Optical Filters - Frequently asked questions now answered

1) What does emc stand for?
2) Do all displays emit electromagnetic (emi) and radio frequency (rfi) interference?
3) How does a shielded window work?
4) Why use a mesh window instead of a conductive coating?
5) What is the difference between the various meshes used in shielded windows?
6) What is Moiré Fringing?
7) Why does a non-dispersive etch not suit CRT displays?
8) What does a circular polarizer do?
8b) Can you use a circular polarizer on an LCD display?
9) What does a linear polarizer do?
10) How do I ground my window to a display:
11) Why choose a plastic window over glass?
12) How do Optical Filters inspect their windows?
13) What about impact resistance if I use a glass window?
14) What type of window is suitable for food production lines?
15) How do I prevent the front of my display from frosting or misting over?
16) The emi shielded window is not offering adequate attenuation.
17) How can I improve the contrast enhancement on my display ?
18) What about privacy?


1) What does emc stand for?
Electro-magnetic compatibility. It is the ability of two electrical or electronic systems or units to co-exist without either disturbing the correct operation of the other.

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2) Do all displays emit electromagnetic (emi) and radio frequency (rfi) interference?
They all emit some level of radiation. What is important however is that these levels do not exceed the levels set out by the relevant standard. These levels are specified as being the acceptable limit to ensure that the performance of an electronic or electrical apparatus will neither disturb that of another apparatus nor be degraded by electromagnetic disturbance.

EMI from an LCD or EL display originates from the internal electronic circuitry and is channeled through the open display. This is why a shield is either required across the aperture or on the circuitry itself.

EMI on a plasma display emanates from the display itself and the reaction between the phosphors in the subpixels and the gas in plasma state. This cannot be dealt with at board level.

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3) How does a shielded window work?
First and foremost, the purpose of a shielded window is to be able to see through it to the display behind. The emi shielding element acts in two ways. Depending on the frequency the wire mesh or conductive coating absorbs and conducts the interference signal. The effectiveness of the shield, measured in dB, is determined by testing the level of the signal passing through the aperture both before and after fitting the shield and taking the difference. 

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4) Why use a mesh window instead of a conductive coating?
This is primarily down to shielding effectiveness. A mesh window typically offers 60-80 dB of attenuation, whilst a conductive coating would offers <40 dB.

Conductive coatings are often favoured for their high light transmission and optical clarity. EmiClare emi shielded windows are the zero-compromise solution to this trade-off. Being a mesh window it has high levels of shielding but the mesh configuration has been optimised for high light transmission whilst eliminating the optical distortion patterns often associated with mesh windows.

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5) What is the difference between the various meshes used in shielded windows?
Conventional meshes are specified by the open area (opi) and their material (copper or stainless steel). Essentially there is very little difference between all of the mesh types. Below 200 MHz a copper wire mesh is preferable.

EmiClare mesh offers high levels of shielding and light transmission across the range.

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6) What is Moiré Fringing?
This is an optically distracting black and white moving pattern caused by optical interference between the mesh and pixel structures. This can be reduced with conventional meshes by rotating the mesh to minimise the optical interference. This can be eliminated with EmiClare mesh which has changed the aperture shape from square to rectangular. If necessary the mesh can also be rotated.

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7) Why does a non-dispersive etch not suit CRT displays?
Most CRT displays are produced with a fine etched surface. Using a filter with a non-glare surface (another etched surface) produces a coloured sparkling effect when viewing the CRT through this filter. This can be dealt with by moving the filter further away from the CRT or by changing to an anti-reflective optical coating.

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8) What does a circular polarizer do?
Circular polarizers are efficient contrast enhancement filters and improve the sunlight readability. Ambient light passing through a circular polarizer is transformed to a polarised circular motion which when reflected from the surface of the display is rotated through 90°and can no longer pass back through the filter as the polarisation of the reflected light is out of phase with the polarising filter.
The effect of this is that the background to the display appears dark blue or black. The signal from the display has not been polarised and appears very bright against this dark background.

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8b) Can you use a circular polarizer on an LCD display?
It is possible with care. But it is important to be aware that the LCDs contain linear polarizers and the polarising element of the circular polarizer must be correctly aligned with the polarizer of the LCD to avoid colour distortion.

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9) What does a linear polarizer do?
Linear polarizers confines light to a single plane. This linearly polarised light can be modified to suppress reflections and to eliminate glare in a variety of applications.

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10) How do I ground my window to a display:
There are several ways of achieving this. Hereunder are a few general suggestions:

	Using a conductive silver busbar termination to the window, the window is then clamped to a conductive bezel. Where the match between the silver busbar and the bezel is not good, a conductive gasket or silver loaded epoxy adhesive can be used.
	A mesh extension is the most the most emi efficient method as it eliminates interface losses from various gasket options.

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11) Why choose a plastic window over glass?
Mechanical Properties

Surface Durability

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12) How do Optical Filters inspect their windows?
Optical Filters have developed the optical inspection standard OIS . The content of this standard is based on our experience of the materials that we use, the process, and the expectations of our customers.

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13) What about impact resistance if I use a glass window?
Laminated glass offers high levels of impact resistance, equivalent to BS6206:1981
Class A and B, which states the requirements for safety glass.
A laminated window has the added safety advantage that if the glass were to break, the shards will adhere to the adhesive layer rather than scatter about.
Glass also has very high scratch resistance.

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14) What type of window is suitable for food production lines?
A polycarbonate laminate or glass in a polycarbonate or polyester sandwich laminate.

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15) How do I prevent the front of my display from frosting or misting over?
ThermaClare is a transparent heated window with the option of an emi shielding element. 

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16) The emi shielded window is not offering adequate attenuation.

	First of all check that the window is fixed correctly. There should be no gaps in the contact between the window and the bezel. Ensure that you are using the correct conductive adhesive / gasket.
	If the window is terminated with a silver busbar at present, you can try using a window with a mesh extension. Mesh ground directly onto the bezel will give better results.

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17) How can I improve the contrast enhancement on my display?

	coloured filters (either a neutral grey or a colour to match the display)
	circular polarizer
	Add a non-glare or anti-reflective finish to the front of the display to reduce the surface reflections.

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18) What about privacy?
A light control film (LCF) or blur film can be incorporated into a window. This controls the position from which a display can be viewed.

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