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