Only that x,y designate optional points in the X,Y axes , and z of equation is {TAIL ~ a distance} of the optical axis direction from an optional point of {HEAD ~ coaxially aspherical surface} to the XY plane 
Only that x , y designate optional points in the X , Y axes , and z of equation is { TAIL ~ a distance } of the optical axis direction from an optional point of { HEAD ~ coaxially aspherical surface } to the XY plane
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH