c) detecting {HEAD ~ light} of such wavelength that is reflected or scattered by {TAIL ~ said test element}, and 
c ) detecting { HEAD ~ light } of such wavelength that is reflected or scattered by { TAIL ~ said test element } , and
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH REL REL OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH