Optical techniques photodetector

The light emitted from the excitation source is dispersed and recorded by multiple photodetectors placed behind suitably located exit slits (Fig. 6.65). Fibre-optical techniques can also be used to collect light at a suitable location in the spectrometer focal plane for transmission to a battery of PMT.s. Calibration can be provided using standard solutions or standard electrodes. Using a computer-controlled data collection system the concentrations of the selected elements can be printed out shortly after the introduction of the sample. A complete system frequently incorporates means for automatic sample exchange. 

The measurement of the time-dependent depolarization of the fluorescence from molecules rotating on a time-scale comparable to the fluorescence decay time, enables information to be derived concerning the molecular reorientation motion. A review of these techniques has been published. A method involving an optical delay line has been used to record time-resolved fluorescence depolarization methods using only 1 photodetector, and thus some of the possible instrumental distortions are removed. ... 

One disadvantage of such control systems is that they control the electrical input to the lamp rather than the actual lamp intensity. Several systems have been described using optical-feedback techniques to directly control the lamp intensity. Usually in these techniques the lamp intensity is monitored with a second photodetector, and a signal from this detector is fed back to the lamp power-supply. 

Interferometry is the most accurate laboratory technique for precise measurement of distance. It should be recognized that direct measurement of the phase of an electromagnetic wave cannot be done for the optical wavelengths, as is done with oscilloscopes for low-frequency electrical signals, and only the light intensity, which is the average power of the electromagnetic field, can be measured with photodetectors. 

---