  Transcranial photobiomodulation is an optical method for non-invasive brain
stimulation. The method projects red and near-infrared light through the scalp
within 600-1100 nm and low energy within the 1-20 J/cm2 range. Recent studies
have been optimistic about replacing this method with pharmacotherapy and
invasive brain stimulation. However, concerns and ambiguities exist regarding
the light penetration depth and possible thermal side effects. While the
literature survey indicates that the skin temperature rises after experimental
optical brain stimulation, inadequate evidence supports a safe increase in
temperature or the amount of light penetration in the cortex. Therefore, we
aimed to conduct a comprehensive study on the heat transfer of near-infrared
stimulation for the human brain. Our research considers the transcranial
photobiomodulation over the human brain model by projecting 810 nm light with
100 mW/cm2 power density to evaluate its thermal and optical effects using
bioheat transfer and radiative transfer equation. Our results confirm that the
near-infrared light spectrum has a small incremental impact on temperature and
approximately penetrates 1 cm, reaching the cortex. A time-variant study of the
heat transfer was also carried out to measure the temperature changes during
optical stimulation.
