Modified Beer's Law – historical perspectives and relevance in near-infrared monitoring of optical properties of human tissue

Studying the interaction of light with human tissue is important in understanding functional changes and oxidative metabolism of tissue (e.g., skeletal muscle, brain). This manuscript focuses on the development of quantitative absorption laws of light energy, from the inception of Bouguer Law to Beer's Law, to gain a perspective on the present application of the absorption concepts in investigating optical properties of light absorbing chromophores of human tissue. Although these absorption laws are applicable to determine the absolute concentration of substances present in solids or solvents, these laws do not take into consideration of either reflection or scattering of incident light that accounts for the loss of transmitted light intensity. Importance of light energy in the near-infrared window of 700–1300 nm compared to that of other regions in the electromagnetic spectrum for determination of optical tissue properties in vivo is emphasized. Since human tissue is a highly scattering non-homogenous medium, limitations of Beer's Law with respect to studying human tissue chromophores are presented. Finally, modifications of Beer's Law to investigate the relationship between changes in absorbency and concentrations of light absorbing chromophores within the medical spectral window (700–900 nm) in human tissue are discussed.Relevance to industryAs light propagates through human tissue, concentration of some chromophores varies in time within the medical spectral window, implying physiological changes in the tissue. Utilization of this optical principle in biomedical (non-invasive) research of the health and disease of human tissue has increased in the last two decades. While impressive in clinical applications, researchers in the field of ergonomics and human factors have been slow in exploiting the potential of examining optical properties of tissue relevant to human performance at the workplace.