Near infrared light
Absorption of light by blood
Cerebral Cortex
The total weight of blood in the body is said to be about 1/12 the total body weights, so human adult weighting 70kg has about 6kg of blood. Blood contains red blood cells (erythrocyte--5 million cells per cubic millimeter), white blood cells (leukocyte--7 thousand cells per, blood platelet (thrombocyte 350 thousand cells per, nourishment, metabolites, water and so on.

Red blood cells account for about 45% of the total blood volume. Hemoglobin, whose molecular weight is around 64,000, easily changes its state by absorption and release of oxygen, thus carries the oxygen throughout the human body.

The color of arterial blood is bright red, while that of venous blood is somewhat darker; these changes in color depend on the content differences of oxy- and deoxy-hemoglobin. Arterial blood is higher concentration of oxy-hemoglobin than that of venous blood. The fact may explain the difference in optical absorption coefficient of oxy- and deoxy- hemoglobin. (see figure 2, column 2)

The optical topography system is able to measure the blood volume concentration change of oxy- and deoxy-hemoglobin separately.