Mobility light sensors

Illumination creates excess electrons and holes which populate the extended and localized states at the band edges and give rise to photoconductivity. The ability to sustain a large excess mobile carrier concentration is crucial for efficient solar cells and light sensors and depends on the carriers having a long recombination lifetime. The carrier lifetime is a sensitive function of the density and distribution of localized gap states, so that the study of recombination in a-Si H gives much information about the nature of the gap states as well as about the recombination mechanisms. 

In addition to the fiber sensors, there have been numerous reports on remote fiberless optical detection of pollution from stationary sources. Mobile remote sensors can have cost advantages over on-site instruments and also are much more versatile. Pollutants in smokestack emissions have been identified by irradiation with a laser from a mobile unit equipped with a telescope, a monochromator, and low-noise detection electronics. The gases and particles in the plume scatter the laser light in various directions. A fraction is scattered back to the receiver and analysed to detect the amount and type of gas in the plume. Many of these methods await their application to fiber-optic sensing schemes which are inherently safer than direct laser spectroscopic schemes. 

The UV absorption system is very similar to that of the DuPont bifunctional detector. Light from a UV lamp is collimated through the cell by a quartz lens that serves as one end of the sensor cell, and is then focused by another quartz lens at the other end of the cell onto a photodiode. The output from the photodiode is processed electronically in the usual manner to provide an output that changes linearly with solute concentration. The ends of the cell, between the cell body and the quartz lens, where the mobile phase from the column enters the... 

The detector cell is 3 mm long terminated at one end by a cylindrical quartz window and at the other by a plano-convex quartz lens that disperses the transmitted light over a significant portion of the light sensitive area to the photocell. The total volume of the sensor cell was about 2.5 pi. Next to the quartz windows are two stainless steel discs separated by a 3 mm length of Pyrex tube. The mobile phase enters and leaves the detector cell through radial holes in the stainless steel discs that connect to the central orifice of the disc. The stainless steel... 

The importance of these amorphous layers derives from their electronic structure. There are no longer sharp bands characterized by a definite band gap, but quasi-continuous changes in the density of states are observed leading to differences between the optical gap and the mobility gap . Thus, interesting optoelectronic properties and applications are reported, e.g., photoconductivity and solar cells [204, 205], optical vnndows for solar cells [206, 207], electroluminescence and light emitting diodes (LED) [207, 208], or thermistors for IR sensors [209]. 

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It is not necessary in every case to make use of a complete instrument with elaborate spectral dispersion. Often the set-up can be restricted to the demands of the actual measurement. A useful arrangement is the combination of a light-emitting diode (LED) with a photodiode possessing an absorption spectrum congruent with the emission spectrum of the LED. Such a small and cheap device is mobile and fulfils the demands of sensor use. An approach in this matter has been devised by Smardzewski (1988). 

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