If the medium under investigation is not homogenous, then these absolute values may not be correct. For example, when measuring the neonatal head, the tissue is not homogenous, but has a layered structure of skin, bone, cerebrospinal fluid (CSF), and brain. This issue has been addressed in several studies using simulation, phantoms, and in vivo experiments. In simulations, it was shown for the neonatal head that a single light bundle is affected to between 8% and 23% by superficial tissue.7 This result depends strongly on the assumptions made concerning the geometry of the neonatal head, such as the source-detector distances8 and the shape and thickness of these layers.7 An additional difficulty is that data on the thickness of the skull, skin, and CSF are not available for preterm infants. For the term neonates, we only found data on skin thickness.9 For future simulations, it would be important to create reliable data on the thickness of these superficial tissues. In phantom studies, it was shown that by measuring at several different source detector distances (multidistance geometry), the effect of superficial tissue can be excluded mathematically.10In vivo, studies in adult subjects indicated that values can be influenced by superficial tissues.11,12 Again, no such studies are available in neonates yet, but according to the simulations, we expect that the influence of superficial tissue on the value is much smaller in neonates or preterm infants compared to adults.