Near infrared light
Absorption of light by blood
Cerebral Cortex
Interpolation is a mathematical way of reasonably filling a gap between measured digital data. In general, the progression value of the measured data is assumed to be continuous and smooth. There are several methods of interpolation in these cases. Here we use "third degree Spline interpolation". In this method, the first and second degree derivative functions of a third degree polynomial are able to pass through each measured value. To do this, we first take three points of the measured values from one side to the other successively (see the figure below). The points have the third degree polynomial apply the conditions noted above, then shift by one point, repeat again and so on. In the actual system, this method is expanded two dimensionally.

The figure shown below, is a simple one-dimensional example of the third degree Spline interpolation used for adequate values.