;+
; NAME:
;  rt_solirflx
; PURPOSE: (one line)
;  return solar IR flux
; DESCRIPTION:
;
; CATEGORY:
;  RT
; CALLING SEQUENCE:
;  flx = rt_solirflx(ww)
; INPUTS:
;  ww - flux in wavenumbers
; OPTIONAL INPUT PARAMETERS:
; KEYWORD INPUT PARAMETERS:
; KEYWORD OUTPUT PARAMETERS:
; OUTPUTS:
;  solar flux in erg/cm^2/s/invcm at 1 AU
; COMMON BLOCKS:
; SIDE EFFECTS:
; RESTRICTIONS:
;  None
; PROCEDURE:
;  calls expm1, which returns exp(x)-1, even for small x.
; MODIFICATION HISTORY:
;  Written 2004 May 8, Leslie Young SwRI
;  Modified from routine in cascase.f, Roger Yelle
;-

function rt_solirflx, ww

    physconstants
    c1 = 8.37d-10 ; 2 pi h c^2 (rsun/au)^2
    c2 = 2.4945d-4 ; (hc)/(kT) for T = 5767.7903

    solirflx = c1*(ww^3.)/expm1(c2*ww)
    return, solirflx

end