. . . . "For example, for a protein or other target analyteimmobilized at a height 330 (referred to as height h in the equations that follow), the quadrature angle .crclbar..sub.q is given by: 0.5*w.sub.o*sin .crclbar..sub.q=.lamda./4 for an incident laser beam having a wavelength .lamda. and a beam widthw.sub.o." .