical for applications outside of the labora-
tory. The theoretical value for relative signal-to-noise reduction is a factor of 2
improvement for every 6 . 3 ◦ C reduction in temperature, although this can be

−

Output

Shift Register

Readout
Amplifier

Input

Spectrometer

Fig. 1.7. Schematic of CCD readout