Illumination Engineering

Abstract

Étendue is one of the most basic yet important concepts in the design of nonimaging and illumination optics. First, it explains the flux transfer characteristics of the optical system, and, second, it plays an integral role in the ability to shape the distribution of radiation at the target. Interestingly, the concept of étendue was only formally accepted in the 1970s through a series of letters to a journal [1-4]. Reference [1] asked for input to a proposal to be submitted to the Nomenclature Sub-Committee of the International Commission for Optics in order to standardize the name for what ultimately became étendue [5]. Initially, the author of this journal letter favored the term "optical extent," with the term étendue not likely due to "the typing disadvantage of é, (and) the pronunciation difficulty of the French u" [1]. Through the series of replies, both in the journal and by private communications, the term étendue gained favor "since the accent will be dropped anyway" [4]. The dropping of the accent is not applicable today since modern computer technology makes it simple to include. This recent history of varied opinions and potential confusion illustrates the complexity inherent in the term étendue, indicating that it has the potential to be interpreted in many ways. In fact, after defining étendue and illustrating its innate conservation in optical systems, we present a series of terms that are analogous or often confused with étendue. In this chapter, a series of proofs and examples show that the flux transfer characteristic is an integral characteristic of étendue through its inherent conservation. The shaping of the distribution from the source to the target is also an inherent part of étendue, but it is often better viewed through the first derivative of étendue-the skewness. Like étendue, skewness is also invariant through an optical system of rotational symmetry. We end this chapter by presenting a discussion of the limitations of étendue, especially in the physical domain of optics, and thus present an alternate expression for development. Note that for this chapter, the radiometric terms of irradiance, radiance, and radiant intensity or simply intensity are used. The photometric quantities of illuminance, luminance, and luminous intensity, respectively, can be substituted. Additionally, we use imaging terms such as "entrance pupil" or "exit pupil" in order to remain in agreement with the imaging community. We find that using such expressions from the imaging community can alleviate potential confusion to those new to the field of nonimaging optics. Alternate terms of entrance aperture or exit aperture can be used instead. © 2013 the Institute of Electrical and Electronics Engineers. Published 2013 by John Wiley & Sons, Inc.