real function rf_hirsbright(r, k, opt)
c * Inverse Planck function for HIRS
c .... version of 01.03.06

c  r = radiance, standard units
c  k = channel number: 1...20
c  opt (1=unshifted Planck coefficients, 2=shifted Planck coefficients)

        integer nb, nt
	parameter (nb=20,nt=2)

        real cwn, fk1, fk2, tc
        real cwn_shifted, fk1_shifted, fk2_shifted, tc_shifted
	common/hirpfc/cwn(nb),fk1(nb),fk2(nb),tc(nt,nb)
	common/hirpfc_shifted/cwn_shifted(nb),fk1_shifted(nb),fk2_shifted(nb),tc_shifted(nt,nb)

        integer k, opt
        real r, expn, tt

        if(opt .eq. 1) then
 
	  expn=fk1(k)/r+1.
	  tt=fk2(k)/alog(expn)
	  rf_hirsbright=(tt-tc(1,k))/tc(2,k)

c         write(*,'(''Opt1: '',i5,7f15.5)') k, cwn(k), fk1(k), fk2(k), tc(1,k), tc(2,k),
c    *           tt, rf_hirsbright

        else if(opt .eq. 2) then

	  expn=fk1_shifted(k)/r+1.
	  tt=fk2_shifted(k)/alog(expn)
	  rf_hirsbright=(tt-tc_shifted(1,k))/tc_shifted(2,k)

c         write(*,'(''Opt2: '',i5,7f15.5)') k, cwn_shifted(k), fk1_shifted(k), fk2_shifted(k),
c    *           tc_shifted(1,k), tc_shifted(2,k), tt, rf_hirsbright

        end if

	return
	end