Thermodynamics of ideal illumination: a novel figure of merit for characterizing illumination efficiency.

Abstract

The advancement of modern lighting technologies has led to many revolutions in lighting efficiency and presentation. The progression from filament bulbs, to CFL, and now LED technologies have produced a bounty of energy-efficient lighting options for design engineers and consumers. As the light-producing elements of a lighting fixture improve, the limiting factor in efficient illumination is no longer the light source, but the optical system itself. There are many characterization methods and standards for defining light for illumination in terms of color and human response. With concerns of how things like light pollution and energy requirements impact our society and the world around us, it is critical to understand how well a lighting fixture can illuminate a desired area while minimizing light lost to the environment and maximizing the total radiative intensity (radiance) of a space. This work presents two figures of merit, one for over-illumination and another for under-illumination, to characterize the optics of a lighting system based on a ray tracing methodology. Five common simplified optical design, with four varying beam angles, were simulated to present these new figures of merit. Results showed that common imaging optical systems such as parabolic and ellipse reflectors struggled to produce a well-lit area without over illumination, while nonimaging alternatives like the compound parabolic and compound elliptical reflectors were able to reach the thermodynamic ideal of a fully illuminated area without light lost to the environment. This work hopes to inform illumination engineers and lighting designers to help improve their optical design to maximize performance and minimize waste.