FDS: Radiation: use Planck mean for zero path lenght

Author: shostikk

Manuals/FDS_Technical_Reference_Guide/Appendices.tex

@@ -424,11 +424,11 @@ \chapter{Gas Phase Absorption Coefficients}
 
 By default in FDS, the pathlength, $L$, is 10~cm. It can also be specified by the user. If $T=T_{\rm rad}$ the intensity does not depend on $\kappa_{n,i,{\rm e}}$, and the value $\kappa_{n,i,{\rm e}}(T_{\rm rad})$ is therefore interpolated from the neighboring temperatures.
 
-In cases with only one band ($N$=1), the smaller of the two absorption coefficients is used:
+In cases of non-zero path length, the smaller of the two absorption coefficients is used:
 \be
    \kappa_{n,i}=\min \Big( \kappa_{n,i,{\rm P}}(Y_i,T),\kappa_{n,i,{\rm e}}(Y_i,T,L) \Big)
 \ee
-If $N>1$ or $L=0$, $\kappa_{n,i}=\kappa_{n,i,{\rm P}}$. Note that the spectral data within RADCAL are used whenever the gas mixture contains water vapor, fuel or combustion products, regardless of the number of radiation bands $N$.
+If $L=0$, $\kappa_{n,i}=\kappa_{n,i,{\rm P}}$. Note that the spectral data within RADCAL are used whenever the gas mixture contains water vapor, fuel or combustion products, regardless of the number of radiation bands $N$.
 
 \textbf{Note on wavenumber, wavelength, and frequency}: some confusion might arise when dealing with the various quantities describing the wave nature of radiation. These quantities are wavenumber $\om$, wavelength $\la$, and frequency denoted here $\nu$. Most users may be familiar with the frequency $\nu$, in units of \textit{hertz}, ${\rm Hz}$, representing the number of cycles per second. While this unit is preferred for radiation waves of low energy such as radio waves, wavelength and wavenumber are preferred for waves of higher energy. Wavenumber and wavelength are related to frequency through \cite{Penner:1959}
 \be