A method of measuring the refractive index profile of a consolidated glass body with a cylindrical surface and refractive index striae in the radial cross-section of the cylindrical glass body, the striae having a mean periodicity or spacing d, the method comprising the steps of: a. scanning the cylindrical surface of the consolidated glass body using an optical beam of wavelength λ that emanates from an aperture, such that an image of the aperture is formed downstream from the consolidated glass body, between the consolidated glass body and at least one optical detector, and sampling the cylindrical surface with the optical beam at multiple sampling locations x i; b. detecting locations where zero-order optical beams corresponding to the sampling locations x i impinge on at least one optical detector after passing through the consolidated glass body; c. determining deflection angles of the zero-order beams corresponding to the multiple sampling locations x i; d. calculating the refractive index profile of the consolidated glass body based on the deflection angles of the zero-order optical beams corresponding to the multiple sampling locations; characterised by: predicting the trajectory of the zero-order optical beams through the consolidated glass body based on (i) the sampling locations x i of the optical beam impinging on the cylindrical surface of the consolidated glass body, and (ii) the locations at which the corresponding zero-order beams impinge on the at least one optical detector; wherein for a glass preform wherein the spacing of the striae d < 12 μm, the optical beam is near-infra red light of wavelength λ, given by 0.7 μm < λ < 2 μm; and for a glass preform wherein the spacing of the striae d > 12 μm the optical beam is light of mid-infrared wavelength λ. a. scanning the cylindrical surface of the consolidated glass body using an optical beam of wavelength λ that emanates from an aperture, such that an image of the aperture is formed downstream from the consolidated glass body, between the consolidated glass body and at least one optical detector, and sampling the cylindrical surface with the optical beam at multiple sampling locations x i; b. detecting locations where zero-order optical beams corresponding to the sampling locations x i impinge on at least one optical detector after passing through the consolidated glass body; c. determining deflection angles of the zero-order beams corresponding to the multiple sampling locations x i; d. calculating the refractive index profile of the consolidated glass body based on the deflection angles of the zero-order optical beams corresponding to the multiple sampling locations; predicting the trajectory of the zero-order optical beams through the consolidated glass body based on (i) the sampling locations x i of the optical beam impinging on the cylindrical surface of the consolidated glass body, and (ii) the locations at which the corresponding zero-order beams impinge on the at least one optical detector; wherein for a glass preform wherein the spacing of the striae d < 12 μm, the optical beam is near-infra red light of wavelength λ, given by 0.7 μm < λ < 2 μm; and for a glass preform wherein the spacing of the striae d > 12 μm the optical beam is light of mid-infrared wavelength λ.