A common path phase shifting interferometer is proposed using a liquid
-crystal cell as a phase modulator. In the proposed common path interf
erometer, the specular component of the object light, separated in the
Fourier transform plane, plays the role of a reference wave for the d
iffracted component of the object. A homogeneous-type nematic liquid-c
rystal cell is used as a phase only modulator, to introduce an arbitra
ry phase modulation between the specular and the diffracted components
to achieve phase shifting interferometry. This makes quantitative mea
surements possible as well as qualitative observations for phase objec
ts like in a Zernike's phase contrast microscope. Finally, experiments
were carried out to show the validity of the proposed method. Experim
ental results show that the present method is quite stable against mec
hanical shocks and air fluctuation owing to its common path arrangemen
t. The accuracy of the present method was experimentally verified to b
e better than 1/30 of wavelength by comparing the results with those o
btained by the heterodyne interferometer.