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Lanthanides in Luminescent Thermometry Chapter 281 357 0.5 A W1 (Saleh et al., 1991). The noncooled detectors present typical signal-to-noise ratio (SNR) of 1/500, whereas in cooled ones this value decreases to 1/1000, corresponding to a maximum relative dI of the order of 0.1% (Saleh et al., 1991). The main advantage of PDA detectors is the simul- taneous access to all the covered spectral range, which is critical to avoid any delay between the acquisition of the two transitions used to measure tempera- ture. On the other hand, their high acquisition rate ($5 Hz) precludes their use in submillisecond regime, which typically is not critical for dynamic nanothermometry applications. CCDs are usually metal–oxide semiconductor structures (SiO2 on Si and a layer of polycrystalline Si). The manufacturers report SNR values between 0.1% and 0.5% making these detectors appropriate for spectrometric measure- ments. The photosensitivity in the visible spectral range is about 2.5–4.0 A W1 (Groom et al., 1999), meaning a fourfold increase in SNR, when compared with PDAs. The use of complementary metal–oxide semicon- ductors and the cooling of the detector can improve the dark–noise value resulting in low SNR values, 0.05% at best. The most obvious advantage of CCDs in comparison with other detecting solutions is that they are usually organized in bidimensional arrays giving access to spectral images. Spectrally resolved measurements can be implemented using bandpass filters for a set of channels (typically three CCDs for blue, green, and red spectral regions) or hyperspectral cameras. The last approach has been gaining a renovated interest since it allows the acquisition of spectral information on submicro- metric sized domains. The most limiting factor for the spreading of the CCD technology is the cost: the price of a laboratory-grade CCD detector is one order of magnitude higher than that of the PDA equivalent in terms of photosensitivity. Finally, PMTs are the most sensitive and the fastest response time detec- tors being constituted by a photocathode (photoemissive cathode), focusing electrodes, an electron multiplier and an anode (electron collector) in a vac- uum tube. When photons impinge on the photocathode it emits photoelectrons that are focused by electrode voltages toward the electron multiplier, and finally collected by the anode as an output signal. The PMTs present typical overall photosensitivity in the visible range around 104 A W1 (Dorenbos et al., 1993), one of the reasons justifying their wide use in most of the laboratory-grade spectrophotometers. Although most suppliers refer higher detection capabilities for these detectors, their incorporation in the actual spectrometers produce intensity fluctuations in steady-state conditions that degrades SNR to values of the order of 0.03%. Although commercially avail- able devices allow covering the electromagnetic spectrum from the deep UV to the far IR, the experimental setups commonly use a set of gratings and moving optics that results in acquisition times of the order of a minute for a single spectrum, Table 2.PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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