\subsection{The Sun as a black body radiator}
\label{Sec:SunBlackBody}
%
The Kirchhoff-Planck function describes
an ideal black body radiator.
For matter of convenience one may define 
%
\begin{equation}
{\rm B}_\lambda^{\mbox{\scriptsize\rm sunshine}}
=
{\rm B}_\lambda^{\mbox{\scriptsize\rm Sun}}
\cdot
\frac{{\rm R}^2_{\mbox{\scriptsize\rm Sun}}}
     {{\rm R}^2_{\mbox{\scriptsize\rm Earth's orbit}}}
=
{\rm B}_\lambda^{\mbox{\scriptsize\rm Sun}}
\cdot
\frac{1}{(215)^2}
\end{equation}
%
Figure~\ref{Fig:Visible} shows the spectrum 
of the sunlight, assuming the Sun is a black
body of temperature $T=5780\,{\rm K}$. 
%
\begin{figure}[htbp]
\ifthenelse{\equal{IJMPB}{\TschStyle}}{%
\centerline{\psfig{file=PictureLarge_sunlight_.eps,width=4.50in}}
}{}%
\ifthenelse{\equal{arXiv}{\TschStyle}}{%
\centerline{\includegraphics[scale=2.00]{PictureSmall_sunlight_.png}}
}{}%
\ifthenelse{\equal{TeX4ht}{\TschStyle}}{%
\centerline{\includegraphics[scale=0.66]{PictureLarge_sunlight_.png}}
}{}
\vspace*{8pt}
\caption{The spectrum of the sunlight assuming the sun
         is a black body at $T=5780\,{\rm K}$.}
\label{Fig:Visible}
\end{figure}
%

To compute the part of radiation
for a certain wave length interval 
$[\lambda_1,\lambda_2]$
one has to evaluate the expression 
%
\begin{equation}
\frac{ \int_{\lambda_1}^{\lambda_2} 
       {\rm B}_\lambda^{\mbox{\scriptsize\rm sunshine}} (5780)\,d\lambda }
     { \int_0^\infty 
       {\rm B}_\lambda^{\mbox{\scriptsize\rm sunshine}} (5780)\,d\lambda }
\end{equation}
%
Table~\ref{table:sunlight} shows the  
proportional portions of the 
ultraviolet, visible, and infrared sunlight,
respectively.
%
\input{GerlichTscheuschner2007a_220_real_greenhouses_sun_as_black_radiator_table008_}
%
Here the visible range of the light is assumed
to lie between 380\,{\rm nm} and 760\,{\rm nm}.
It should be mentioned that the visible range 
depends on the individuum. 

In any case,
a larger portion of the incoming sunlight 
lies in the infrared range than in the 
visible range. 
In most papers discussing 
the supposed greenhouse effect 
this important fact is completely 
ignored.   
