Electromagnetic interference (EMI) has long been a curse for manufacturers and users of electronic equipment. The effects of EMI are well known, but it is less known why they are becoming a growing problem.

There are two main reasons why optical products in particular should be shielded from EMI: Firstly, more and more devices are mobile or portable and used in uncontrolled environments. On the other hand, stricter regulations on electromagnetic compatibility (EMC) are being introduced.

In 2016, the European Union adopted a directive on EMC conformity (2014/30/60). Severe sanctions will be imposed for non-compliance. In the worst case, a non-compliant product will be withdrawn from the market and replaced. A fine of up to 6500 € and three months imprisonment are threatening. Product-specific EMI regulations have also been issued, for example the Medical Device Directive (MDD). For the user, this has the advantage that devices function everywhere without interference or disturbances. Reliability has become the norm.

These changes have meant that shielded windows are no longer only of interest in military, aerospace or test and measurement applications, where it has always been important to protect the function of equipment from EMI. In today’s circumstances, it would only be fair that in all areas where integrated displays are used in optical devices, they should be as free from electromagnetic interference as possible.

The problem is that in many devices with indicators or displays there is the inevitable gap between the housing and the display – a gap that is automatically susceptible to EMI.

Although most devices are manufactured with integrated shielding against EMI (metal housings, metallized plastic moldings, conductive seals, and even additional environmental seals help shield electromagnetic radiation), any housing with an opening is susceptible to EMI. The larger the indicator or display, the greater the problem.

A solution in such cases is a shielded window above the opening, i.e. in front of the display, to fully restore the EMI shielding of the conductive housing. The quality of the window and the material can vary considerably.

A shielded window is a laminated transparent panel of acrylic, polycarbonate and polyester or a glass substrate containing a conductive wire mesh. The braid maintains electrical continuity and weakens the EMI signal that would otherwise pass through the gap between the display and the enclosure.

This wire mesh provides a compromise or balance between EMI attenuation and sufficient light intensity to allow the human eye to read the display in different environments. The mesh size is critical to how effective the shielding is. A denser mesh will certainly provide a high level of shielding, but may reduce the light transmission from the backlight.

Other factors that contribute to the efficiency of EMI shielding are the electrical conductivity of the crossing wires and the techniques and materials used to connect the wires to the frame. Glass is the best material for shielded windows because it is optically transparent and offers high scratch resistance. Polycarbonate or acrylic windows, on the other hand, are lighter and more resistant to impact and breakage. They can also be used at temperatures up to +100 °C. Depending on the application, water-repellent, oil-repellent and anti-fog coatings can be applied.

Chomerics Europe, a division of Parker Hannifin, takes all these factors into account in the development of its shielded windows. A product manufactured in Grantham (England), where optical windows have been manufactured for more than ten years, guarantees the best optical performance and the use of optically matched adhesives and optically high-quality polycarbonate substrates. Chomerics designs the wire mesh in copper or stainless steel and blacks it by anodising, minimising reflections and improving the optical performance of the window.

The latest product in the battle against EMI is WIN-SHIELD Mesh 100P, which can be used in glass or plastic windows, shielded vents and displays without compromising airflow or display readability. The wire mesh can be easily adapted to different shapes. If necessary, it can also be stretched over small ventilation openings in a housing.

The advantage of Mesh 100P is the high shielding of 55 to 60 dB (100 MHz to 1 GHz) and a surface conductivity of <0.1 W/sq. The 61% open area copper mesh is coated with an inert blackened polymer and provides 100 openings per inch (OPI) and a wire diameter of 0.056 mm (0.002 inch). Not only does the braid reduce glare and improve viewing, its coating is designed to not fade, age or react with humidity. With an operating temperature range of -40 to +70 °C, the shield is available in sheet form (max. 30,000 mm x 2,000 mm) or as a customized punched part. Chomerics also offers WIN-SHIELD C, a monomer material that combines electromagnetic shielding with optical clarity. Applications include display filters in test equipment, medical devices, public information displays as well as in military radio systems and computers. Unlike other materials for shielded windows, which consist of an electrically conductive medium such as a laminated wire mesh on a substrate, WIN-SHIELD C is a one-component material that guarantees 90% light transmission and can be manufactured in thicknesses between 1.5 and 6 mm. The operating temperature range is -60 to +100 °C and the material can be used in a variety of demanding environments. Also available are touch screens, which are easy to use when EMI shielding is required. They are suitable for mobile, portable devices, rugged LCDs, medical equipment, shielded cabinets and racks, and digital signage. The EMI shielding is integrated into the touchscreens and has no air gaps or cavities thanks to optically suitable, pressure-sensitive adhesives. The EMI shielding can be up to 0.007 inches (0.177 mm) thin and includes conductive foils with 4 and 8 W/sq and a coated stainless steel braid with 50, 80 or 100 OPI. We would be happy to advise you on this and help you make the right choice.

Ref: CH/244/A