Professor Ron Hui, author of Photo-Electro-Thermal Theory for LED Systems: Basic Theory and Applications takes part in a Q&A about his unique book. Read Chapter One for free until 31st December 2018 here.

Q1: What inspired your book?

Answer: Humans have not been very advanced in lighting technology. For tens of thousands of years we generated light by starting a fire based on wood, oil, wax, gas or fossil fuels etc. Artificial lighting was only possible when the British Scientist, Sir Humphrey Davy, accidentally shorted two carbon electrodes connected to two terminals of a stack of battery cells and observed the glow of the discharge arc about 200 years ago. Light bulbs and fluorescent lamps were commercialised in 1890’s and 1940’s respectively, and are still used today.

LED technology is a true technological revolution. Three Japanese scientists were awarded the Nobel Prize in Physics in 2014 for their studies on blue LED chips. LED chips cannot be used on their own without proper LED driver design (i.e. power control) and thermal designs.

LED technology involves complex interactions of 4 elements, namely light intensity, heat, power and colour. In 2007, there was no LED system theory that could unify these 4 elements under one mathematical framework. This prompted me to investigate the fundamentals of LED science and then propose the Photo-Electro-Thermal (PET) Theory for LED Systems in 2008.

The PET theory initially linked up the interactions of luminous flux, power and heat in one equation. It was extended to cover colour in 2012. It is now a comprehensive LED system theory that can be used as a theoretical tool, as well as a design tool for LED systems.

This aim of my book is to provide education on the engineering sciences of LED systems to both the academic and industrial world. For universities, PET theory can be incorporated into Power Electronics, Energy Conversion or Light Science courses at both of the undergraduate and graduate levels, while industrial lighting designers can use it as a design tool to predict and optimise LED system performance.

Q2: Can you share any interesting anecdotes from the book?

Answer: The PET theory from this book offers new understanding and insights for both researchers and practicing engineers. While writing this book, I received an LED demo-kit from a large electronic company. The small LED system was comprised of an LED driver with control integrated circuit and a 3W white LED package mounted on a small heatsink. Naturally, but incorrectly, the design engineer designed the driving power at 3W, which is the rated power of the LED package. After applying the PET theory to this system, I sent an email to the company and suggested that they reduce the driver power from 3W to 2W if they wanted to double the light output. They took my advice and were stunned with the results predicted by the PET theory.

The reason is simple. Light and heat are enemies in LED technology. When the LED power is small, the junction temperature of the LED chip is relatively low. So the luminous efficacy (i.e. lumen per watt) is high. As the LED power increases, the junction temperature of the LED chip will rise resulting in a lower luminous efficacy. The luminous flux follows a parabolic curve with increasing LED power. After the maximum luminous flux point, increasing the LED power will only reduce the light output.

Q3: How do you think this technology will change our lives in the future?

Answer: Before LED technology became widely available in the lighting market, traditional light sources, which are much less efficient, consumed about 18% of global electrical energy. The LED revolution can potentially reduce this energy consumption substantially, leading to reduced greenhouse gas emission.

Before the PET theory was available, most of the LED systems were not optimally designed because design engineers simply do not have the proper knowledge of LED system designs. The massive failures of many large-scale LED street lighting projects from 2003-2008 are examples of improper LED system designs. It is hoped that this book will provide not only a theoretical understanding of the LED systems for the academic world, but also a general LED system design and modelling tool for lighting industry worldwide.

Read Chapter One for free until 31st December 2018 here.