In the embodiment of FIG 6A, the heads 510 b, 510 r and 510 y of the laser sensors are positioned in {HEAD ~ an equilateral triangular placement} which facilitates {TAIL ~ a three point plane distance determination} for measuring the height of a surface such as a wafer surface 
In the embodiment of FIG 6A , the heads 510 b , 510 r and 510 y of the laser sensors are positioned in { HEAD ~ an equilateral triangular placement } which facilitates { TAIL ~ a three point plane distance determination } for measuring the height of a surface such as a wafer surface
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH