Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1989-02-28
1990-11-13
Willis, Davis L.
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
356363, G01B 902
Patent
active
049697447
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an optical device for measuring an angle between a beam of light and a measuring device.
The most common method of measuring angles is probably with the aid of a circular scale whose graduations are observed in one way or another. Automatic systems which operate on the basis of this principle are also to be found. Another principle is one which is based on reflecting light from a rotatable mirror onto a scale and observing the scale with the aid of a telescope and the mirror, this method being invented by Poggendorff in 1829. A third method is the auto-collimation method.
The present invention, however, relates to an interferometric method which approaches the principle of the so-called interference for equal inclinations named after Haidinger (1795-1871) and applied by Fabry and Perot in the interferometer named after these people and used for measuring wavelengths accurately. Although this method has certain complications, of which more will be said in the following, the theory which lies behind these known interferometers can be used to explain the present invention.
The theory of this interferometer has been relatively well developed over the past 80 years. An etalon comprising two partially reflecting and precisely parallel surfaces is irradiated on one side with the light to be studied and an optical system located on the other side enables the intensity of transmitted light to be studied in mutually different directions, this system in the simplest of cases comprising a positive lens and a screen positioned at the focal point of the lens. A number of interference rings will then appear on the screen, where the innermost ring corresponds to a wavelength which depending on the contribution afforded by the size of the ring, essentially coincides with the number of half wavelengths of the light making up the thickness of the etalon. The theory of the Fabry-Perot etalon is described, for instance, by Born & Wolf in "Principles of optics", 6th ed., 1980, chapters 7.5-7.6. and by Tolansky in, inter alia, "An introduction to interferometry".
In the case of such interferometric methods as these it is the innermost interference rings that are of interest. The invention instead preferably utilizes higher order interferences, which can be studied when the etalon is turned away from its conventional position in which the incident light is normal to the plane of the etalon. In contradistinction to the conventional Fabry-Perot interferences, the fringes which can then be observed are essentially vertical lines and with regard to terminology can be more precisely termed as Haidinger fringes or "fringes for equal inclination".
Interferometric measuring processes for measuring lengths or distances are known generally to the art. Reference can be made in this respect to, for instance, the Michelson interferometer and similar constructions which, due to the advent of the laser with its great coherence length, can be also used in practice to measure relatively long distances. Two Fabry-Perot etalons which are moved relative to one another in mutually parallel relationship have also been used to this end: cf. U.S. Pat. No. 4,558,950. The present invention, however relates to the use of a corresponding etalon as an angle detector, wherewith the Haidinger fringes which are obtained with rotation form a kind of natural "scale lines" moving past a detector. These "scale lines" define a monotonous but non-linear scale for defining the rotation of a plane-parallel plate in relation to a light cone projected from a laser device.
In the case of a standard Fabry-Perot interferometer, the space between the two partially reflecting surfaces is air, implying that each of the surfaces is carried by a respective slightly wedge-shaped glass plate. However, since the absolute value of the wavelength is of no interest in the case of the present invention, there is preferably used a plane-parallel glass plate, such plates being commercially available and used as interference filters. In certain cases very
REFERENCES:
patent: 4558950 (1985-12-01), Ulrich et al.
Koren Matthew W.
Polymetric AB
Willis Davis L.
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