Optical density – openfnirs

Definition: Optical density (or Absorbance) is defined as the (decadic) logarithm of the ratio of incident to transmitted radiant power through a (non-scattering) sample. This quantity is used, e.g., in spectrophotometry to characterize a dissolved chromophore in a non-scattering solution or an optical filter. Absorbance is defined as: $A = -log_{10} T = -log_{10}(\frac{I_d}{I_0})$ where A is the absorbance or optical density, T is the transmittance, $I_{d}$ is the intensity of the transmitted light after the light traveling a pathlength d, and $I_{0}$ is the intensity of the incident light. For a single chromophore, T can be written as $T = e^{-\mu_a d} = 10^{-\epsilon cd}$ where a $\mu_a$ is the absorption coefficient, $\epsilon$ is the molar decadic absorption coefficient, c is the concentration of the chromophore. This equation leads to $A = \epsilon cd = \frac{\mu_a d}{log_e 10}$ $\mu_a = log_e 10 \cdot \epsilon c = 2.303 \epsilon c$ In fNIRS, optical density (OD) is sometimes used as a synonym for (decadic) attenuation (termed A as well). In this context, attenuation is relevant instead of absorbance since it also includes the effect of light scattering.

Category: Physics

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Abbreviation or symbol: A, OD

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References:

https://doi.org/10.1117%2F1.NPh.8.1.012101

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