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. 2018 Aug 7;1464(1):5–29. doi: 10.1111/nyas.13948

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© 2018 The Authors. Annals of the New York Academy of Sciences published by Wiley Periodicals, Inc. on behalf of New York Academy of Sciences.

This is an open access article under the terms of the http://creativecommons.org/licenses/by/4.0/ License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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Continuous wave devices measure light attenuation due to scattering and absorption based on intensity measurements of the input (I _IN) and output (I _OUT) light. Attenuation is evaluated by computing the logarithm of the ratio between I _IN and I _OUT that is related to changes in hemoglobin concentration. The first attenuation measurement is subtracted to the following attenuation values to remove the effect of scattering, melanin, and water concentrations (differential spectroscopy). The changes in attenuation ΔA are related to the changes in chromophore concentrations (Δc, either HbO₂ or HbR) by the modified Beer–Lambert law. Note that d represents the source–detector distance, ε is the extinction coefficient of the chromophore at a certain wavelength λ, and the DPF is the differential pathlength and indicates the increase in the photon path due to scattering.