Worker using a tablet at an industrial workplace

Optical Fundamentals – Why Optical Bonding Works Physically

The superior readability of an optically bonded display, especially in bright ambient light, is not magic, but the result of simple physical principles. The key lies in reducing reflection—specifically, in minimizing internal reflections at the interfaces between different materials and components. 

Every time light passes from one medium into another with a different refractive index, a portion of the light is reflected. The refractive index is a dimensionless number that describes how fast light travels through a material. Air has a refractive index of about 1.0, while glass and most polymers have a refractive index of about 1.5.

a comparison with optical bonding and without it on a display
left: without optical bonding, right: with optical bonding

In an air-gap display, there are two critical transitions:

  1. from the inside of the cover glass (n ≈ 1.5) to air (n ≈ 1.0)
  2. from air (n ≈ 1.0) to the surface of the display (n ≈ 1.5)

Approximately 4% of the light is reflected at each of these interfaces.

These reflections have two negative effects:

  • The useful light emitted by the display is attenuated on its way to the viewer’s eye, resulting in lower perceived brightness and poorer readability.
  • Ambient light entering from outside (e.g., sunlight) is reflected at these internal surfaces and superimposes on the useful light, drastically reducing contrast.

The image appears “washed out” and is barely recognizable in direct sunlight.

Schematic comparison of an air-gap display and an optically bonded display

Optical bonding solves this problem by replacing the air gap with a transparent optical adhesive whose refractive index (typically n ≈ 1.45–1.5) is very close to that of glass and the display surface. This eliminates reflections at the two critical internal interfaces. The light beam effectively “sees” only a single block of material and passes through the layers without significant internal reflections. The only remaining reflection is at the outer surface of the cover glass, which can be further minimized by an anti-reflective coating (AR or AG coating).

The result is a dramatic improvement in optical performance:

  • Contrast is increased by a factor of 5 to 10.
  • Perceived brightness increases by about 8%.
  • Color brilliance is significantly improved.

The image is sharp, clear, and transparent, and remains highly legible even in direct sunlight. This is not only a matter of comfort but a critical safety feature, as important information must always be reliably visible, particularly in many heavy-duty HMI applications. Displays with optical bonding offer clear advantages here over conventionally constructed systems without optical bonding.

Note on the technical information in this chapter

The technical values, test profiles, and performance data stated in this chapter are provided for technical classification purposes. Actual results depend on the specific display configuration, material system, bonding process, application profile, and scope of qualification. Product-specific values are validated on a project basis using data sheets, measurements, test reports, and customer specifications.