Wavefront measurement with a phase-shifting lateral-shearing Sagnac interferometer operating in broadband light

A phase-stepping lateral shearing Sagnac interferometer for wavefront measurement is described. Phase shifting is implemented using a polarisation modulator operating on the wavelength independent Pancharatnam's phase allowing accurate measurement of phase maps even with broadband light. We demonstrate the interferometer by measuring a range of different wavefront shapes using both laser and white light.