The invention relates to electromagnetic shielding panels and, in particular, to translucent electromagnetic shielding panels comprising a pane of coated glass or plastics.
Glass and transparent plastics are translucent to short wave electromagnetic radiation, i.e. radiation having a frequency in the range 20 to 10000 MHz, so that stray emissions from electronic data processing equipment can escape through windows and be detected outside. Such escape prejudices the security of the information being processed. Moreover, entry of extraneous electromagnetic signals, such as radar signals, through the windows of rooms housing electronic data processing equipment can effect the proper operation of the equipment.
It is known that the transmission of short wave electromagnetic radiation through windows can be prevented or at least reduced by providing the window with a continuous electroconductive layer and earthing the layer all around the periphery of the window to provide an electromagnetic shielding window.
A number of glass products incorporating electroconductive layers have been offered for use in electromagnetic shielding windows. One such product is glass carrying an indium tin oxide coating having a sheet resistance in the range 4 to 6 ohms per square. Such coatings may be deposited by reactive sputtering but, in order to obtain the high conductivity (low resistance) required, it is necessary to heat the glass substrate. Suitable apparatus for coating large areas of glass is expensive; the coatings are therefore generally deposited in relatively small areas and are expensive to apply.
Another known product is laminated glass incorporating a layer of fine metal mesh in a plastics interlayer between two panes of glass. Unfortunately, the mesh not only reduces the light transmission through the laminate but also obscures the view. Moreover, because the final metal mesh is expensive, laminates incorporating the mesh are expensive to produce.
It has also been proposed to apply a polyester film carrying a sputtered coating of a metal or metal alloy such as titanium, stainless steel, bronze, sterling silver or 24 carat gold to glass with the coated face of the film towards the glass to provide an electromagnetic shield. Unfortunately, it is difficult to make electrical contact with the coating on the film without damaging the film, so that there are problems in earthing the coating.
There is a need for a high performance translucent electromagnetic shielding panel which can be produced economically with good optical properties.