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