Application of Coatings on Glass

Antireflection coatings on plastics deposited by plasma polymerization process

“…Usually, silica (SiO 2 ) with a refractive index of about 1⋅45 and titania (TiO 2 ) with a refractive index of about 2⋅2 are chosen as the low and high refractive index materials, respectively. This is because these materials can be readily deposited in thin film form, are hard and transparent in the visible region, and are compatible with each other in a multilayer stack since they have opposite (tensile and compressive) intrinsic stresses which balance each other to produce low stresses in the composite multilayer (Pulker 1987(Pulker , 1999Macleod 2001;Baumeister 2004). Accordingly, a four-layer ARC comprising of alternate layers of TiO 2 and SiO 2 , to be deposited on a carbonyl SiO 2 layer on the PC substrate, has been designed using FILMSTAR thin film design software (FTG 1997).…”

“…Moreover, it is very difficult to find a suitable coating material with a refractive index lower than 1⋅30. Two-layer or three-layer ARCs, comprising of films of two or three materials and with thicknesses of one quarterwave or non-quarterwave at some design wavelength, can provide a low reflectance zone over a short range of wavelengths (Pulker 1987(Pulker , 1999Macleod 2001;Baumeister 2004). However, most ARCs applied to spectacle lenses comprise 4-6 layers of two materials with high and low refractive indices (compared to the refractive index of the substrate), in an alternate sequence, with certain non-quarterwave thicknesses, and provide a low reflectance zone covering the visible spectrum (Samson 1996;Cerac 1997Cerac , 1998Cerac , 2000.…”

“…The simplest ARC comprises a single layer of a transparent material whose refractive index is the square root of the refractive index of the substrate (for PC, √1⋅59 = 1⋅26) and whose optical thickness (refractive index, x, physical thickness) is one-quarter of the wavelength at which the antireflection is required (Pulker 1987(Pulker , 1999Macleod 2001;Baumeister 2004). However, this ARC will result in zero reflectance only at this wavelength, while at other wavelengths the reflectance could be more than that of the bare substrate.…”

“…Thus, different techniques are required for the deposition of optical coatings on plastic as compared to glass substrates. In the case of plastic substrates, the required energy is usually provided to the depositing material particles through ionassisted deposition processes-a fairly expensive although well-established technology (Pulker 1987(Pulker , 1999; Macleod *Author for correspondence (kmk_srivatsa@mail.nplindia.ernet.in) 2001; Baumeister 2004). Plasma polymerization processes enable the deposition of optical thin films at room temperature, at any rate below 100°C, and are therefore, especially suitable for plastic substrates (Samson 1996;Shi 1996;Martinu and Poitras 2000).…”

“…The purpose of an ARC is to minimize the amount of radiant energy that is reflected from the surface of the optical element on which the ARC is applied. Typically, ARCs comprise one or more optical thin films deposited sequentially on the surface of the optical element, to reduce the net reflected light intensity by destructive interference between the light waves reflected/transmitted from the surfaces and interfaces of the films and the substrate (Pulker 1987(Pulker , 1999Macleod 2001;Baumeister 2004). ARCs are widely applied on the surfaces of spectacle lenses, because unwanted multiple reflections of incident light from the lens surfaces are thereby largely avoided, and so the visibility of the wearer is improved and a better cosmetic appearance is achieved.…”