Electronic Design

Optical Bonding And Surface Treatments Improve Displays

Why should you consider optical bonding for your portable design? Do you need anti-glare or anti-reflective enhancements in the product? Should you plan on a more powerful backlight for your thin-film-transistor (TFT) LCD display? Start by evaluating the ambient light levels your product is most likely to encounter.

REFLECTION MANAGEMENT
Everyone has experienced unwanted glare obscuring information they need to see on a cell phone, TV, GPS navigation system, or kiosk display. Such reflections can be mild inconveniences in consumer items or severe safety hazards in missioncritical equipment, such as cockpit avionics displays.

Glare on vehicle navigation systems or vehicle PC screens can contribute to accidents if it impedes time to perception and causes drivers to spend critical time away from the primary task of piloting the vehicle. On a marine vessel, glare can obscure vital navigation information about oncoming hazards. Fortunately, reliable techniques can combat glare. Reflection management is a key concept that designers of products used in brightly lit ambient environments should understand.

Many devices that are intended for outdoor use employ transmissive TFT LCDs. These devices generally incorporate a protective overlay or touchscreen atop the TFT that serves to protect the soft polarizer surface from damage and seal the device from dust or liquids. While necessary for durability, these overlays impair optical performance by creating external and internal reflective surfaces that decrease the display’s legibility in bright ambient environments.

However, additional steps like optical bonding and antiglare (AG) or anti-reflective (AR) surfaces can be added to the overlays to maintain the product’s performance. Optical bonding and AG and AR surface treatments also improve display legibility without impacting power consumption, making reflection management key to beating power budget and battery weight design constraints in mobile devices.

Typically, most protective windows and touchscreens adhere to TFT displays with double-sided tape, leaving an air gap between the polarizer surface and the overlay. Yet light reflects when it travels between various mediums with differing indexes of refraction. If you place a transparent protective window on top of a display, light will reflect back at the viewer where the interface of air and glass meet. Light will also reflect back when it exits the medium.

A simplified model demonstrates that 4251 nits (cd/m2) of specular illumination reflect from a protective window and the surface of a typical transmissive TFT when an air gap is present (Fig. 1). Note that in this example there is no AR surface treatment. Yet optical bonding and an AR coating on the front surface of the protective window can reduce specular reflection by eliminating the air gap (Fig. 2).

Reducing reflections from 4251 nits down to 239 nits is fundamental to increasing readability. A designer can now begin to contemplate a backlight to overpower the luminous intensity of the ambient reflection.

Reflections also occur as light emitted from a backlight source transits various interfaces as it exits the display toward the viewer. These additional reflections further lessen the total emission of usable light if an air gap exists between the display and protective overlays.

Furthermore, reflected ambient luminescence is brightness that competes against the luminescence of the information you desire to see. By reducing unwanted internal and external reflections, you minimize the intensity of backlighting required to overpower the ambient light.

CONTRAST RATIOS HELP DETERMINE LEGIBILITY
Display-usability experts generally say a 7:1 minimum contrast ratio is required for data legibility on full-color graphic displays, while 3:1 suffices for black alphanumeric data on a white background. Consider the formula for contrast ratio:

(White brightness area + total specular and diffuse reflections)/ (black brightness area + total specular and diffuse reflections) = contrast ratio

As an example, for a display in a dark room with an air gap between it and a protective window with a contrast ratio (CR) of 333:1 and a luminance of 1500 nits, we get for a full ambient daylight (diffused) environment of 10,000 fC (107,600 Lux):

CR = (1500 + 4351)/(3 + 4351) = 1.3

This CR is too low for display information to be legible. When the same display is bonded to the protective window and an AR coating is applied on the front, the CR will improve:

CR = (1500 + 240)/(3 + 240) = 7.1

This improvement will make even color images legible. The display in Figure 1 requires an impractical 26,000 nits of brightness for color images to be legible, while the display in Figure 2 requires only 1500 nits. Legible black-on-white alphanumeric data requires 8502 nits for the display in Figure 1, as opposed to 478 nits for the display in Figure 2 (see the table).

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STOP UNWANTED REFLECTIONS
Optical bonding is the lamination of the TFT surface to the protective window or touchscreen overlay to eliminate the air gap. Transparent adhesive materials such as silicone, urethane, and epoxy usually couple the display and glass overlay into one optical path that optimally has a single index of refraction.

Also, optical bonding increases the overlay’s durability. The only significant drawback to optical bonding is cost. It is a timeintensive enhancement to standard TFT displays, making it more suitable for devices where bright ambient light viewability or environmental ruggedness is necessary.

Acting as light absorbers, AR treatments consist of multiple layers of thin-film materials deposited on a surface whose arrayed properties cause destructive interference to cancel reflections. The AR coating or a polymer film with AR coating can adhere directly to the surface of the protective window.

Unfortunately, AR-enhanced glass easily smudges, and it is very difficult to clean. AR coatings also wear off through repeat cleanings or touches, so it isn’t a viable contender for touchscreens. These coatings find wider use in avionics applications.

AG surface treatments consist of etching the reflective surface of the display overlay, which results in non-coplanar micro surfaces that scatter reflected light in many directions. While the sum of reflected light is undiminished, the eye then perceives an effective reduction in glare because it senses only the portion of light that reflects collinearly between it and the reflecting micro surfaces.

Overall, AG surfaces are easier to clean and more durable than AR surfaces, making them the best choice for managing external surface reflections in touchscreen applications. AG treatments aren’t the best choice for bright daylight situations, though a mild concoction like AG110 is a good compromise.

FINAL WORD
Products displaying data in bright ambient environments will benefit from optical bonding and reflection-reducing surface treatments. Remember, air gaps and reflective surfaces decrease a display’s legibility, and increasing the backlight output isn’t the best design practice in battery-powered and other applications where power consumption is critical. Reflection management techniques can increase contrast ratios without the additional power necessary for brighter backlights.

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