The solution of the radiative transfer equation for spherical ionized blob : $$\frac{dI_{\nu}}{ds} = j_{\nu}-\alpha I_{\nu}$$

and solution is (Ref: http://en.wikipedia.org/wiki/Radiative_transfer} $$I_{\nu}(s) = I_{\nu}(s_0)e^{-\tau_{\nu}(s_0, s)}+ \int_{0}^{s} j_{\nu}(s')e^{-\tau_{\nu}(s', s)} ds'$$

This is solved using $I(0)=0$. where $\tau_{\nu}(s', s)=\tau_{\nu}(s)-\tau_{\nu}(s')$. It is not clear to me why $\tau_{\nu}(s', s)$ is written this way.

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See here –  Vijay Murthy Jun 27 '12 at 8:34