Optical window in biological tissue

The near-infrared (NIR) window (also known as optical window or therapeutic window) defines the range of wavelengths from 650 to 1350 nm 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.

The absorption coefficient (${\displaystyle \mu _{a}}$) is defined as the probability of photon absorption in tissue per unit path length. Different tissue components have different ${\displaystyle \mu _{a}}$ values. Moreover, ${\displaystyle \mu _{a}}$ is a function of wavelength. Below are discussed the absorption properties of the most important chromophores in tissue. The molar extinction coefficient (${\displaystyle \varepsilon \,}$) is another parameter that is used to describe photon absorption in tissue. By multiplying ${\displaystyle \varepsilon \,}$ by the molar concentration and by ln(10), one can convert ${\displaystyle \varepsilon \,}$ to ${\displaystyle \mu _{a}\,}$.

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