If you picture an embedded HMI in cross-section, it’s two glass panels with the ITO sensor in between. For perfect readability, disturbing reflections at the single layers must be avoided. In optical bonding, this task is performed by a translucent adhesive that has the same refractive index as glass (LOCA). Optically, the whole system becomes a single panel. Further advantages: No penetration of moisture and dust and thus no condensation between the panels. Higher color brilliance and better contrast. We have mastered the process of bubble-free bonding for years.
The cheaper solution is to join the two glass panels in the clean room using a surrounding adhesive tape frame. The air gap between the glass panels is maintained, which is why this process is also known as air-gap bonding.