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Illustration of the path (shown in red) followed by the NIR photons from the light source to the detector through the different layers of the head. The penetration depth of the light is proportional to the source–detector distance (d 1: deeper channel; d 2: superficial channel). A channel is composed by the pair source–detector and is located at the midpoint between the source and the detector and at a depth of around the half of the source–detector separation.
