Linear diode array

Most present-day fiber-optic sensors use linear diode arrays combined with optical gratings and measure the absorption, transmission, fluorescence, and reflection in UV, visible, and NIR regions (see Table 3.1). Light travels to the sampling probe via one fiber-optic cable and returns to the instrument via a second. Laser excitation permits long-distance transmission of excitation radiation to get a useful signal from the sample. 

Diode-array spectrophotometry exploits a different principle to conventional spectrophotometry (Figure 7-7). In this, the cuvette is illuminated directly by the light source, and the monochromator is positioned between the cuvette and the detector. In place of an exit slit on the monochromator, the detector consists of a linear diode array which enables light at all wavelengths to be detected simultaneously. The advantage of this construction is that rapid measurements can be made of the complete spectrum the disadvantage is that the sample is continuously exposed to high intensity illumination. 

S-Series Solid State Linear Diode Array", E.G. G. Reticon Brochure, 1978. 

A solid-state linear diode-array spectrometer has been applied to the simultaneous determination of relative amounts of hemoglobin, carboxyhemoglobin, and oxyhemoglobin in whole blood [128]. The fiber is incorporated in a hypodermic syringe needle and the analytical wavelength is set between 520 and 620 nm. Derivative spectra, signal averaging, and least-squares calculations were applied to extract the maximum possible information from the spectra. 

FIGURE 7-33 A reverse-biased linear diode-array detector, (a) cross section and (b) top view. 

Most spectral information is obtained if a dispersive grating (polychro-mator) disperses the probe light on a linear diode array, such that an entire spectrum can be measured at once. A second diode array should then be used to record the reference beam (an example is given in Fig. 5 ). Pulse compression is needed in both the pump and probe beams to compensate for group velocity dispersion (GVD) in the entire setup. 

Ideally, the velocity of the counter flow should be exactly equal and opposite to the velocity of the analyte band so that band broadening from pressure driven flow is minimized. The practical implementation of such a system requires a detector capable of imaging the entire width of the separation window such as a linear diode array or charge-coupled device (CCD) camera. 

In recent years, multichannel detectors have become more and more popular, especially intensified linear diode arrays and CCD cameras. These devices offer a sensitivity similar to that of PMTs, while a complete spectrum can be recorded simultaneously. Apart from the substantial reduction in measurement time, another advantage is the fact that in the case of signal fluctuations (e.g., due to photodegradation, hole burning) all peaks in the spectrum are equally affected. For time-resolved detection a pulsed excitation source and a fast gateable intensified camera can be used. 

This has a diode pixel density of 1024 x 256 giving an effective size of 27 pm by 27 pm. This device is capable of generating 74 500 spectra per second and is up to 100 times more sensitive than conventional linear diode array detectors. 

Fig. 4.68a,b. Measuring interference ring diameters with a linear diode array (a) correct alignment (b) misaligned diode array... 

If such a linear diode array with N diodes and a length L = N b H- d) is placed in the observation plane of a spectrograph (Fig. 4.1), the spectral interval... 

Automatic X-ray inspection using linear diode array imaging systems is discussed as a quality control method for tyres. 

FFT fast Fourier transform ILDA intensified linear diode array... 

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