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