Playing Ping Pong with Light: Directional Emission of White Light

TL;DR

This paper presents a multi-layer thin film design on white LEDs that enhances forward-directed white light emission by using a physics-guided Bayesian optimization approach to identify angle and wavelength filtering mechanisms.

Contribution

It introduces a novel multi-objective optimization framework and demonstrates how multi-layer thin films can control light directionality in white LEDs.

Findings

Enhanced forward emission of white light achieved

Multi-layer thin films filter light by angle and wavelength

Bayesian optimization effectively identifies optimal film structures

Abstract

Over the last decades, light-emitting diodes (LED) have replaced common light bulbs in almost every application, from flashlights in smartphones to automotive headlights. Illuminating nightly streets requires LEDs to emit a light spectrum that is perceived as pure white by the human eye. The power associated with such a white light spectrum is not only distributed over the contributing wavelengths but also over the angles of vision. For many applications, the usable light rays are required to exit the LED in forward direction, namely under small angles to the perpendicular. In this work, we demonstrate that a specifically designed multi-layer thin film on top of a white LED increases the power of pure white light emitted in forward direction. Therefore, the deduced multi-objective optimization problem is reformulated via a real-valued physics-guided objective function that represents the hierarchical structure of our engineering problem. Variants of Bayesian optimization are employed to maximize this non-deterministic objective function based on ray tracing simulations. Eventually, the investigation of optical properties of suitable multi-layer thin films allowed to identify the mechanism behind the increased directionality of white light: angle and wavelength selective filtering causes the multi-layer thin film to play ping pong with rays of light.