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1800 Series

Screened Optical Windows

ADC (allyl diglycol carbonate) trade name CR39 has a very hardsurface hence its use as spectacle lenses. ADC is normally cast and has good impact resistance and is highly formable but is easily broken if scratched or notched. ADC is the most expensive substrate, it meets UL94HB flammability rating.

Fine wire mesh

Wire mesh provides the highest level of shielding while maintaining excellent optical properties. Wires used as the EMI shield are stainless steel or copper. The stainless steel mesh ranges from 50 wires per inch up to 250, copper 70, 100 and 145. Wire diameters are 0.025mm or 0.056mm for stainless, 0.056mm for copper. The greater wire density gives better shielding performance but light transmission and optical clarity is degraded. The optimum wire count to achieve good shielding and optical characteristics is around 80-100 wires per inch. The wire is plated and blackened to fuse the wire crossovers ensuring consistent EMC performance and the blackening reduces reflection from the mesh enhancing the optics. Wire meshes can cause moiré fringing on some displays in particular CRT's, to eliminate this the wire is orientated in the substrate at an angle to be determined during the window design, this maybe from 11 to 45 degrees dependent on the display. Glass has a very durable surface and will withstand hightemperatures making it suitable for the application of vapour coatings such as ITO (indium tin oxide) for EMI shielding and or anti- reflection coatings that will reduce first surface reflections to less than 0.5%. Glass can be moulded or sagged to suit CRT applications but is not easily machined. Glass can be fully laminated with fine wire meshes and circular polarizers when required. Screened optical windows are used to provide a transparent EMIshield for optical devices such as CRT‘s, LED‘s, LCD‘s, Plasma displays, Electro mechanical displays and any aperture that needs to display information or indicator lamps. Screening or shielding of optical windows is achieved by two basic methods. (1) Using a very fine woven wire mesh entrapped between or embedded in a clear optical substrate such as Acrylic, Polycarbonate or Glass. (2) Transparent vapour deposited conductive coatings applied to the surface of the clear optical substrate such as Indium Tin Oxide or Gold. Polycarbonate has very high impact resistance, more than 16times that of Acrylic and 200 times more than glass making it most suitable for rugged applications. Light transmission is not as good as other substrates at 85%, however has the advantage of having UL94VO flammability rating over 2.4mm thickness, special grades as thin as 0.25mm are also available that meet UL94VO. Hard anti- scratch, chemical resistant coatings can be applied to the surface as an optical flat or anti-glare. Fine wire meshes can be fully laminated between two layers.

Window design and mounting

Windows are custom items and drawings or detailed specification are required. Sizes available range from 1cm
2 up to 1 X 2 metresfor architectural use. Windows can be moulded to suit CRT's, machined and fabricated, silk screened or printed with logo's, information etc. Termination can be by mechanical clamping or bonded with conductive adhesive.

Optical Substrates

Acrylic is a very versatile substrate and can be supplied in manycolours to match display outputs to improve contrast enhancement, including clear which exhibits 92% light transmission through to infra-red transmitting opaque materials. Acrylic is very easily machined and formed making it suitable for moulded CRT applications and front panels needing cut outs, holes, steps etc. Multiple layers can be fully laminated with the fine wire meshes in between, together with different colour combinations and the inclusion of circular polarizers. Acrylic has a UL94HB flammability rating. Hard anti-scratch, chemical resistant coatings can be applied to the surface as an optical flat or anti-glare.
Kemtron Limited , Prospect House 1-3 Brickfields Road, South Woodham Ferrers, Chelmsford, Essex CM3 5XB, EnglandTelephone +44(0)1245 325555 Facsimile +44(0)1245 325590 Email: info@kemtron.co.uk Internet: www.kemtron.co.uk