Poor Optical Properties

Poor optical properties result from many causes that can be divided into two problem groups material and processing. Two material problems that cause poor optical properties are contamination and phase separation of incompatible materials. Contamination occurs when foreign matter (such as dirt, degraded particles, or water) is in the extrudate, creating specks or streaks in the film. Phase separation of incompatible materials from improper blending of polymer types or including an incorrect additive can lead to a reduction in clarity. 

Processing problems that result in poor optical properties include die lines, low cooling rates, melt fracture, and, for coextrusion, interfacial instabilities. Die lines, as discussed previously, are bothersome because they can reduce both mechanical and optical properties. The cooling rate has a significant effect on polymer crystallization. Because crystallinity in polymers generally reduces optical properties, cooling rates are often increased to improve film clarity. Melt fracture will usually have an effect similar to die lines with respect to optical properties. Any occurrence that creates texture on the film surface will reduce film transparency. Similarly, if the texture occurs at the boundary of two layers in a coextruded film, as happens with interfacial instabilities, the same reduction in optical properties will result. 

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Several gemstone species occur in various colors, depending on the presence of impurities or irradiation-induced color centers. Examples are the beryl, comndum, and quart2 families. Quart2 has poor optical properties (RI = 1.55, DISP = 0.013), but becomes of gemological interest when it exhibits attractive colors. Any material can have its color modified by the addition of various impurities synthetic mby, sapphires, and spinel are produced commercially in over 100 colors (2). Synthetic cubic 2irconia has been made in essentially all colors of the spectmm (11), but only the colorless diamond imitation is produced commercially in any quantity. 

In the 1980s, CdSe quantum dots vere prepared by top-dovm techniques such as lithography ho vever, size variations, crystal defects, poor reproducibility, and poor optical properties of quantum dots made them inadequate for advanced applications. Introduction of bottom-up colloidal synthesis of CdSe quantum dots by Murray et al. [3] and its further advancements brought radical changes in the properties of quantum dots and their applications in devices and biology. The colloidal syntheses of CdSe quantum dots are broadly classified into organic-phase synthesis and aqueous-phase synthesis. 

The first synthetic polymers to be used as paint varnishes were acrylic and vinylic resins. Poly(vinyl acetate) (PVAc), commercialized under the name Mowilith by Hoechst and Vinylite by Union Carbide, has been used in conservation as an adhesive since 1932 and in 1937 it was proposed as a picture varnish by Stout and Cross [63]. PVAc was soon rejected as a varnish because, despite its light stability and good solubility in organic solvents, it demonstrated poor optical properties in terms of colour saturation and the tendency to pick up dirt due to its low glass transition temperature. 

Poly(trimethylene terephthalate). Poly(trimethylene terephthal-ate) (PIT) is a crystalline polymer that is used for fibers, films, and engineering plastics. The polymer has an outstanding tensile elastic recovery, good chemical resistance, a relative low melting temperature, and a rapid crystallization rate. It combines some of the advantages of poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT). Disadvantageous are the low heat distortion temperature, low melt viscosity, poor optical properties, and pronounced brittleness low temperatures. 

FEP and PFA despite being melt-processible are crystalline (between 50 and 70%). The crystallinity results in poor optical properties (low clarity) and a very poor solubility in organic solvents. The latter makes the preparation of thin optical coatings exceedingly difficult.10 TEFLON AF, an amorphous fluoropolymer, contains in its molecule a bulky dioxole ring, which hinders crystallization. As a result, the polymer has an exceptionally high clarity and excellent optical properties. Its refractive index is the lowest of any plastic.11... 

In spectrophotometry, the Schlieren effect can alter the analytical results because it influences the propagation of the radiation beams involved (Fig. 4.1). Collimation of the light beam plays an important role in this respect, as an effect analogous to a shadowgraph is observed when a spectrophotometer with poor optical properties is used [83]. 

The crystal quality of the InGaN QWs becomes poor mainly due to the lattice-constant mismatch and the difference of the thermal expansion coefficient between InN and GaN with increasing the In composition [4,5]. Therefore, in order to improve the external quantum efficiency (i/ext) of the InGaN-based LEDs and LDs, it is important to elucidate and optimize the effects of the various growth conditions for the InGaN active layer on the structural and optical properties. Recently, we reported a fabrication of efficient blue LEDs with InGaN/GaN triangular shaped QWs and obtained a substantial improvement of electrical and optical properties of the devices [6,7]. 

Further, it may often be desired to identify poorly formed crystals such as needle-like crystals without definite faces, or skeletal growths, or even completely irregular fragments. This can only be achieved by measuring some properties of the crystal material itself, properties which are independent of the shape of the crystals. Of such properties, by far the most important and the most convenient for measurement are the optical properties, especially the refractive indices. An elemen-... 

Further modification of the coumarin ring system in esculin (1) led to the development of 7-amino-4-methylcoumarins with an A-alkyl group at the 7-position. The sole representative of this series still on the market is 7-diethylamino-4-methylcoumarin (63) [91-44-1] [101], Despite its poor lightfastness, the good optical properties of this compound have kept it in use for the brightening of wool, cellulose acetate rayon, and polyamides. Compound 63 is produced by cyclocondensation of 3-diethylaminophenol with ethyl acetoacetate in the presence of zinc chloride [102],... 

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