Why can near-infrared light read only the outer centimetre or two of the brain? · Grey Matter

Near-infrared light is absorbed little by tissue, but it is scattered every 25 to 200 µm, so after a few millimetres each photon has changed direction many times and carries no image.

Why can near-infrared light read only the outer centimetre or two of the brain?

Near-infrared light is absorbed little by tissue, but it is scattered every 25 to 200 µm, so after a few millimetres each photon has changed direction many times and carries no image. fNIRS works with that diffuse light: a detector a few centimetres from the source collects photons that wandered down through scalp and skull into the outer cortex and back, and the deepest of them reach only about 1.5 to 2 cm. The method therefore sees the cortical surface under the skull at centimetre resolution, and cannot see deep structures at all.