Abstract: The device for measuring membranes comprises a light-emitting device which is arranged on one side of a transparent material (40) and which comprises a monochromatic light source (42), a fixed polarizer (44) and a photoelastic modulator (46) which gives two reference frequencies f0 and 2f0, a scattering strip (60) placed on the other side of the transparent material, means for moving the emitted light beam (62) over the entire length of the scattering strip, and a device (64) receiving the scattered light which lies on the same side as the emitting device in relation to the transparent material and comprises an analyzer (48), an avalanche photodiode (50), two synchronous-detection amplifiers (54, 56) having as references the two frequencies which are given by the modulator, and an acquisition bench (58) for determining the phase shift of the electrical signal collected at the output of the photodiode, when stresses are exerted in relation to the case in which stresses are not exerted.

Abstract: The invention relates to a process and to a device for measuring the thickness of transparent materials. More particularly, but not exclusively, the invention concerns the thickness measurement of glass materials and, even more precisely, the thickness measurement of flat glass, in particular float glass. According to the invention, a light beam with modulated frequency is focused, two light beams or rays reflected by each of the surfaces of the transparent material are received, interference is created between them, the number of oscillations per modulation period of the interference signal is determined, the path difference (&dgr;) between the two beams and the thickness (e) of the transparent material are deduced and the phase shift (&phgr;) of the said interference signal is determined. This determination of the phase-shift between the two signals each coming from one of the surfaces of the transparent material can then be used to deduce other characteristics of the said material.

Abstract: A method and apparatus for measuring the thickness of a transparent material. A light beam from a laser diode has a modulated optical frequency. The emitted light shines on opposite surfaces of a wall of the material and is reflected from both of the surfaces. The two beams are used to form an interference. The difference of step .delta. of the interference signal is then determined. The thickness of the transparent material is determined from this difference of step. The apparatus may be used with bottles of glass or plastic material, whether clear or colored and on flat glass.