Implementing programmable phase control using a liquid-crystal spatial light modulator (LC-SLM) over a bandwidth approaching a PHz presents specific challenges associated with calibrating the phase response of the device, which is characteristically nonlinear with both wavelength and applied voltage. Here we introduce a powerful new calibration protocol, which not only maps the phase response of the device as a function of wavelength and voltage across nearly an octave (400–770 nm), but also provides sufficient information to recover the thickness of the liquid-crystal cell and the Sellmeier equation of the liquid-crystal.