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Weak near infrared light of about 2mW from a laser diode is illuminated onto head from optical fibers attached to the scalp. This light passes through the skull and reaches the cerebral cortex. It penetrates to a depth of about 30mm, and is scattered by hemoglobin in the blood. The light is partially reflected back through the scalp. The reflected light back on the scalp contains the information about the cerebral cortex. A 30mm distance from the original illuminated point is suitable to detect. In adults, the light attenuates to a level as low as a hundred millionth or even a billionth of the irradiated light. The reflected light is sent through optical fibers and received with a highly sensitive avalanche photodiode detector.
The figure shown below is the molecular absorption coefficient of oxy- and deoxy-hemoglobin.
The brain activity is characterized by the firing of neurons. This results in active energy metabolism and causes secondary increase in blood volume to supply glucose and oxygen to the tissues. Thus, the change in concentration of hemoglobin is an important index to know the brain activity.
The reflected light is measured every 0.1 seconds, and the latter measured data is subtracted from the former to determine the concentration change of hemoglobin.
The intensity of the reflected light depends on the respective concentration of two kinds of hemoglobin. Two kinds of laser diodes (695nm and 830nm) are used to separate the two kinds of hemoglobin concentration changes independently. It is necessary for the spectral line width of each diode to be narrow enough to determine precisely the two results. Thus, laser diodes are used.
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