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