Detectors photodetector

Photodetectors. The second major class of infrared detectors— photodetectors—has found negligible application in analytical instrumentation except for the use of PbS photoconductive detectors in the near infrared. The principal reasons for this are ... 

There are important figures of merit (5) that describe the performance of a photodetector. These are responsivity, noise, noise equivalent power, detectivity, and response time (2,6). However, there are several related parameters of measurement, eg, temperature of operation, bias power, spectral response, background photon flux, noise spectra, impedance, and linearity. Operational concerns include detector-element size, uniformity of response, array density, reflabiUty, cooling time, radiation tolerance, vibration and shock resistance, shelf life, availabiUty of arrays, and cost. 

When equation 12 is vaUd, the detector is said to be a background-limited infrared photodetector (BLIP). When this is the case, attempts often are made to improve D by cold shielding which reduces ( ). The ideal D is shown in Figure 3 as a function of wavelength with background photon flux as a parameter. The line of termination in the lower left corner represents TN values for a 180° (27T) detector field of view, 300 K ambient background... 

Eig. 17. Cross-sectional schematic of a microbolometer photodetector. Micromachining is used to constmct small, very low mass detectors. The dimension... 

Although iastmmentation is discussed ia many of the analytical articles, there are only a few places ia the Eniyclopedia where it is the primary emphasis (see Analytical methods, hyphenated instruments Automated instrumentation). However, articles relating to materials used either ia or as iastmmeatal compoaeats such as eaergy sources (see Lasers), sampling devices (see Eiber optics), and detectors (see Biosensors Photodetectors SsENSORs) abound. 

Sohd-state multi-element detector arrays in the focal planes of simple grating monochromators can simultaneously monitor several absorption features. These devices were first used for uv—vis spectroscopy. Infrared coverage is limited (see Table 3), but research continues to extend the response to longer wavelengths. Less expensive nir array detectors have been appHed to on-line process instmmentation (125) (see Photodetectors). 

Light detectors fall into two categories photoconductors and photodetectors. Photoconductors are devices whose resistance decreases upon exposure to light. Cadmium sulfide (CdS) and cadmium selenide (CdSe) are the most commonly used photoconductor materials in the visible spectrum. They are still mostly produced by sputtering but CVD is used increasingly (see Ch. 12, Sec. 4.0). 

Instrument Productivity. The number of samples that can be processed by one instrument in a given period of time is determined by the length of time each sample has to be read by the photodetector or other read-out device, or by the number of cuvets per unit time which pass the detector. For example. 

Example 5.7 We have to design a servo-controller for a mixing system. A blue dye for making denim is injected into a stream of water. The injected dye is blended into the pipe flow with the aid of in situ static mixers. A photodetector downstream is used to monitor the dye concentration. The analog output of the detector is... 

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