Derivation of Flux Density ($F_{\nu }$) from Brightness Temperature ($T_{b}$)

An NoRH image indicate distribution of brightness temperature (unit K). To know the intensity (unit SFU/sterad) from each pixel,
IDL> fi=norh_tb2flux(data,index,/intensity) $<$CR$>$
The flux density (unit SFU) of the whole image, the procedure is as follows.
IDL> fi=norh_tb2flux(data,index) $<$CR$>$
The flux density from the partial area of an image, $i0<i<i1$ $j0<j<j1$ is
IDL> box=[i0, i1, j0, j1] $<$CR$>$
IDL> fi=norh_tb2flux(data,index,box=box) $<$CR$>$
or by giving the $(x, y)$ coordinate (origin solar disk center; unit arcsec), for the area $x0\le x\le x1$ and $y0 \le y \le y1$,
IDL> abox=[x0, x1, y0, y1] $<$CR$>$
IDL> fi=norh_tb2flux(data,index,abox=abox) $<$CR$>$