Microcavity refers to a phenomenon where an object is exposed to a force that is equal to its own frequency and amplifies that frequency. It is also known as resonance. In the context of OLED displays, microcavity is used to amplify light efficiency and ensure brighter image quality. 

To better understand the principle of microcavity, the principle of interference must be understood because this serves as the basis of resonance. When different waves overlap, they can be combined to increase or reduce the amplitude of the original waves. This phenomenon is referred to as interference. In the illustration above, the bottom left shows two different waves creating a larger amplitude (in the vertical direction), which indicates constructive interference, while the bottom right shows the waves creating a smaller amplitude, which is destructive interference. This nature of waves also applies to light. The constructive interference in light can create strong, or brighter, light. 

In the structure of OLED, light is emitted from the emission material layer (EML). Light coming from the EML is transmitted and reflected within the EML, where the complex phenomenon of interference occurs. The light that was emitted downward hits the metal layer of the anode and is reflected upward. In contrast, the light emitted upward hits the cathode. Some of the light that reaches the cathode is reflected in the direction of the screen facing the viewer, while the rest is reflected downward. 

These different rays of light create the interference effect. To induce constructive interference, the EML is set at a certain thickness so that a unique frequency is created to induce resonance of light. In other words, the EML thickness is set at a level that is optimal for the organic light-emitting material to create the optimal level of resonance frequency. By using this approach, light efficiency can be boosted by 1.5 to 2 times.