Frequently Asked Questions

What is an optical filter?
Why should I use an optical filter?
What are common applications for optical filters?
What causes my display to “wash out” in direct sunlight?
How do I improve the sunlight readability of my display?
How much will I improve the sunlight readability by using filters or display enhancements?
What is EMI/RFI?
Why is EMI/RFI important?
What are some common methods to provide optical electromagnetic compatibility?
What is the typical shielding effectiveness of conductive coatings versus optical fine wire mesh?
What substrates are recommended for optical filters?
Why would you select one substrate over another?
What types of coatings are typically applied to optical filters?
What is display contrast and how do optical filters improve it?
How much transmittance loss can be expected when adding an EMI/RFI shielding filter to my display or touch screen? 
What is the best way to install an optical filter into a display assembly? 
For EMI/RFI filters, what is the recommended method for grounding the filter to create a conductive ground plane?
How can I determine the optical or EMI/RFI shielding performance of the filter? 
What optical filters are used for displays exposed to high and low temperatures?
What are privacy filters used for?

What is an optical filter?
An optical filter is fabricated from a visually transparent substrate such as polycarbonate, acrylic or glass and is used to admit light into an optical system and to exclude dirt and moisture.

Why should I use an optical filter?
An optical filter is used to improve electronic display performance such as contrast enhancement and glare reduction, provide impact and environmental protection, and allow the display to operate in high and low temperatures.

What are common applications for optical filters?
Common examples of optical filters include the protective overlays on a cell phone, GPS or PDA. Optical filters have wide ranging applications such as ATMs, kiosks and displays used in outdoor applications such a marine navigation, digital signage, and mobile industrial and military computers.

What causes my display to “wash out” in direct sunlight?
In high ambient light, when reflected sunlight exceeds the luminance (brightness) of the display, the display becomes unreadable.

How do I improve the sunlight readability of my display?
The two primary methods to improve sunlight readability of LCDs are to reduce reflections (e.g., using an antireflective display filter) at the display/user interface and/or to increase display luminance via the addition of passive enhancements or supplemental lighting (e.g., active enhancements to the Cold Cathode Fluorescence Lights (CCFLs) or Light Emitting Diodes (LEDs).

How much will I improve the sunlight readability by using filters or display enhancements?   Although display systems vary, every uncoated glass or plastic surface in the display system will have at least a 4.25% reflection loss. Adding an antireflective coating to these surfaces will reduce reflection loss from 4.25% to less than 0.5% which equates to almost a 90% reduction in each treated surface. (See IMO-Bond)

For passive enhancements, the addition of brightness enhancement films or reflective films can improve display luminance from 10% to >50%, depending on the component and the display.  Likewise, adding additional CCFLs or LEDs can increase luminance significantly (in some cases by several hundred percent). However, adding supplemental lighting generally increases power consumption and adds heat to the display system.

Note: In a display system with a resistive touch screen, internal reflections, due to the conductive layers of the touch screen, can add an additional 15 to 20% reflection loss and corresponding decreased sunlight readability.

What is EMI/RFI?
EMI is electromagnetic interference, and RFI is radio frequency interference.

Why is EMI/RFI important?
EMI/RFI may cause disruption or erroneous readings to exposed electronic devices. In addition, Electromagnetic Compatibility (EMC) is often a requirement for military, federal and international standards (e.g., MIL-461 or FCC compliance).

What are some common methods to provide optical electromagnetic compatibility?
The two most common solutions are using conductive coatings – e.g., Indium Tin Oxide (ITO) – or using optical fine wire mesh to create a conductive ground plane as a return path for radiated radio frequency energy.

What is the typical shielding effectiveness of conductive coatings versus optical fine wire mesh?
For conductive coatings, the shielding effectiveness generally corresponds to the coating resistance. The lower the coating resistance, the better the shielding effectiveness.  Most conductive coatings for EMI/RFI shielding are 30 ohms/sq or less. Optical fine wire mesh is often conductively plated to improve its shielding effectiveness and may have a surface resistance of 0.1 ohms/sq or less. Consequently, the shielding effectiveness of optical fine wire and printed mesh is usually 50-100% better than conductive coatings in the 10-30 ohms/sq. range, especially at the high frequencies. See VCF and OFW datasheets for specific performance data of conductive coatings and optical fine wire mesh.

Because shielding effectiveness varies widely depending on the system, grounding configuration, installation and test methods, it is advisable to discuss your specific application personally with a Dontech sales engineer.

What substrates are recommended for optical filters? 
In most display systems important design criteria include: a) optical clarity; b) environmental durability (including thermal range, chemical, impact and scratch resistance); c) weight; d) cost; e) lead time; f) manufacturability.

Consequently, common substrates for optical filters include glass (soda lime, borosilicate, chemically strengthened), acrylic (Plexiglas™), and polycarbonate (Lexan™). Additional optical materials such polarizers, vacuum deposited coatings and UV or thermally cured hard coatings can be applied to the optical substrates for the modification or enhancement of optical properties.

Why would you select one substrate over another?  
Optical system performance and cost are the two primary factors dictating the selection of an optical substrate. For instance, a glass substrate provides excellent optical transmittance and hardness, but it is twice the weight of plastic and not impact resistant unless strengthened. Plastics are also easier to machine and are, therefore, better suited to complex shapes. Plastics have a much higher coefficient of thermal expansion than glass and often require surface hard coatings to provide chemical and scratch resistance. Therefore, it is advisable to discuss your optical filter application directly with a Dontech sales engineer.

