The transmittance measurement for each slice thickness was obtained by averaging measurements from seven animals (green curve in Fig. 3). Specifically, the transmittance values were , , , , , , , and () at thicknesses of 300, 400, 500, 600, 800, 1000, 1200, and 1500 μm respectively. These in vitro values were much larger than the light intensities obtained from live and postmortem animals (paired -test, ). Note that the in vitro experiments were normalized with the light intensity received by the photodiode without any tissue present, whereas in vivo measurements were normalized to the subdural light intensity. Using the Fresnel equation [, where for air and for neural], the reflection for a light beam entering the tissue perpendicularly was calculated as 2.55%, and neglected in the in vitro plot. This means the reflected light at the surface of entry was lumped into the attenuation by the tissue.