Visible and Ultraviolet Light

Scintillators are also used in the detectors of CT scanners. Here an electronic detector, the photomultiplier tube, is used to produce an electrical signal from the visible and ultraviolet light photons. These imaging systems typically need fast scintillators with a high efficiency. 

Sunblocks are opaque substances such as zinc oxide, titanium dioxide, and iron oxide that protect by forming a shield on the skin, which reflects and scatters incident radiation. In essence, sunblocks provide physical protection against sun exposure, including both visible and ultraviolet light. Sunscreens are substances that chemically absorb ultraviolet light in the top layer of the epidermis, protecting the underlying layers. 

The sunblocks zinc oxide, titanium dioxide, and iron oxide are inorganic chemicals that are not absorbed into the skin. These substances consist of opaque particles that reflect both visible and ultraviolet light. In addition, zinc oxide blocks virtually the entire UVA and UVB spectrum and thus offers overall protection. The particulate nature of these sunblocks enhances their effectiveness at reflecting sunlight. The smaller the particle size, the greater the surface area available for reflection, and the more effective the sun protection offered by the formulation. 

Table 1.1 shows the properties of visible and ultraviolet light. 

When a molecule absorbs a quantum of light, it is promoted to an excited state. Because the energy of visible and ultraviolet light is of the same order of magnitude as that of covalent bonds, another possibility is that the molecule may cleave into two parts, a process known as photolysis. 

An extraordinary variety of reactions of organic compounds are known to occur under the influence of visible and ultraviolet light. Some of these, such as the photochemical halogenation of alkanes and photosynthesis in green plants, already have been discussed (see Sections 4-4D and 20-9). It is notour purpose here to review organic photochemistry in detail — rather, we shall mention a few types of important photochemical reactions and show how these can be explained by the principles discussed in the preceding section. 

Cooper, W., and M. Fielden Graft polymerization in emulsion using visible and ultraviolet light. J. Polymer Sci. 28, 442 (1958). 

In addition to the absorbance of visible and ultraviolet light, fulvic acids are major fluorophores in natural waters. Analysis of the fluorescence of... 

Coumarin groups are commonly incorporated into photoactive polymers because of their favorable photochemical characteristics and the ease of integration into the polymer matrix. The addition of the appropriate coumarin moiety into the monomers themselves or as an additive in the polymerization process can result in the polymeric product possessing beneficial visible and ultraviolet light absorption properties. 

Photochemistry involves the interaction of visible and ultraviolet light with cellulose whereas radiation chemistry involves its interaction with high-energy radiations, such as from y-radiation. Light promotes the deterioration of cellulosic products, particularly cotton fabrics, and certain dyes or other additives greatly accelerate this process known as phototendering. Ionizing radiation is used to sterilize medical and bioproducts many of which are cellulosic. Several authoritative reviews are available [511-514]. 

Hewetson, J.F., Rivera, YE, Poli, M. A., and Hines, H.B. 1990. Modification of palytoxin activity and structure by visible and ultraviolet light. Tbxtcon 28(6) 612. 

Another important characteristic of semiconductors is their optical absorption properties. The optical properties of semiconductors are crucial to their uses as photovoltaic energy conversion devices, photonic detectors, light-emitting diodes, solid state lasers, and optical switches see Semiconductor Interfaces). Some semiconductors absorb both visible and ultraviolet light, while others only absorb photons in the... 

Figure 4 Photodegradation (decreasing drug A levels versus increasing phenol levels) induced by various International Conference on Flarmonization visible and ultraviolet light exposures for a solution containing 0.57mM drug A, lOmM citrate (pFI 6), and 136 mM sodium chloride. A = 0.3 x 10 Lux hour B = 0.6 x 10 Lux hour C = 0.9 x 10 Lux hour D = 1.2 x 10 Lux hour E = 1.2 x 10 Lux hour followed by 50 W hr/m F = 1.2 x 10 Lux hour followed by 100 W hr/m G = 1.2 x 10 Lux hour followed by 150 W hr/m FI = 1.2 x 10 Lux hour followed by 200 W hr/m. ...

Determination of the Total Pre- and Aftereffect. Measurement in the nonstationary state requires a method of altering the initiation rate of the system instantaneously, and a precise determination of the small amount of conversion involved. The first condition in most cases is met by photochemical initiation methods. This was, to our knowledge, never done on emulsion systems, because the vast and varying dispersion of visible and ultraviolet light by the system during the reaction prevents the homogeneous illumination of the reaction vessel. These difficulties can be successfully met by applying y-radiation. 

Irradiation also affects the course of more conventional separation processes. Visible and ultraviolet light have been found to affect plutonium solvent extraction by photochemical reduction of the plutonium (12). Although the results vary somewhat with the conditions, generally plutonium(VI) can be reduced to pluto-nium(IV), and plutonium(IV) to plutonium(III). The reduction appears to take place more readily if the uranyl ion is also present, possibly as a result of photochemical reduction of the uranyl ion and subsequent reduction of plutonium by uranium(IV). Light has also been found to break up the unextractable plutonium polymer that forms in solvent extraction systems (7b,c). The effect of vibrational excitation resulting from infrared laser irradiation has been studied for a number of heterogeneous processes, including solvent extraction (13). 

With the introduction of ultraviolet absorption procedures (ASTM D-2008, ASTM D-2269), the test finds less use but still provides a useful method to determine possible contamination of mineral oil with impurities transparent to both visible and ultraviolet light and hence not detectable by color or by ultraviolet absorption measurements. 

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