If one were to simply compare the visual appearance of an optically bonded LCD display to that of a traditional “air-gapped”, non-bonded display one will quickly observe that the optically bonded LCD looks noticeably better. This significantly improved visual aspect will be true in any type of ambient light condition, not just in direct ‘sunlight’ environments. So, why does an optically bonded display always produce a much better viewable image, what other major resulting benefits occur, and how does optical bonding work in general? The answers are abundant and many relate to optical physics as well as human vision. This paper intends to explain all of these effects and benefits.

This variation is measured on the Refractive Index (RI) scale. The polariser material typically used by the Original Equipment Manufacturer (OEM) has Refractive Index of 1.45; air has a value of 1.00; and the various types of glass substrates such as borosilicate and soda-glass have an average Refractive Index of closer to 1.50. The existence of an Refractive Index mismatch of more than 0.10 units between contacting surfaces is enough to cause significant light reflection to occur at the interface between those substances.

The greater the Refractive Index discord, the greater the interface light reflection levels. Consider this, the three reflective layers of Refractive Index mismatches typical of most LCD “air gap” monitors have several optical effects. First, is external ambient light shining on the display surface at an angle of incidence greater than zero degrees. A subsequent result is specular reflections at each of the three traditional interfaces; (1) Polariser to Air, (2) Air to Glass and (3) Glass to Air. The second effect is light generated by the LCD’s backlight. This generated light causes internal specular reflections at the interfaces of (A) Air to Glass and (B) Glass to Air. The third effect is external ambient light shining onto the display surface at zero degrees of incidence resulting in ‘diffuse’ reflections (generally referred to as “glare”) at the interfaces of (X) Air to Glass and (Y) Glass to Air.

The cumulative effect of the internal specular reflections of A & B alone result in an average loss of 9.0% light transmission (luminance) from the display’s backlight(s). Depending on the angle of incidence and intensity of external light, both specular and diffuse reflections can cause image “washout” (see "Reflection Washout" image below). This is the point where reflected light intensity is greater than the emitted light intensity from the display image. As the level of reflected light increases, the contrast ratio of the display image decreases below the level of 5:1, and no longer is visible to the human eye. It is for these reasons that ‘air gapped’ LCD products are not considered as sunlight viewable. The use of anti-reflective (AR) coatings on the front and back surfaces of the cover glass substrate, and even on the surface of the front polariser, serves to help minimise these reflection levels by index matching the glass and polariser surfaces closer to the 1.00 Refractive Index of air. Although the usage of multiple anti-reflective (AR) coatings (commonly referred to as ‘passive enhancements’) improves the viewability of ‘air gapped’ display products, the limited efficiency of these AR coatings (see Image 2 below) still permits reflections to occur at all interface surfaces. So, in the end, passively enhanced only displays are marginal at best for achieving LCD image readability in direct sunlight or very high ambient lighting conditions.

The ultimate solution to the problem of internal and external light reflections and its limitations on human vision is to eliminate all of the Refractive Index (RI) interfaces between the display’s polariser and the LCD product’s cover window. This technique is typically referred to as “active enhancement” and is achievable by a unique use and application of special silicon based optical material which has a Refractive Index close to 1.44. This RI value is very close to the polariser index of 1.45 and within the tolerance range of the nominal glass index of 1.50. VarTech takes this advanced silicon material and utilises a unique bond design (described below) to join the glass and polariser surfaces which results in an elimination of the ‘air gap’. The ending result is an optical Refractive Index match of materials which allows for uniform light transmission and very low reflection. By utilising display cover glass with a front surface anti-reflective coating treatment, the following effects are achieved:

External light strikes the display cover glass at various angles of incidence. About 5% of this light (2) is reflected off of the AR coated surface. More than 93.5% of this external light (1) passes through the now index matched bonded solution. As light passes through the front polariser, it becomes polarised light. When this polarised light then reflects off of the LCD cell, its polarisation axis is rotated to where it is then absorbed and blocked by the front polariser material.

In addition, due to the operating switching state of the liquid crystal cell sub-pixels, much of this external light will pass directly through the rear polariser and reflect off of the internal backlight films. At this point this reflected external light (Z) is fundamentally the same as the internal light generated by the display backlight (B).

The end result is that the external light is optically directed, polarised, and utilised to enhance the colour brightness of the display image. This enhanced colour brightness maintains high contrast ratio levels, independent of the luminance intensity of the external light source. With the contrast ratio maintained and reflection “washout” eliminated the display image is easily seen and thus becomes truly readable in direct sunlight conditions.

VarTech’s VBOND technology combines an innovative bonding process with an industry-leading proprietary adhesive to optically bond an anti-reflective glass, plastic or touch sensor directly to the front of an LCD display. VarTech’s bonding technology enhances display performance by improving sunlight readability up to 400% and impact and scratch resistance up to 300%. It is ideal for use in consumer, military, marine, and other industrial applications requiring outdoor viewability and the durability to withstand impact, vibration, extreme temperatures, altitudes and dust.

Optical Bonding is the affixing of two optical elements to one another, using a liquid adhesive. In this way, we differentiate bonding from lamination; a lamination process is currently performed by alternate “bonding’ companies as its solution for the ‘air gap’ problem. By lamination, we are referring to the affixing of two optical elements to one another using a pressure sensitive adhesive. Bonding is suitable for use with elements which are rigid and may be substantial in size, while lamination is suitable for affixing a thin membrane, such as an antireflective-coated plastic film, to a more or less rigid substrate, such as an LCD, OLED, Plasma display, touch sensor or anti-vandal shield.

Using the qualifier "optical", implies that the bonded adhesive material is transparent, has a suitable refractive index and is made under adequate control that there are no significant variations in optical properties within a single bond. On a practical side, the adhesive must also provide adequate bond strength, have a reasonable pot life after preparing, not present any health or safety issues, be available at reasonable cost from reliable sources and cure to the finished bond condition using temperatures and time which are friendly to flat panel display manufacture.

Additionally, when considering the optical bond as a useable material, it is important to analyse the impact of each substance and component to be used jointly as well as the associated properties of each material. Introduction of new materials into the process must be accompanied by a study of such things as environmental stability of the adhesive may be affected by the new materials and cure time(s).

VarTech has devised an innovative bonding formula to address these necessities.

An alternate adhesive agent that some bonding companies use is an epoxy based formula. This makes a much more rigid bond than silicone. However, it is not re-workable in the event of any issues during production or use (including in-warranty or post-warranty period repair situations). The biggest drawback, however, is the ‘yellowing’ effect. This type of material exhibits a severe yellowing over time when exposed to high ambient (solar) lighting conditions. Because of this tendency to yellow, VarTech does not use this type of adhesive.