Nonimaging optics: a tutorial

Nonimaging optics is the theory of thermodynamically efficient optics and as such, depends more on thermodynamics than on optics. Historically, nonimaging optics that work as ideal concentrators have been discovered through such heuristic ideas as “edge ray involutes,” “string method,” “simultaneous multiple surface,” and “tailored edge ray concentrator,” without a consistent theoretical definition of what “ideal” means. In this tutorial, we provide a thermodynamic perspective of nonimaging optical designs to shine light on the commonality of all these designing ideas, or what “ideal” nonimaging design means. Hence, in this paper, a condition for the “best” design is proposed based purely on thermodynamic arguments, which we believe have profound consequences. Thermodynamics may also be the most intuitive way for a reader who is new to this subject to understand or study it within a certain framework, instead of learning from sporadic designing methodologies. This way of looking at the problem of efficient concentration and illumination depends on probabilities, the ingredients of entropy, and information theory, while “optics” in the conventional sense recedes into the background. We attempt to link the key concept of nonimaging optics, etendue, with the radiative heat transfer concept of view factor, which may be more familiar to some readers. However, we do not want to limit the readers to a single thermodynamic understanding of this subject. Therefore, two alternative perspectives of nonimaging optics will also be introduced and used throughout the tutorial: the definition of a nonimaging optics design according to the Hilbert integral, and the phase space analysis of the ideal design. The tutorial will be organized as follows: Section 1 highlights the difference between nonimaging and imaging optics, Section 2 describes the thermodynamic understanding of nonimaging optics, Section 3 presents the alternative phase space representation of nonimaging optics, Section 4 describes the most basic nonimaging designs using Hottel’s strings, Section 5 discusses the geometric flow line designing method, and Section 6 summarizes the various concepts of nonimaging optics.