Optical bonding is a process of affixing the touch panel (or just the protective glass) to the display using a liquid adhesive, gel or dry (film). In general, this process improves the parameters of the module – the optical performance, as well as durability.
The air-gap is the reason for the phenomena of reflection of sunlight. So, when the display is off, the Visual Area (V.A) is never black – it is gray.
When removing the air gaps, we reduce the number of internal reflecting surfaces, which increase of optical performance. This phenomenon improves readability, by reducing the reflection and correct contrast.