Charge-Coupled Devices CCDs

Photodiode arrays are now increasingly replaced by charge-coupled device (CCD) arrays, which consist of an array of small MOS junctions on a doped silicon substrate (Fig. 4.101) [4.121-4.124]. The incident photons generate 

Readout noise at 50 kHz [electron charges] Dark charge [electrons/(h pixel)] 

Quantum efficiency peak Front illuminated Backward illuminated 

The dark current of cooled CCD arrays may be below 10 electrons per second and diode. The readout dark pulses are smaller than those of photodiode arrays. Therefore, the sensitivity is high and may exceed that of good photomultipliers. Particular advantages are their large dynamic range, which covers about five orders of magnitude, and their linearity. 

The disadvantage is their small size compared to photographic plates. This restricts the spectral range that can be detected simultaneously. More information about CCD detectors, which are becoming increasingly important in spectroscopy, can be foimd in [4.124,4.125]. 

The disadvantage is their small size compared to photographic plates. This restricts the spectral range that can be detected simultaneously. More information 

Quantum efiSdency peak Front illuminated Backward illuminated Spectral range [mm] 

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A more recent, and superior, type of detector, which also benefits from the multiplex advantage, is the charge-coupled device (CCD). The CCD, as used for spectroscopy, has been developed from the CCD detector used in a camcorder. 

Charge coupled device (CCD) defectors are being used increasingly in fhe visible and ulfraviolef regions. Af presenf fhese are very expensive buf 1 have anticipated fheir increasing importance by including a brief description in Chapfer 3. 

An x-ray area detector can be used to collect the intensities of many reflections at a time. The crystal must be oriented in many different settings with respect to the incident beam but the detector needs to be positioned at only a few positions to collect all of the data. A charge coupled device (CCD) is used as the area detector on the Siemens SMART single crystal diffractometer system. The SMART detector consists of a flat 6-cm circular phosphorescent screen that converts x-ray photons to visible light photons. The screen is coupled to a tapered fiber optics bundle which is then coupled to a one inch by one inch square CCD chip. The CCD chip has 1024 x 1024 pixels each of which stores an electrical charge proportional to the number of... 

Sihcon charge coupled devices (CCDs), commonly used in soHd-state video cameras and in research appHcations, are being appHed to low light level spectroscopy appHcations. The main advantage of area array CCDs over linear photodiode detectors is the two-dimensional format, which provides simultaneous measurements of spatial and spectral data. 

The main detectors used in AES today are photomultiplier tubes (PMTs), photodiode arrays (PDAs), charge-coupled devices (CCDs), and vidicons, image dissectors, and charge-injection detectors (CIDs). An innovative CCD detector for AES has been described [147]. New developments are the array detector AES. With modem multichannel echelle spectral analysers it is possible to analyse any luminous event (flash, spark, laser-induced plasma, discharge) instantly. Considering the complexity of emission spectra, the importance of spectral resolution cannot be overemphasised. Table 8.25 shows some typical spectral emission lines of some common elements. Atomic plasma emission sources can act as chromatographic detectors, e.g. GC-AED (see Chapter 4). 

The spectroscopy system uses a dispersive element and a detector which is either a charge-coupled device (CCD) or a diode array. A computer is required for instrument control and for intensive data processing. 

Such detectors are frequently charge-coupled devices (CCD) known from digital cameras... 

Because of the double exposure, the preparation suffers from increased bleaching and photodamage. Furthermore, split-imaging on charge-coupled-device (CCD) systems (see Textbox 1) is not an option. Nevertheless, excitation ratioing may be an economic choice for laboratories that have an old Fura-imaging setup. These microscopes often allow very fast excitation switching... 

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