As shown in Figure, the beam coming from the light illuminates the slit (cross reticle). After passing through a polarization beam splitter (PBS), the beam is reflected and collimated by the collimating lens with a focal length as f. The emergent light is a parallel light. When the beam reached the mirror, it will be reflected back into the collimating lens, passing through the NPBS, finally, focused on the detecting area of the CCD Camera.
If the mirror moves a small displacement as θ, as the principle of reflection law, the reflected beam from the mirror will move 2θ. When it focused on the CCD, the beam position moved for a displacement Δ. Then we can get the relation between the small angle θ, the focal length f and the beam displacement Δ on the CCD, as follows:
Generally, the angle is small enough, so that Eq.(1)can be approximately written as
The same as,
In Eq. (3), f is a design parameter of the collimating lens, Δ is accurately detected by the CCD camera. So we can measure the displacement of the mirror θ with high precision.