At {HEAD ~ any time} after or during {TAIL ~ staining}, the sample is illuminated with a wavelength of light selected to give a detectable optical response, and observed with a means for detecting the optical response 
after
At { HEAD ~ any time } after or during { TAIL ~ staining } , the sample is illuminated with a wavelength of light selected to give a detectable optical response , and observed with a means for detecting the optical response
OTH HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH