Touch screen filter

Touch screen filter

This is an article describing the EMI and Optics filters that can be manufactured and incorporated onto touch screens.
A number of filters and light guides into touch panels to achieve a variety of applications. In addition, all of these transparent devices are available as non-touch filters. The filters fall into two categories of 1) reducing electromagnetic emissions or interference (EMI) and 2) optics to manipulate the physics of light.

EMI filters
Electromagnetic Interference filters are designed to reduce emissions in the radio frequency(RF) range of the electromagnetic spectrum (see below). At the lowest levels ofemissions, the US Federal communications division, and CE in Europe, requires manufacturersof electrical equipment to limit the levels of RF emission. The FCC has twocategories, Class A for electrical equipment used in offices, and Class B, for home use,which is more restrictive, and does not allow any interference with radio or televisionoperations. At the other end of concern are so-called “Tempest” limits, which seek tostop enemies from scanning remote devices and conducing espionage, and are so restrictive,one cannot even know what is needed to meet the filter requirements. In betweenare filters, primarily for military or aerospace environments that seek to stop interferencewith navigation instruments or limit the ability of “enemy sniffers” to find andtarget a location.
Within the RF range, the filters can be tuned for specific frequencies, and the filter thatis good at blocking one frequency, may be less effective for another. Touch International, Inc. offers two EMIfilter types, low ohm ITO and blackened mesh.

