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| Working Group 5.44 |
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Interferometry on Prismatic Bodies
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When the length of gauge block shaped samples is measured under air by interferometry, the air refractive
index must be determined accurately.
There are two alternative strategies: 1) determination of the refractive
index using an empirical equation involving the environmental parameters for pressure, temperature, humidity,
CO2 content, which have to be measured precisely, 2) interferometrical determination of the air
refractive index by comparing the optical path length through the air with a vacuum path as indicated in Fig. 1,
left. For this purpose a vacuum cell can be inserted into the Precision Interferometer.
Fig. 1, right shows a typical interferogram. The cell is positioned nearby the sample so that the
measured air refractive index represents the appropriate values effecting the wavelength under air and thus
the determined sample length.
This is an advantage compared to the determination of air refractivity using an empirical equation which requires
much better homogeneity of air parameters within the interferometer chamber.
Fig. 1
The determination of the interference fractions is similar as in measurements of the length of gauge blocks,
i.e. the interference phase is averaged within regions of interest (circles, see Fig. 1, right), and
subsequent subtraction of the averages obtained in vacuum and in air. In this framework it is essential to
apply a zero correction taking account for inhomogeneity of the quartz plates. In the Precision Interferometer
this correction can be measured directly by performing the refractometer measurement under vacuum conditions.
This facility dramatically reduces the uncertainty of the refractometer measurement.
Besides the necessity to determine the refractive index of air when the length of samples is to be determined
interferometrically, the refractometer can be used in the framework of fundamental investigations regarding the
mentioned correlation between the air refractive index and environmental parameters. As an example, one version
of existing empirical equation bases on measurements using the Precision Interferometer and allows the evaluation
of the refractive index of air with very small uncertainty.
In opposite to the above mentioned vacuum cell refractometer the "length refractometer" bases
on the observation of the interference of a sample under vacuum and, subsequently, under gas.
Fig. 2 shows the beam geometry which is the same as for "normal" length measurements
by optical interferometry. (see: Precision Interferometer).
Fig. 2
The presents of the test gas causes a length change of the sample by compression. This, and other corrections,
have to be taken into account when the refractive index of the gas is evaluated. Thus, this method is appropriate
only at relatively small pressures (up to 5 kPa).
When water vapor is used as test gas, the adsorption onto surfaces (blue skin at Fig. 2) compensates in
this geometry. This is an advantage compared to refractometer cells (Fig. 1) where this effect is additive.
- Schödel, R.; Walkov, A.; Abou-Zeid, A.:
"High accuracy determination of water vapor refractivity by length interferometry"
Optics Letters, 31, 1979 - 1981 (2006)
- Bönsch, G.; Potulski, E.:
"Measurement of the refractive index of air and comparison with modified Edlén's formulae",
Metrologia 35, 133 - 139 (1998)
© Physikalisch-Technische Bundesanstalt
Page created: 26-Sep-2006, last update: 03-Apr-2008 11:49 AM,
K. Eggert |
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