Light photodiode array

We have also studied the digital signal intensity versus light intensity behavior of the photodiode array detector used in method B. Non-linear effects have been observed for Vidicon multichannel analyzers when employed in measuring the intensity of picosecond duration light. Photodiode arrays, in contrast, have rarely been employed in picosecond spectrometer, and, we are not aware of a previous study of the linearity of these devices for the detection of high intensity picosecond duration light. 

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... 

Fig. 7.18 (continued) (c) Light scattering apparatus used to detect polymer-polymer demixing 1 - HeNe laser, 2 - sapphire window, 3 - polymer film, 4 - photodiode array, 5 - copper block, 6 - resistance thermometer, 7 - temperature controlled jacket, (Reproduced with permission from Zywocinski, A., et al. J. Polymer Sci. Polym. Phys. 33, 595 (1995))... 

The photodiode array detector (PDAD) measures absorption of light waves by a sample. This is considered the most powerful of the ultraviolet spectrophotometric detectors. The optical system focuses light from a deuterium source through the sample flow cell onto several photodiodes. These act as capacitators by holding a fixed amount of charge. When light strikes the photodiodes, they discharge a certain amount of current. 

A PMT mounted at right angles to the incident laser beam permits continuous recording of the light scattered at that angle. In addition, a 512-element linear photodiode array (not shown) is mounted on the ring electrode... 

Note that the number of diffraction peaks decreases with time as the droplet diameter decreases, and the number density of peaks is very nearly proportional to the droplet size. The intensity of the scattered light also decreases with size. The resolution of the photodiode array is not adequate to resolve the fine structure that is seen in Fig. 21, but comparison of the phase functions shown in Fig. 22 with Mie theory indicates that the size can be determined to within 1% without taking into account the fine structure. In this case, however, the results are not very sensitive to refractive index. Some information is lost as the price of rapid data acquisition. 

The reaction cell has a White cell optical system (see Chapter ll.A.lc) with a pulsed xenon lamp light source. Once the radicals are formed, they are detected by their absorptions in the UV using the Xe lamp and a monochromator-photomultiplier or photodiode array detector. Thus the absorption spectra of the free radicals generated in the system can be measured and the absorption at a particular wavelength used to follow their reaction kinetics. 

The light then enters a polychromator, which disperses the light into its component wavelengths and directs the light at the diode array. Each diode receives a different wavelength, and all wavelengths are measured simultaneously. Resolution depends on how closely spaced the diodes are and how much dispersion is produced by the polychromator. The spectrum in Figure 21-26 (in the next chapter) was produced with a photodiode array detector. 

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