 Dual polarisation interferometry is an emerging technique to directly measure conformational change and formula_4.In the less extensive technique of equilibrium unfolding, the fractions of folded and unfolded molecules (denoted as formula_13 and formula_14, respectively) are measured as the solution conditions are gradually changed from those favoring the native state to those favoring the unfolded state, e.g., by adding a denaturant such as guanidinium hydrochloride or urea.