Photodiode array construction

Light detection can also be achieved by semiconductor photodiodes or by photodiode array detectors. Their sensitivity, so far, is lower than that of PMTs but they possess the great advantages of much smaller dimensions and lower demand on power supply. These features make them attractive, especially for the construction of portable chemiluminometers. The sensitivity of these detectors... 

Hamano et al. (1982) have fabricated an a-Si H photodiode array linear image sensor. The sensor structure is shown in Fig. 5. The sensor is constructed by first forming individual electrodes on a glass or a ceramic substrate. Then l-/mi-thick undoped a-Si H is produced at 230°C by glow-discharge decomposition of silane and finally 1500-A-thick ITO common electrode, which also acts as an antireflection coating, is deposited by dc sputtering. 

We employ method B to study effects of this type. In this mode, our apparatus yields relative high-resolution fluorescence spectra at different time windows after excitation of the sample by the 355 nm pulse. The spectra are acquired by the upconversion method. The upconverted fluorescence spectrum is recorded simultaneously at all monitored wavelengths by an optical multichannel analyzer. It is constructed from a poly-chromator (HR320 Instruments SA) and an intensified silicon photodiode array detector (Princeton Applied Research Model 1412). The detector is interfaced to our Cromemco computer. 

The common detector for AAS is the photomultiplier tube (PMT). The construction and operation of a PMT has been described in Chapter 5. While PMTs are the most common detectors, solid-state single and multichannel detectors such as photodiode arrays (PDAs) (discussed in Chapter 5) and charge-coupled devices (CCDs) (discussed in Chapters 5 and 7) are increasingly being nsed in AAS spectrometers. Many small systems, particularly those dedicated to one element snch as a dedicated CVAAS mercury analyzer, use solid-state detectors instead of PMTs. Multielement simnltaneous AAS systems also use multichannel solid-state detectors to measure more than one wavelength at a time. 

Whereas IR, NMR. and mass spectroscopy are used mainly for the elucidation of structure and the identification of substances, UV-VIS spectroscopy enables quantitative determinations to be carried out much more precisely and reproducibly. Therefore, its primary areas of application are in quantitative analysis and clinical medicine, in the determination of drug concentrations, in the quantification of pharmaceuticals and as detectors in chromatographic processes (HPLC, TLC) (- Liquid Chromatography) [4], [51, Furthermore, mixtures as well as pure substances can be studied and the components determined quantitatively by methods of multicomponent analysis [6]. Since modem spectrometers operate very rapidly and can be constructed in the form of photodiode arrays, they have the advantage over other analytical methods of being usable not only for observing... 

Photodiodes produce an electric field as a result of pn transitions. On illumination a photocurrent flows that is strictly proportional to the radiation intensity. Photodiodes are sensitive and free from inertia. They are, thus, suitable for rapid measurement [1, 59] they have, therefore, been employed for the construction of diode array detectors. 

Fortunately, such a microscope can now be fitted with a multi-element detector to measure multiple areas simultaneously. One multi-element detector approach is to construct quasi-lD arrays from the same Hgi xCdxTe photo-conductive material as for a single-element detector, providing essentially the same spectral range and performance. Each element can be individually biased and read-out by an AC coupled amplifier having bandwidth compatible with FT-IR scanner velocities of 1 cm/s or above. Larger area, 2D arrays are usually made with photovoltaic elements i.e. photodiodes) to avoid the current biasing... 

A CMOS photodiode is sensitive to all visible wavelengths of light. To construct an array of pixels with channels sensitive to individual primary colors (e.g., red, green, and blue), a filter is placed above each pixel, creating a color filter array (CFA) mosaic. Photons outside of each filter s passband are absorbed, and by adjusting the thickness of the filters, the selectivity can be modified. There are a variety of different CFA patterns, the most popular being the Bayer pattern, which is shown in the photomicrograph of Fig. 7. 

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