The first derivative of the pseudo-absorption spectrum of a water-loaded cotton wool (water–CW) phantom, which mimics muscle tissues, was used to determine the light path length in the near-infrared (NIR) region. The light path length increased as the density of the turbid medium decreased. It is independent of both water content in the range of 75–85% (by weight) and the diffuse reflecting reference used to determine the pseudo-absorbance. The path length determination procedure was verified by measurements of diffuse reflectance in chicken breast tissue for which the path length of 1.8 mm (differential path length factor, DPF = 2.1) was found to be similar to the path length of NIR light of 1.5–2.2 mm (DPF = 1.8–2.6) in a water–CW phantom of density similar to chicken breast. We conclude that the NIR light path length can serve as a characteristic of muscle tissue density.

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