page_title: Near-infrared window in biological tissue
section_title: Water
hierarchical_section_title: Near-infrared window in biological tissue / Absorption properties of tissue components / Water
caption_attribution_description: English: Figure 3: The molar extinction coefficients of eumelanin and pheomelanin.
context_page_description: The near-infrared window defines the range of wavelengths from 650 to 1350 nanometre where light has its maximum depth of penetration in tissue. Within the NIR window, scattering is the most dominant light-tissue interaction, and therefore the propagating light becomes diffused rapidly. Since scattering increases the distance travelled by photons within tissue, the probability of photon absorption also increases. Because scattering has weak dependence on wavelength, the NIR window is primarily limited by the light absorption of blood at short wavelengths and water at long wavelengths. The technique using this window is called NIRS. Medical imaging techniques such as fluorescence image-guided surgery often make use of the NIR window to detect deep structures.
context_section_description: Although water is nearly transparent in the range of visible light, it becomes absorbing over the near-infrared region. Water is a critical component since its concentration is high in human tissue. The absorption spectrum of water in the range from 250 to 1000 nm is shown in Figure 2. Although absorption is rather low in this spectral range, it still contributes to the overall attenuation of tissue.
   
  Other tissue components with less significant contributions to the total absorption spectrum of tissue are melanin and fat.
caption: Figure 3: The molar extinction coefficients of eumelanin and pheomelanin.[5]
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