Optometric optical coating system refers to applying single layer or multiple layers of optical thin film of certain thickness on the surface of lens by using physical and chemical processes to offer some new excellent features that the lens do not have previously. It can be used for mass production of anti-reflection thin film on the lens of glasses.
Currently, it primarily includes: 1. anti-reflection film; 2. top film.
1. Anti-reflection film (transmittance-increasing film)
It is multi-layer film. The transmittance of neither resin nor glass lens can reach 100%; there will be some light that is reflected by the two surfaces of lens; the lens with higher refractivity has higher reflectivity. 4% for resin, 4.3% for glass. The reflection by lens can reduce the transmittance of light and form interfering image on the retina, thus affecting the imaging quality and affecting the appearance of wearer. For examples, the lens show obvious spirals; it reflects light when taking a photo; you cannot see the eyes of wearer etc. Anti-reflection film is the most commonly used optical coating; based on the principle of light interference, it allows the reflecting lights from lens and coating to interfere each other thus eliminating the reflection. Different films selectively eliminate the reflection of light in the corresponding band so as to remove the interfering light and increase the transmittance of visible light thus enhancing the imaging quality. It enables clearer imaging and reduces eye strain. It can also selectively reduce the transmittance of harmful rays in some bands e.g. ultraviolet light, X-ray etc. in order to reduce the damage to eyes by these rays and protect the eyes from hurting by them. Currently, the anti-reflection film on resin lens has up to nine layers, which have reduced the reflectivity down to about 0.2% and increased the transmittance of resin lens to be higher than 99%. It greatly enhances the imaging quality and achieves optimal corrective performance.
2. Top film (water-proof film, anti-fog film)
Due to technical demand, the anti-flection film has large intermolecular gap so that the surface of lens is prone to retain dirt. The component of top film has small molecular particles and small intermolecular gap so that the surface of lens is more smooth and cleaner, having additional features such as water proofing, fog protection, dust protection and contamination protection.
Choice of optometric optical coating process and quartz crystal:
Optometry (glasses and sunglasses; primarily plastic substrate): – Most of designs are intended for SiO2 and TiO2 coating by evaporation; therefore the electron beam gun becomes the preferred evaporation source, which needs working power supply up to 4-6 kW. We can use the resistive evaporation source to facilitate the adhesion of thin film (Cr) or water-proof film (polymer with high vapor pressure); such evaporation source is typically very small, and only needs very low power supply (2 kW). The system generally includes a heating device, but it is designed only for initial heating to facilitate air removal; so it also needs very low power (1-3 kW). In the system, the low-energy bombardment and the modification to substrate surface by the ion source before completion of the coating process is decisive to enhance the adhesion on substrate.
In order to exhaust large amount of water vapor released from the substrate, it is critical to exhaust water vapor at high rate. Therefore, these systems are generally equipped with auxiliary cryogenic pumps. A cryogenic pump is typically of size between 700 mm to 1,250 mm; a large system generally makes use of a set of diffusion pumps for air exhaust. A small system mostly makes use of a cryogenic pump or molecular pump. In order to maximize the capacity in a batch, the work piece racks are generally arranged in spherical cap configuration with revolution only; but when you coat the non-plano lenses with thin perimeter, all are arranged in reverse way. In order for the uniformity of color, goggles (sunglasses and safety glasses) need special shield. In these applications, it is sufficient to control the film thickness at an accuracy about 2%; the deposition rate and the final parameters both are controlled by QCM. Usually, it is sufficient to use gold crystals.