{HEAD ~ The reflected laser radiation} is collected by lens L2 and focussed via beam splitter BS1/F1, mirror M1, beam splitter BS2 and {TAIL ~ a second mirror M2} on to the photodetector 5 which is in the form of a 1 mm.×1 mm. PIN photodiode 
{ HEAD ~ The reflected laser radiation } is collected by lens L2 and focussed via beam splitter BS1 / F1 , mirror M1 , beam splitter BS2 and { TAIL ~ a second mirror M2 } on to the photodetector 5 which is in the form of a 1 mm.×1 mm . PIN photodiode
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH OTH REL REL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH