Лако-красочные материалы — производство

Optical systems have a higher resistance to the environment and better long-term stability when the surfaces in the ray path are kept small. When the optical cement is applied in a very thin layer, the elements can be placed with very high accuracy. An optical cement is a highly transparent adhesive that bonds the optical surfaces of two optical components. Unlike air gaps, the optical cement influences the optical path of the devices in service, due to its refractive index. This adhesive must therefore be highly transparent and free from particles. Usually, the refractive index of the optical cement is similar to that of the glass used. Decementing may occur due to thermal strains or moisture aging that may induce internal reflections and reduced trans­mission, leading to a complete failure of the system. It is therefore crucial that measures are taken to reliably prevent decementing.

When replacing compact mounts with optical cements, the measuring accuracy and the accuracy of image are improved. Loss of light will be reduced, and the system will be lightweight. Compared to structural adhesives, optical cements must meet totally different requirements because they are applied within the optical path. The key characteristics of optical cements are:

• spectral transmission

• refractive index and dispersion

• viscosity

• lack of strains and flexibility

• build-up of adhesion without primer

• resistance to UV light

• fluorescence

Special optical cements are available for devices used in space research, in medical devices required to be designated ‘sterile’, and in sensor technology. The optical cleanliness and resistance to aging of these cements may differ considerably. Selecting the proper optical cement requires a consideration of the temperature and moisture conditions to which it will be exposed. As glass surfaces are subject to moisture aging due to their chemical structure, immediately before cementing the glass components are cleaned in aqueous ultrasonic equipment. The removal of any surface contaminants is a prerequisite for uniform wetting and adhesion. The resistance to moisture aging at elevated temperatures depends largely on the chemical composition of the optical glasses used; for example, glasses with a high resistance to moisture, acids and bases will have a higher stability with regard to moisture aging. Frequently, the glass surfaces to be cemented are optically coated. Usually, the optical cement adheres better to the hydrophilic coating than to non — coated glass surfaces.

Owing to its flexibility, the optical cement acts as a buffer when bonding optical glass elements with different coefficients of expansion. When cementing different types of glass, the optical cement layers can be applied in variable thicknesses, depending on the viscosity of the cement and the pressure applied when joining the components. Viscous and more flexible optical cements are used when cementing large optical elements made from different types of glass with different coefficients of thermal expansion. However, in practice, thicker cement layers induce undesired wedge errors.

In industrial production, UV-setting acrylics, urethanes or epoxy cements are mainly used owing to their easy metering and rapid curing characteristics. Besides flexible two-part epoxy systems, two-part silicone systems are employed for special requirements, particularly for components that are very sensitive to strains. These cements are also used whenever a high spectral transmission is required below 400 nm and/or whenever radiation is very intense.

8.8.4