{HEAD ~ The spatial pattern} of {TAIL ~ densities} in the media is sensed by shining a light through the media and sensing the transmitted light intensity 
of
{ HEAD ~ The spatial pattern } of { TAIL ~ densities } in the media is sensed by shining a light through the media and sensing the transmitted light intensity
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH