Instead of mechanically rotating a polariser to analyze the ellipticity of the reflected lightbeam, the incident light is modulated by the Birefringence Modulator. This element is operated in longitudinal resonant vibration at a frequency of 50 kHz, but has no mass motion. Because there are no moving parts, there are no beam deflections which plague rotating element instruments.

The Birefringence Modulator is a balanced 3 component device. A gauge crystal provides feedback on the oscillation amplitude which leads to a high long term stability that is independent of environmental parameters such as temperature. The exact reproducibility of the modulation amplitude eliminates the need for frequent recalibrations or elaborate cross-checking of the phase shift. This is especially important for spectroscopic measurements where the modulation amplitude is tracked across the spectrum.

With a modulation frequency of 50 kHz the instrument can work with response times as short as 1 ms. The high frequency modulation moves the signal away from 1/f noise region. Phase lock detection further reduces the noise, leaving photon shot noise as the only limiting factor to the instrument sensitivity. To eliminate signal offsets, both Re(r) and Im(r) can be subjected to a low frequency analyser modulation which removes spurious optical signals. The incidence and reflection arms are driven by stepping motors that can be synchronized in a sample feedback loop to maintain the ellipsometer at the Brewster angle in dynamic situations.