Is a rose a rose? To crudely paraphrase Shakespeare, “What's in a name?” Can the same flower be described and characterized differently when viewed from different perspectives? Different scientific and technical fields tend to develop variations of the nomenclature for the same physical quantities. Within each field there is generally a degree of consistency, but interdisciplinary topics that bring together scientists, engineers, and practitioners from disparate fields often face inconsistencies in the symbols, units, and terminology utilized for quantitative analysis. The field of biomedical optics has not been immune to this problem, and publications in the field have invoked a variety of terms and symbols. In some cases, the same terms and symbols, as used in different fields, carry subtle but crucial differences in their meanings. An important example is the different meaning of the term intensity in physics (power per unit area), in radiometry (power per unit solid angle), and in heat transfer (power per unit area, per unit solid angle). Another example is that of the molar extinction coefficient for optical absorption of hemoglobin, which is typically defined using the logarithm to base-10 in chemistry and biology, or to base-e in physics, and may refer to one functional heme group or to the full molecule (four heme groups), depending on the specific physiological or biochemical characterizations. This may lead to a mismatch by as much as a factor of 9.2 (i.e., 4 × ln (10)) among the numerical values reported for the molar extinction coefficient of hemoglobin according to different conventions. For a long time, the three fields that most commonly make use of and describe (and teach) the methods of quantitative optical measurements have been physics, astronomy, and electrical engineering (or its subfield, optical engineering), although chemistry and biology also make use of optical characterization techniques. Because biomedical optics is a broad interdisciplinary field, which is based on contributions from researchers in a variety of specialty areas, it is important that a common language be used to describe and characterize its key quantitative parameters.

In this chapter, we define the nomenclature used in this book for the quantities that describe the optical radiation field and its interaction with biological tissue.