{TAIL ~ The reflected laser radiation} is collected by lens L2 and focussed via beam splitter BS1/F1, mirror M1, {HEAD ~ 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 
{ TAIL ~ The reflected laser radiation } is collected by lens L2 and focussed via beam splitter BS1 / F1 , mirror M1 , { HEAD ~ 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
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