Symbol I_0

The passage around this formula

…filtered through the atmosphere’s outer layers before reaching the detector. Each gas absorbs a characteristic set of wavelengths, governed to first approximation by the Beer–Lambert relation, $I(\lambda) = I_0(\lambda)\, e^{-\tau(\lambda)}, \qquad \tau(\lambda) = \int n(z)\,\sigma(\lambda, z)\, dz$. where $I_0(\lambda)$ is the incident spectrum, $\tau(\lambda)$ the wavelength-dependent optical depth, n(z) the number density of an absorbing species at altitude z , and $\sigma(\lambda, z)$ its absorption cross-section, which itself depends on temperature and pressure. The instrument records I($\lambda$) ; the…

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