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08:05 May-20-2001
Leonid Mihaljov
A new method for improving the accuracy of measurements made with a laser interferometer

ref.: http://www.ndt.net/article/v05n06/klein/klein.htm

Dear Sirs,

Let me inform you about our new method for improving the accuracy ot measurements made with a laser interferometer. I am looking forward to cooperation with laser interferometers manufacturers, users or metrology organisations on this project. Should you require further detail, I would of course be happy to supply this upon request.

With best wishes
Data EVM Oy

Leonid Mihaljov


A new method for improving the accuracy of measurements

made with a laser interferometer

Leonid Mihaljov, Data EVM Oy, Finland, email: leonid.mihaljov@pp.inet.fi

Bjorn Hemming, Heikki Lehto and Antti Lassila, Centre for Metrology and Accreditation, (MIKES), Finland, email: heikki.lehto@mikes.fi

The accuracy of the length measurements performed by laser interferometer depends on various factors, such as the refractive index of the air. The refractive index of air is a function of air pressure, air temperature, humidity, and concentrations of gases such as carbon dioxide. Since distance measurements made using laser interferometers are based on the wavelength of light, it is essential to know the real-time refractive index of the medium on the whole laser beam path to obtain sufficiently accurate results. We have developed an acoustic method for determination of the refractive index correction. Experimental measurement data presented in this paper are results of a co-operational project of Data EVM and Mikes.

There are methods for determination of the refractive index of air (n), such as interferometric refractometers or updated Edlén equations with good environmental sensors [2]. With these methods one can reach small uncertainties in good environmental conditions. However, in normal laboratory or workshop conditions these methods are not always suitable. The properties of air are very indeterminate due to local temperature variations of air along the measurement line. Fast changing properties of the air cannot be measured reliably with common laser interferometer environmental sensors. They allow measurement of the refractive index of air only at a single point adjacent to the laser beam path. Another disadvantage is that, the response times of air property sensors, are such that the measured results always lag behind the actual values. As result of above mentioned facts a non correct refractive index of air correction is used. In workshop conditions errors in order magnitude of 2 µm/m are normal.

Description of the acoustic method
The new method consists of determining the properties of air along the whole laser beam path by measuring

the speed of sound travelling the same path, and using the obtained value of the speed of sound to calculate the correction to the distance value measured by the laser interferometer. There are experimental formulas for calculation of the speed of sound in the air as function of temperature, humidity, pressure and CO2 concentration [1].

The device according to this method is constructed so that laser light and the sound waves, used for the speed of sound measurement, propagate simultaneously symmetrically along same axis and over same length (Fig.1).

Fig. 1

Advantages of the new method
The method allows real-time, real-place and sensitive correction to the laser measurements.

At short distances measurements can be made 8 times/s. The speed of travelling sound wave is affected immediately by the changes of physical properties of air without response time. The sound transducers are fixed directly to the laser-optics symmetrically to the beam. The relative effect of a change in the properties of air, is two thousand times stronger (pressure excluded) on the speed of sound in air than on the refractive index of air. So it is possible to achieve very precise distance measurements with the laser interferometer by correcting the original laser interferometer result, through the measured speed of sound. These are essential differences and overwhelming advantages of the this method compared to slower air thermometers and other instruments currently used in laser interferometers.

Experimental results
The following test measurements of the prototype designed and built by Data EVM Oy (pat. pending) were made at MIKES in October 2000. Measurements were performed simultaneously by laser interferometer and prototype over fixed length of approximately 2.3 m. Fig. 2 shows excellent short time correlation between optical length change measured by laser and acoustical length change measured by prototype. The long time change can be explained by the material temperature changes or changes in air pressure.

Fig. 2

The measurement was repeated (Fig.3) when conditions were disturbed by directing a heating fan of 2 kW towards measurement beam. In spite of heating and air flow correlation remained good.

Fig. 3

The new method has shown very promising results improving the accuracy of length measurements made by laser interferometer under normal and difficult environmental air conditions. Next step developing the system will be integration of this prototype to commercial laser interferometer and systematic studies of reliability and accuracy.

1. O. Cramer, The variations of the specific heat ration and the speed of sound in air with temperature, pressure, humidity and CO2 concentration, Journal of Acoustical Society of America 93 (5), May 1993, 2510-2516.

2. K. Birch and M. Downs, An Updated Edlén Equation for the Refractive Index of Air, Metrologia 1993, 30, 155-162. K. Birch and M. Downs, Correction to the Updated Edlén Equation for the Refractive Index of Air, Metrologia 1994, 31, 315-316.


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