What types of coatings are typically applied to optical filters?  
Various coatings are applied to optical filters to improve their optical or environmental performance. The two general categories are thin film, vacuum deposited coatings and thick film, laminated coatings. Because some plastics, such as polycarbonate, are not very scratch or chemical resistant, it is necessary to apply additional thick film hard coatings to improve the environmental durability of the filters. The two most common categories of thick film coatings are thermally and UV-cured coatings. To improve optical performance, plastic substrates are often coated with antiglare, antireflective, or antiglare/antireflective coatings. Most antiglare coatings are produced by texturing or etching the coated surface to scatter or diffuse reflected light.

Thin film, vacuum deposited antireflective (AR) coatings can be applied to glass, polycarbonate, acrylic, polyester and other optical substrates. These AR coatings reduce specular (mirrored) reflection be reducing the refractive index mismatch between air and the substrate.  Consequently, AR coatings increase the amount photopic energy transmitted through the optical filter.

For EMI/RFI shielding, substrates are typically thin film coated with an Indium Tin Oxide (ITO) or Transparent Conductive Oxide (TCO) coatings. See Optical Coatings Section.

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What is display contrast and how do optical filters improve it?  
Display contrast is the difference in the light intensity between the on and off components (typically white and black pixels) produced by the display. Contrast enhancement is the technique used for creating a greater disparity between display active output and the inactive background. Typically, the greater the contrast ratio, the easier it is to read the display.

For monochrome displays, colored selected substrates centered at the peak output of the display optimize light transmission while darkening the background and increasing display contrast. Band pass filters made from colored acrylic, polycarbonate and glass are made for electroluminescent, vacuum florescent and colored LEDs.

Additional methods to improve display contrast include linear and circular polarizers, neutral density filters, and multi-band pass filters for full color LCDs. Antiglare or antireflective coatings may also be applied to optical filters to minimize surface reflections and can degrade display contrast.

Finally, optical bonding of the display cover filter or touch screen to the LCD can increase contrast appreciably by virtually eliminating internal reflection loss between the display and the filter. See Optical Bonding.

How much transmittance loss can be expected when adding an EMI/RFI shielding filter to my display or touch screen?
Luminance or transmittance loss depends on the type of optical EMI/RFI shielding, as well as the optical substrates used. For transparent conductive coatings on glass, unenhanced ITO at 10 ohms/sq will have photopic transmission of roughly 85%. By adding a dielectric (antireflective coating) enhancement to both sides of the glass, transmission can be improved to over 92%. This is approximately the transmission of clear glass. Consequently, transparent conductive coatings can be used for a variety of EMI/RFI shielding applications while preserving virtually 100% of the display’s contrast and luminance.

Polycarbonate, acrylic and polyester substrates will have slightly less optimal performance due to higher haze and less thin film coating efficiency.

Optical fine wire and printed mesh, generally provide a higher level of EMI/RFI shielding, but physically obstruct a portion of the display’s luminance. The transmissivity of the mesh corresponds to wire diameter of the mesh and the strands per inch and is often expressed as open area of the mesh. Total transmission of a mesh filter is determined by the mesh open area, the substrate absorption and reflection loss. In general, the higher the mesh counts, expressed as stands or wires per inch, the lower the transmission. At higher mesh counts, the EMI grid is denser and generally shielding effectiveness is increased. However, light transmission is reduced. Download our Mesh Datasheet and VCF Datasheet for more details.

What is the best way to install an optical filter into a display assembly?   
Most optical filters are attached to either the frame of the display or to the bezel or display enclosure. In most cases tapes or gaskets with pressure sensitive adhesives are used. Other configurations include clamping the filter in place, using fasteners or screws, optically bonding the filter to the display, or using liquid adhesives to perimeter bond the filter.

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For EMI/RFI filters, what is the recommended method for grounding the filter to create a conductive ground plane?   
EMI/RFI filters have conductive coating or mesh layer that needs to be properly terminated in order provide effective shielding. The mesh or conductive coating extends to the perimeter of the part. A continuous conductive bus is typically used to make contact with the conductive layer. The bus is then terminated to the display frame, bezel or enclosure via a conductive tape, adhesive, or gasket. In some cases, the conductive mesh is extended beyond the edges of the filter and is used for grounding to the enclosure, bezel or display.

How can I determine the optical or EMI/RFI shielding performance of the filter?   
The best way is to obtain representative samples of your preliminary optical filter design to test with your display.  Optical performance and EMI/RFI attenuation measurements can then be made.  In the case of EMI/RFI testing, a certified test laboratory may be required.

What optical filters are used for displays exposed to high and low temperatures?   
For displays in direct sunlight or in high ambient temperatures, an infrared blocking filter can be added when convective or active cooling of the display system is insufficient. The IR filter reduces the thermal loading on the display and prolongs the liquid crystals from going isotropic in high ambient temperatures.

For low temperature applications, typically below -20 degrees C, a heating element for the LCD or CCFL’s is often required.  Transparent heaters, constructed of glass or plastic, can be used as the protective cover of the LCD. Thin film conductive coatings or embedded wires are used in the heater fabrication. Voltage is applied to the heater and a thermistor is used to measure surface temperature and provide feedback as a temperature control function.

What are privacy filters used for?   
A privacy filter is used to restrict the off-angle viewing of display information from observers. Common applications include ATM displays used in banking, medical monitors where HIPA regulations restrict access to patient health information and in-flight entertainment systems. Privacy filters are made using micro-louvers or prismatic films that block or blur off-angle viewing of the display.