An apparatus and method for use in assisting a human heart are disclosed. The mass has optical elements adapted to change a length of an optical path in response to movement of the mass. The housing further has a side portion disposed at one side of the main body portion. The metrology model optimizer optimizes an optical metrology model of the first patterned structure. Costs to be minimized may include the deviation of the triangle surface normal from the edge normal, the degree of bending between adjacent triangles, or the departure of the approximated surface from the original surface.