An optical system for improving alignment measurement accuracy is discussed. The optical system includes first and second optical elements. The first optical element may be configured to change a first beam having a first polarization state into a second beam having a second polarization state. The second optical element may be configured to provide total internal reflection of the second beam and to change the second beam into a third beam having a third polarization state. The first, second, and third polarization states may be different from each other.