According to some embodiments a method of measuring the refractive index profile of a consolidated glass body having a cylindrical surface comprises the steps of: (a) forming an image of a slit behind the glass body; (b) optionally pre-scanning the cylindrical surface of the test glass body or a reference glass body and analyzing the data within a fixed window to determine the likely location of the zero-order, un-diffracted beam while ignoring other diffracted beams; (c) optionally adjusting the optical power to improve the intensity of the data within the fixed window in order to improve the analysis; (d) predicting the trajectory of the zero-order beam through the preform based on the sampling location xi (incidence position) of the light impinging on the cylindrical surface and the location at which the zero-order beam impinges on the detector; (e) measuring the cylindrical surface of a glass body while using the estimated location of the zero-order beam and adjusted optical power to set the center of a floating window and the beam power at each measurement point; (e) determining deflection angles of the exiting zero-order beam within the floating window at each sampling location; (e) calculating the refractive index profile of glass body by utilizing a transformation function which determines refractive index at each location based upon the measured deflection angle function of the beam.