ESE March10 pg14_15_Layout 1 26/02/2010 13:01 Page 2
ENCLOSURES 15
such as LED, LCD, vacuum fluorescent and plasma screens. They performance, the wire is plated and blackened. This also reduces
can also form the front panel of an enclosure, providing impact specular reflection from the mesh, enhancing the optics. Wire
protection, contrast enhancement, display colour matching, anti
mesh can cause moiré fringing on some displays, so to eliminate
reflection and anti glare.
this, the wire is orientated at an angle in the substrate from 11 to
45 degrees, depending on the display.
substrate selection
Transparent vapour deposited conductive coatings such as ITO
and gold do not shield as well as wire mesh. Their advantage is that
There is a choice of optical substrates available. Acrylic is available
optical clarity is maintained and there is no degradation in resolution
in many colours to match display outputs for improved contrast.
or fringing. The coating can be applied in various
Options include clear, said to exhibit 92 per cent light
thicknesses to achieve a resistance of
transmission, through to infra-red transmitting opaque materials.
five, 10 or 25 ohms/ in
2
.
Acrylic can also be machined and formed for front panels with cut
outs, holes or steps. Multiple layers can be laminated with fine
wire mesh together with different colour combinations and
www.kemtron.co.uk
circular polarisers. It can also be coated with anti scratch,
chemical resistant formulations as an optical flat or anti-glare and
has a UL94HB flammability rating.
Shielded windows provide EMI
Polycarbonate boasts high impact resistance - more than 16 times
protection while enabling users
that of acrylic and 200 times more than glass, making it suitable for
to view display devices
rugged applications. Light transmission is not as good at 85 per cent,
however it does have UL94VO flammability rating over 2.4mm
thickness. Anti scratch, chemical resistant coatings can be applied as
an optical flat or anti-glare and fine wire mesh can be laminated
between two layers.
Glass has a durable surface and will withstand high temperatures,
making it suitable for vapour coatings such as ITO or anti reflection
coatings that reduce surface reflections to less than 0.5 per cent.
Glass can be laminated with fine wire mesh and circular polarisers.
Allyl diglycol carbonate (ADC), trade name CR39, has a hard
surface. It is normally cast and has good impact resistance, but is
easily broken if scratched or notched. ADC is the most expensive
substrate and meets UL94HB flammability rating.
shielding choices
Fine wire meshes, in stainless steel or copper, provide the
highest shielding while maintaining optical transparency.
Stainless steel mesh ranges from 50 up to 250 wires per inch,
and or copper mesh from 70 to 145 wires per inch. Wire
diameters are 0.025 or 0.056mm for stainless, 0.056mm for
copper. 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 to 100
wires per inch.
To fuse wire
Greater w
ire densit
y gives b
etter shiel
ding
crossovers for
performa
nce but li
ght transm
ission is
degraded
consistent EMC
electronics-sourcing.eu MARCH 2010
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