Near infrared light
Absorption of light by blood
Cerebral Cortex
Infrared light is a type of the radio wave ( electro-magnetic wave), which has a longer wavelength than visible light, and is usually defined as the waves between the end of visible light (about 770nm) and milli-wave {1.000.000nm(1mm)}.
It is present everywhere; for example, sunlight contains infrared light.
In general, the infrared light that is closest to visible light is called "near infrared light" (between from the end of visible light to 2500nm).

These waves are generated easily by heat, which features the direct transferring of the heat.

The waves are generated by the oscillation and/or rotation of atoms and molecules found in all materials.

The Optical Topography system used two discrete and very narrow wavelengths light within the near infrared range. These are 780nm and 830nm and generated from special infrared laser diodes respectively.