  In this paper we demonstrate that the weak temperature dependence of
structure factor of supercooled liquids, a defining feature of the glass
transition, is a consequence of the averaging of the scattering intensity
either due to the use of an incoherent radiation source or explicit angular
averaging. We show that the speckle scattering at individual wavevectors,
calculated from a simulated glass former, exhibits a Debye-Waller factor with a
sufficiently large temperature dependence to represent a structural order
parameter capable of distinguishing liquid from glass. We also extract from the
speckle intensities a quantity proportional to the variance of the local
restraint, i.e. a direct experimental measure of the amplitude of structural
heterogeneity.