Resistive and Capacitive Touch Screen Filters
The thin film ITO used in most touch screens will aid any device in meeting FCC ClassA and Class B requirements. This is because the electronics attached to the touchscreen act to “absorb” the EMI and ultimately “bleed” the noise to ground. Thus a secondarybenefit of resistive or capacitive touch screens is to act as a simple EMI filter.
Low Ohm Thin Film
One way to trap many emissions in the RF range is to use a very low ohm transparentthin film on the back of the touch screen. Many RF emissions can be trapped by puttingthe EMI emitter (device) in a copper box and connecting the box to ground. Usinga low ohm ITO and connecting this to the EMI shield in the device’s cabinet is similar toenclosing the device in a copper box. The transparent low ohm transparent ITO is thepart of the “box” which sits over the display.
In the past, because of its low resistance, gold was used as the transparent thin film,but it had a number of problems, including a deep coloration of the display. Most recentlyTouch International has developed a very low ohm material, which is in the rangeof 4 ohms per square, has a neutral color, and a light transmission rate of around 90%.TI uses silver bus bars for conductivity, copper conductive tape, and wire mesh forlarger form factors (50, 80,100, OPI). This level of EMI “clamping” is generally goodenough for most aerospace applications. Touch International offers .5, 2, 4, 8, 10, and100 Ohm films, which can be exposed or sandwiched. The Ohm thin film is ideal formedical, military, and aerospace.
Mesh Filters
A second, more effective method is to use a tarnished silver mesh of either 50 holesper-inch or 100 holes-per-inch, which has been bonded between, usually, two piecesof glass. The bonding process serves to hold the mesh flat so that it does not furtherdisturb the optics. The bonding also maintains the mesh holes at a uniform size, anotherrequirement for clamping this frequency of emission.Although the science is beyond this white paper, the size and frequency of the holesand the high-conductivity of the silver mesh are excellent at stopping a certain range ofemissions. The light transmission is good because nothing blocks the light passingthrough the holes. However, the mesh does block some small portion of the screen,and resulting “morea effect” can be a mild annoyance, though less than being discoveredand attacked by the enemy forces.
Filters for Light Optics
These filters are for manipulating emissions in the 400nm to 700nm range, we call visiblelight, and in the 880nm to 1200nm, we call infra-red. Instead of blocking theseemissions, these filters mostly seek to manipulate the properties of light. Touch Internationaloffers a variety of finishes, polarizes, retarders, EMI Windows, Broad Band PET,anti-reflective, and light control film for privacy. TI filters have been used in POS, tablets,kiosks, aerospace, and hospital applications.
Privacy Filters
Privacy filters stop off-axis viewing of the display.The film is manufactured by creating chemical louvers that let the light come straightthrough, but block the light on either side so the image cannot be read.
DBEF Filters
Display Brightness Enhancement Film are micro prisms, formed onto a sheet, whichcollect and focus the backlight into bright spots. This filter makes a display which isperceived to be brighter than the rating of the backlight.
Contrast Enhancement
Touch International can incorporate color filters into the touch screen. The purposesvary widely. For example, radar scopes fitted with touch panels have used a specificcolors filter to adjust the persistence of the phosphor. Some airline cockpits use LEDdisplays, and a neutral gray can enhance the ability to read the display. Night visionsystems can make use of filters incorporated into the touch to “tone down” the brightnessof some displays. Most of Touch International’s customers for the color and contrastenhancement filters are for the aerospace sector.
With the growing number of outdoor touch applications and with the advent of HR(highly reflective) LCDs, the production of touch screens using anti-reflective coatingsare increasing. The amount of light which passes through a solid is impacted both byits color absorption and the bending of light at the air/surface interface. For example, aglass window passes about 95% of the light; while the color absorption is minimal, thebending of the light going in and coming out of the glass “uses” up about 5% of thelight. One can see the impact of this phenomena as a reflection from the first surface.Oddly, by adding purple tinged anti-reflective materials to glass, the light transmissionrate can exceed the ordinary 95%.
Almost all touch screens put a silica coating on the first surface to diffuse the reflectivelight and reduce the mirror effect. This is called an anti-glare coating, not to be confused with an anti-reflective coating. Anti-glare coatings are relatively inexpensive andenhance the scratch resistance of the touch panel. However, anti-glare coatings diffusethe image and consequently reduce the sharpness of the display image, reducing visualclarity. The amount of diffusion is measured as the haze factor.
Anti-reflective (AR) coatings are very thin. Measured in ratio of the size of a lightwave, these coatings literally trap light within the AR coating. When the touch screendoes not reflect light, the image behind the touch panel will be brighter and certainlymore easily read. However, creating anti-reflective coatings, which is an extremely preciseprocess, is very expensive and can easily make a touch panel 200 to 400 percentmore expensive than a standard touch panel. Today, all four surfaces-- the backside,two interior, and front-- can be AR treated, though any combination can also be selected.Obviously, the fewer the surfaces treated, the lower the cost.
Until recently the first surface (front) could not be treated with an anti-reflective coating.This was because the AR coatings are delicate (would not take the wear of millions oftouches), and the thickness of the oil from a fingerprint altered the thickness of thecoating and nullified the effect. However, the first generation of hard, fingerprint resistantAR coatings has just become available. Because of the cost, AR coatings are usedfor relatively few applications, such as those that would expose expensive instrumentationto outdoor environments.
Persistence Filters
Decay rates of phosphors, particularly those on radar-scopes, can distract controllers.By index matching the phosphor colors, the decay rate can be accelerated so as to notbe noticeable.
Color Matching
As more devices incorporate alphanumeric displays, designers can find that the colorof each display is quite different. Color filters can be added to the touch screen whichwill adjust the color of the various displays to appear to have the same color.
Flame retardant
Touch International has had much success within the Aerospace markets. One of themain requirements for this market is the FAR 25.853 vertical burn. A cross section of atouch screen is exposed to flame and must self extinguish within 12 seconds or 60seconds depending on the diagonal length. One of the main issues is that the typicalmaterials used in standard touch screens once ignited tend to "wick", or continue toburn once lit. The base material structures that Touch International can implementwithin a custom design have been formulated to pass the 12 and 60 second verticalburn test dictated by the FAA without deleterious effects to the light transmission rate.
Vandal guard
Many touch screens are used in unsupervised public access devices. There are rareoccurrences in which bricks have been thrown at information terminals for buses, andeven gunshots to gaming terminals. Chemically bathed and strengthened glass, 8 and16 hours. Standard is 3 or 4 mm, but can go to 6mm. Touch International produces anumber of touch panels and replacement glass to address these problems (standard ortempered glass). In order of “toughness”, here are options
Chemically Strengthened Glass
Chem. strengthening is a process by which glass has small surface ions (sodium) replacedwith big ones (potassium) under extreme heat for sixteen hours, and results inglass being put into tension. Because of this extra surface tension, the glass strength isfive times more resistant to breakage than that of standard float glass (chart fromthicker glass can be heat tempered, so a part with extremely high vandal resistance(even bullet proof) can be manufactured.
Safety Glass
Safety glass is made by autoclaving two surfaces together under extreme heat andpressure. Between the two (usually) glass layers is a material which holds the piecestogether so that if broken, sharp shards do not fly off and present a danger (thus“safety”). Both tempered and chem. strengthened glass can be used to give this productexceptional strength.
Scratch Resistant Glass
Some applications may require a surface which cannot be scratched or sand blasted.
Tin-Oxide coatings
Tin Oxide is harder than glass and has up to 7H hardness. Crystals of Tin Oxide areuse in sanding abrasives. This is the same coating that is use in the large format capacitiveglass, so this product has a natural resistance to scratching.
Diamond Evaporative Coatings
A diamond-like substance can be deposited by chemical vapor deposition (CVD).When combined with an ion scrub, the resulting surface is almost as hard diamond, themost scratch resistant material known. The material has a slight coloration, and is thin,but offers excellent scratch resistance.
Sapphire Glass
Sapphire glass is not glass, but is a grown Aluminum Oxide crystal. Second to diamondin scratch resistance, this material is clearer and thicker than the carbon baseddiamond material. Sapphire glass is most commonly used on store scanners in which itis said that the stainless steel bezel will scratch before the sapphire glass will.

ee also

*Touch screen

External links

* [ Touch Screen White Papers] - Collection of white papers on touch screens and touch screen components written by touch screen industry pioneer Gary L. Barrett.

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