Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
1999-07-02
2001-04-10
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S485000, C356S239100
Reexamination Certificate
active
06215556
ABSTRACT:
BACKGROUND OF THE INVENTION
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.
The general quality requirements demanded by customers and the savings which can be made by keeping to the bottom of the thickness tolerance range require very rigorous monitoring of thickness in the mass production of flat glass.
Of the techniques normally used for measuring thickness, the most precise methods which can be used in transparent media are optical methods. Among these, interferometric techniques, previously limited to laboratory measurements, have progressively found industrial applications.
For example, document FR-A-2 435 019 proposes a technique for measuring the thickness of a thin film which consists in exposing the thin film to infrared light spectroscopically split by rapid scanning over a range of wavelengths which is predetermined as a function of the nature of the film so as to create a spectrum of interference fringes between the reflected rays, the extreme points of which are determined. The technique is limited to thicknesses necessarily smaller than 30 &mgr;m. It consists in counting the interference fringes of rays reflected by the surfaces of the film. Such a method cannot be used for measuring the thickness of flat glass on a float glass production line whose thickness varies from less than 1 mm to 2 cm.
Another document, WO 95/22740, describes an interference method for determining the wall thickness of bottles during their manufacture.
The process is characterized in that a light beam with modulated optical frequency is emitted, in that two light beams or rays, reflected by each of the surfaces of a wall of a material, are received, in that interference is created between them and in that the path difference &dgr; of the interference signal is determined. A laser diode is used as the illumination source, and this is modulated by modulation of the optical frequency of the beam. Of the rays scattered by the two walls, two parallel rays are selected. The device of the invention makes it possible to take measurements 0.3 msec apart on each sensor. It is thus possible to explore every millimetre of the periphery of a bottle in rotation.
This technique, in which measurements are taken using isolated rays scattered by the surfaces, requires relatively powerful lasers (>30 mW), which may present drawbacks. It will be difficult to use the same method with parallel reflected beams because of the prismaticity of the support and, in particular, of float glass which is always prismatic in the edge zones.
Although providing good precision on the absolute thickness measurement, the method of WO 95/22740 is less well suited to following local thickness variations. This type of measurement is, however, very important for detecting the drifts in the nominal thickness of flat glass on its production line as early as possible. Furthermore, the method of WO 95/22740 does not make it possible to measure thicknesses smaller than 0.7 mm.
SUMMARY OF THE INVENTION
The object of the invention to which the present patent application relates is to develop the techniques above while improving their performance.
The invention proposes a process for measuring the thickness (e) of a transparent material with refractive index (n), in which a light beam with modulated optical frequency is focused, where two light beams or rays, reflected by each of the surfaces of the transparent material, are received, where interference between them is created, where the number of oscillations per modulation period of the interference signal is determined and where the path difference (&dgr;) between the two beams and the thickness (e) of the transparent material are deduced, and in which the phase shift (&Dgr;&phgr;) of the said interference signal is also determined.
This determination of the phase shift between the two signals recorded in succession can then be used to deduce other characteristics of the said material. It may in particular be applied to the precise measurement of local thickness variations, in particular of a strip of float glass. Similarly, it is proposed to apply it to measuring the thickness of a thin transparent material, preferably more than 0.2 mm.
The process of the invention is characterized in that the light beam with modulated optical frequency is emitted by a laser diode with distributed Bragg reflector (DBR).
Another characteristic is that the beams reflected by the surfaces of the transparent material are received after specular reflection and, lastly, another is that the focused light beam converges before reaching the surfaces of the transparent material so that it is divergent at the surfaces of the said transparent material which it reaches.
All these characteristics, taken in isolation or as a group, make it possible to obtain the local thickness variations by receiving the interference signal on a detector followed by:
digitizing the signal if necessary;
obtaining the ratio of the interference signal to the modulation of the intensity;
band-pass filtering the ratio;
determining the extrema of the resultant signal for the measurement at time k;
determining the time between the corresponding extrema of two successive measurements (k and k+1);
calculating the ratio of the preceding time to the corresponding period and multiplying it by 2 &pgr; to obtain the phase shift &Dgr;&phgr;;
calculating the thickness variation by the formula:
Δ
e
=
λ
0
·
Δ
ϕ
4
n
π
with:
&lgr;
0
=wavelength of a laser diode without modulation,
&Dgr;&phgr;=phase shift,
n=refractive index.
By virtue of this method of evaluating the thickness variations, the process of the invention makes it possible to monitor them with precision better than 1.10
−8
m. Such precision very advantageously makes it possible to measure, for example, dioptric defects of float glass. The dioptric defect is, as is known, mathematically connected with the second derivative of the thickness profile.
The invention also relates to the device intended to implement the process. It has, in particular, a light source with DBR laser diode, means for receiving an interference signal and a computer which successively:
digitizes the signals;
takes the ratio of the interference signal and the intensity modulation;
band-pass filters the digitized ratio;
determines the extrema;
determines the time between the corresponding extrema of two successive measurements (k and k+1);
takes the ratio of the preceding times to the corresponding periods and multiplies it by 2 &pgr; to obtain the phase shift;
calculates the thickness variation by the formula:
Δ
e
=
λ
0
·
Δ
ϕ
4
n
π
In one variant, the device has fibre-optic waveguides for transporting the light emitted by the laser diode and/or reflected by the surfaces of the transparent object. This technique makes it possible to work in hostile environments, such as in heat or in dust. It is thus possible to take measurements as soon as the sample to be monitored, for example float glass, leaves its processing device, in particular the float bath.
One variant also has avalanche photodiodes as the reception means.
The figures and the description which follow will make it possible to understand how the invention operates and to appreciate the advantages thereof.
REFERENCES:
patent: 3720421 (1973-03-01), Kasahara et al.
patent: 3736065 (1973-05-01), Cushing et al.
patent: 4254337 (1981-03-01), Yasujima et al.
patent: 4958930 (1990-09-01), Robertson, Jr.
patent: 5657124 (1997-08-01), Zhang et al.
patent: 6067161 (2000-05-01), Marcus et al.
patent: 2 435 019 (1979-08-01), None
patent: WO 95/22740 (1995-08-01)
Grente Pascal
Zhang Jingwei
Font Frank G.
Natividad Phil
Pennie & Edmonds LLP
Saint-Gobain Vitrage
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