      subroutine PLANK
     I   ( DNU1, DNU2, TEMP, DELNU,
     C     PI , 
     O     B )

!-----------------------------------------------------------------      
      implicit real*8 ( a-h,o-z )
!-----------------------------------------------------------------      
      
      NBND = INT ( ( DNU2 - DNU1 ) / DELNU )
      B = 0.0D0 

      do INB = 1, NBND
         WNU = DNU1 + ( INB - 0.5 ) * DELNU
         
         B = B + DELNU * PI * ( 1.1911D-8 * WNU**3 ) /
     \        ( EXP( 1.439D0 * WNU / TEMP ) - 1.0D0 )

      end do

!-----------------------------------------------------------------      
      return
      end
