Iridescence INDEX

Colorants have been developed for special effects. Pearlescent or iridescent give an attractive appearance. They are often used for coating paper (e.g., cosmetic packaging). The pigment consists of thin platelets, less than 1 pm thickness, which have a high refractive index, e.g., mica coated with Ti02. 

The pearlescent effect in natural pearls arises because of the interaction of incident light with alternating layers of high refractive index (CaCOj) and low refractive index (protein), reflected and diffracted light from which leads to the observed iridescent colours. 

In one example, the colloidal structure, is made by sedimentation of polystyrene beads, giving voids in the range 120-1000 mn, and the voids are filled with TiO generated from titanium tetrapropoxide. The polystyrene bead lattice is then removed by calcining to give an iridescent material, but not with a full photonic band gap. In this case one of the controlling factors is the refractive index of the matrix, which needs to be greater than 2.8. 

In practice, platelet crystals are synthesized with a layer thickness d calculated to produce the desired interference colors (iridescence) [5.206], [5.207], Most nacreous pigments now consist of at least three layers of two materials with different refractive indices (Fig. 73). Thin flakes (thickness ca. 500 nm) of a material with a low refractive index (mica) are coated with a highly refractive metal oxide (e.g., Ti02, layer thickness ca. 50-150 nm). This results in particles with four interfaces that constitute a more complicated but still predictable thin film system. The behavior of more... 

The degraded film specimens often impart an iridescent color. This effect may be attributed to the interference of visible light due to diffused reflection between the protuberances formed on the surface of the films, which is interpretable in terms of a thin-layered gradation of the refractive index near the film surface. 

Thirdly, he can cultivate mental agility when his first thought misses the indexer s in such matters as designating properties or products. He can search Bactericides as well as Germicides, and he can switch from specific to general terms, or vice versa. If Luminescence does not serve, he can proceed to its special cases such as Ghemiluminescence, Fluorescence, Iridescence, or Phosphorescence. ... 

They were named zeolite ( boiling stone ) in 1756 by Cronstedt, a Swedish mineralogist, who observed their emission of water vapor when heated. At the other size limit, opals constitute another example of a naturally occurring nanostmctured material. These gems are made up mainly of spheres of amorphous silica with sizes ranging from 150 nm to 300 nm In precious opals, these spheres are of approximately equal size and can thus be arranged in a three-dimensional periodic lattice. The optical interferences produced by this periodic index modulation are the origin of the characteristic iridescent colors (opalescence). 

Coextrusion is an important process for manufacturing layered plastic composites, such as film, sheet, and tubing (Middleman, 1977 Schrenk and Alfrey, 1983 Schrenketal., 1963). A costly alternative would be the fabrication of individual layers of plastics followed by conventional lamination and coating. An additional advantage of the coextrusion technique is its ability to handle extremely thin films on the order of 10 xm. One of the most colorful applications of this technique is the iridescent film, which consists of hundreds of very thin layers of alternating low and high refractive indices. A typical example of this type of film consists of 116 layers of poly(methyl methacrylate) (PMMA) with refractive index 1.49 and 115 layers of polystyrene (PS) with refractive index 1.59 and total film thickness of about 20 o.m (Radford et al., 1973). 

---