Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1995-12-15
1997-08-12
Font, Frank G.
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
356349, 356357, G01B 902
Patent
active
056571240
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention:
This invention concerns a method and a device for measuring the thickness of transparent materials. More especially, but not exclusively, the invention is aimed at measuring the thickness of materials of glass and, more specifically, measuring the thickness of glass containers, such as bottles or flasks.
2. Discussion of the Background:
The general quality requirements demanded by clients and the reduction in weight of bottles require a very rigorous control of the thickness of these bottles.
In the glassmaking industry it is usual, for obtaining thickness measurements, to use the measurement of electrical capacitance as a base. In effect, since glass is a dielectric material, measurement of the capacitance of the glass by means of electrodes enables the thickness of the glass to be locally determined.
Such a technique is described, notably, in Patents EP 300 616 and EP 363 114. It consists of applying a pick-up, in the form of an elongate electrode, onto a bottle while the bottle revolves about its axis.
Such a technique has the disadvantage of requiring the pick-up to be held against the wall of the bottle. This inevitably leads to rapid wear of the pick-up, amplified by the rotation of the bottle. The pick-up may also be entirely destroyed if the bottle has a large defect which strikes the pick-up.
Another technique is described in the French Patent published under number 2 129 416. This technique consists of establishing a high-frequency electro-magnetic field and measuring the variation in intensity of this field caused by the introduction of a dielectric material. The voltage induced in the probe gives an indication of the variation in intensity of the field and is proportional to the thickness of the dielectric material.
This technique does not, however, lead to high accuracy in measurements of thicknesses. It is more especially used for determining thicknesses below a threshold value without, however, giving an exact measurement.
Furthermore, it also requires contact with the bottle, which can result in premature wear of the measuring head, as has been described for the preceding technique.
This premature wear or accidental destructions therefore require a frequent replacement of the detectors or measuring heads and lead to high costs of maintenance of the control system.
Moreover, each deterioration and/or replacement of a pick-up or measuring head may necessitate a rechecking of a series of articles which has been inadequately checked and an interruption to production during the pause in the control, which therefore results in reduced output.
SUMMARY OF THE INVENTION
A first objective of this invention is to find a method of measuring thickness of a transparent material, notably an article of glass, which shall give a high accuracy of the measurements.
A second objective is to avoid the risk of premature wear or destruction of the measuring tools, notably during friction of the articles being measured on these tools.
Another objective of the invention is to adapt the method of measurement to the measuring of thicknesses of articles in continuous movement, over the entire article.
The inventors have considered a transparent material to be assimilable to an interferometer; the light incident perpendicularly upon such a material is reflected by both faces of the material. The two reflected rays interfere with a difference of step, denoted .delta., which is expressed as follows: thickness of the transparent material.
Furthermore, the principles of optical interferometry, when the light source is monochromatic, lead to an interference signal received by the photo-detector which can be expressed as follows: ##EQU1## where I.sub.0 and .lambda. represent, respectively, the light intensity and the wavelength of the monochromatic source and K denotes the interference contrast.
By determining the difference of step, it is then easy to determine the thickness of the transparent material. However, a limit appears for the application of this method lin
REFERENCES:
patent: 5402230 (1995-03-01), Tian et al.
Grente Pascal
Zhang Jingwei
Font Frank G.
Kim Robert
Saint Gobain Cinematique et Controle
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