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