To add: Water has an absorption band that is clear to most visible light and the near IR, but it starts to absorb as you go deeper into IR. Many tissues and the

To add: Water has an absorption band that is clear to most visible light and the near IR, but it starts to absorb as you go deeper into IR. Many tissues and their light absorption curves can be seen here: https://en.wikipedia.org/wiki/Near-infrared_window_in_biological_tissue https://en.wikipedia.org/wiki/Near-infrared_window_in_biolog...