Modelling of high luminance white light solid-state sources

Efficiency is always a major motivator behind technological advances. In lighting, it has prompted the wide adoption of solid-state light sources. When coupled with fluorescent materials, solid-state sources such as light emitting diodes are one of the most efficient white light sources available. For high brightness applications, such as projection systems, car headlights and large outdoor area illumination, laser diodes are a very promising alternative for LEDs thanks to their higher efficiency at high optical density output. Commercial applications of laser lighting (such as car headlights) are already entering the market, and this trend is predicted to continue as we strive for more efficient and higher brightness light sources.

Despite its promising features, there is a major obstacle on the path towards the wide adoption of laser lighting. The fluorescent materials used to generate white light in combination with laser diodes are known to self-heat when exposed to the very bright laser diode light. As fluorescent materials heat up, their colour conversion efficiency rapidly decreases. This always leads to a non-stable white light system, and occasionally to the irreversible degradation of the fluorescent material itself. Surprisingly, the tools that are available to design such types of phosphor-converted light sources do not account for these effects. This means that any predictions obtained with these simulation tools regarding the performance of such white light sources are incorrect. To accurately model the phenomena previously described, it is necessary to model not only the system's optical properties but also their thermal properties -- and, perhaps most importantly, how they interact with one another.

The goal of this research was on developing an efficient and general opto-thermal simulation framework to tackle the fluorescent self-heating issue. This way, it becomes possible to simulate the performance of a white light source based on laser diodes while considering all the relevant optical and thermal dependencies that affect fluorescent colour converting materials. Such a comprehensive opto-thermal simulation framework requires a great many deal of inputs, like the colour converting material's fluorescent and non-fluorescent optical properties and how these change with temperature and optical power. Thus another goal of this doctoral work was focused on developing inverse methods to extract these parameters from experimental measurements of scattering and fluorescent samples. Taking both the inverse method and the simulation framework together, this research provides an important step forward towards developing a comprehensive solution that permits designing novel high-luminance solid-state white light sources that can avoid the current opto-thermal limitations.