Light absorption, near infrared

Upconversion is a process where low-energy light, usually near-infrared (NIR), is converted to higher-energy (visible) light via multiple absorptions or energy transfer processes. 

Amorphous (vitreous) selenium, vacuum-deposited on an aluminum substrate such as a dmm or a plate, was the first photoconductor commercially used in xerography (6). It is highly photosensitive, but only to blue light (2). Its light absorption falls off rather rapidly above 550 nm. Because of the lack of photoresponse in the red or near infrared regions, selenium photoreceptors caimot be used in laser printers having He—Ne lasers (632.8 nm), or soHd-state lasers (680—830 nm). 

Photometric Moisture Analysis TTis analyzer reqiiires a light source, a filter wheel rotated by a synchronous motor, a sample cell, a detector to measure the light transmitted, and associated electronics. Water has two absorption bands in the near infrared region at 1400 and 1900 nm. This analyzer can measure moisture in liquid or gaseous samples at levels from 5 ppm up to 100 percent, depending on other chemical species in the sample. Response time is less than 1 s, and samples can be run up to 300°C and 400 psig. 

Ultraviolet and infrared spectroscopy indicate that quinoxaline-2,3-dione type structures are preferred to tlie tautomeric 3-hydroxy-quinoxalin-2 One or 2,3-dihydroxyquinoxaline forms. The light absorption properties (UV) of quinoxaline-2,3-dione have been compared with those of its NN -, ON-, and OO -dimethyl derivatives (79, 80, and 81), and also its N- and 0-monomethyl derivatives (43 and 82). The parent dicarbonyl compound and its mono- and di-A -methyl derivatives show very strong carbonyl absorption near to 1690 cm split into two peaks. 

Compounds 3 and 4 are claimed31,32 to exhibit good solubility, while 5 is stated33 to possess excellent light-resistance, However, to date, no similar product has been able to replace CVL in the marketplace. One further example of the flexibility of the synthetic route in Scheme 3 is the preparation of compound 6,34 which is reported to show light absorption in the near infrared region and is thus suitable for recordings readable by lasers. 

The book is divided into seven chapters. Chapter 1 describes photo-chromic materials which have critical applications in memory technology. These compounds generally are activated by light. Chapter 2 covers leuco quinones which, in many cases, when oxidized, have their absorption maxima in the near-infrared region. Chapter 3 describes leuco dyes of a common group of compounds—oxazine, thiazine, and phenazines—that have found applications in color photography. Chapters 4-6 describe arylmethine-type compounds that can be triggered to dyes by common chemistry. Chapter 7 describes a special class of leuco dyes, namely, tetra-... 

Even more elegantly, the local resolution is improved by irradiation with very intense focused femtosecond laser pulses outside the absorption range of the fluoro-phore (e.g., in the near-infrared). The very intense focus of the laser beam—and only this—will excite the fluorophore by nonresonant two-photon absorption. Artifacts by scattered primary radiation are ruled out and the local resolution is comparable to a confocal microscope. In addition, the damage of the sample by laser light absorption is reduced to a minimum. 

In spite of the advent of visible light lasers the interest in near-infrared (NIR) absorbers continues because they do offer certain advantages, especially if they show little or no low absorption in the visible region, for instance their use in invisible security markings. The near-IR absorbers may be classified according to their... 

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