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