Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
2001-05-21
2002-12-10
Bruce, David V. (Department: 2882)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S484000, C356S512000
Reexamination Certificate
active
06493092
ABSTRACT:
This invention relates to a method for interferometric measurement, which method comprises emitting waves onto a reference surface and onto a measured object which each reflect a part of the emitted waves, and receiving both the reflected parts of the waves by one and the same receiver, in which a representation of the measured object is generated from the reflected parts of the waves in the form of an interferogram, from which the form of the measured object is determined.
In addition to the method this invention also relates to an apparatus for interferometric measurement which includes a light source for emitting light, a reference surface for reflecting a first part of said light and a receiver which is arranged to receive the first part of the light and a second part of the light, reflected by an object, a representation of the measured object being generated in said receiver in the form of an interferogram from which the form of the measured object is determinable.
Optical interferometry is often used for non-contact measurement of the form of surfaces. For this purpose light which has been separated into two parts by the use of a prism, a semitransparent mirror or some other device is employed. One of these parts of the light illuminates and is reflected by a plane reference surface. The other part of the light illuminates and is reflected by the surface, the form of which is to be measured. The two reflected parts of the light are brought together to illuminate photographic film or some form of light recording electronics. The image thus produced is called an interferogram. From the interferogram the form of the surface can be determined. One example of an apparatus for Producing interferograms is shown schematically in FIG.
1
.
In those cases where the light employed is monochromatic, i.e. all the light is of one and the same wavelength, the interferogram consists of light and dark regions. The lightest regions are generated where the two reflected parts of the light interfere constructively, i.e. where they are in phase. This is the case when the total distance (from the light source to the recording medium) is the same for the two parts of the light or when it differs by a whole number of wavelengths. The darkest regions are generated where the two reflected parts of the light interfere destructively, i.e. where they are in opposite phase. This is the case when the total distance (from the light source to the recording medium) for the two parts of the light differs by half a wavelength in addition to a whole number of wavelengths.
In those cases where the light employed is white, i.e. when the light is composed of light of different wavelengths, the interferogram consists of differently coloured regions. Light of one wavelength in the two reflected light components may interfere constructively in the same point where light of a different wavelength interferes destructively. In this way the spectral composition of the reflected light changes, i.e. the reflected light has a hue (dominant wavelength) as opposed to the case for the light source employed.
By analysing the changes in the interferogram, in intensity in monochromatic interferograms, in hue and possibly also intensity and saturation in interferograms recorded with white light sources, the position and form of the surface can be determined, although not always unambiguously.
When a monochromatic light source is employed, the surface can be translated in steps of half wavelengths (i.e. the total distance the light travels changes in steps of whole wavelengths) without changes in the interferogram. The determination of the position of the surface thus is ambiguous. Besides there is an ambiguity in the determination of the form of the surface—an indentation in the surface may result in the same interferogram as a protuberance in the surface.
By employing a light source which emits white light, these ambiguities can be eliminated. However, the interval within which the form and position of the surface can be determined becomes very small, in the order of microns. Outside of this narrow interval so many interferences occur, both constructive and destructive, that the interferogram cannot be interpreted due to low colour saturation. In order to determine the position and form of a surface unambiguously, it is necessary to calibrate the instrumentation with the aid of a surface in an accurately known position and having an accurately known form (e.g. a spherical ball with known diameter, fastened in a fixed jig).
There are a number of important technical applications for measuring position and form within a very narrow interval. With a monochromatic light source it is then possible to eliminate ambiguities in the determination of the form of the surface by recording several (at least three) interferograms of the same surface with different displacements of the two light parts (e.g. a semitransparent mirror can be moved, see FIG.
2
). This is called phase-stepping interferometry.
There are two kinds of phase-stepping interferometry. In the one kind, temporal phase-stepping, the interferograms are generated consecutively in time and it is thus assumed that the position and form of the surface is not changed during the time required to record all the necessary interferograms. In the other kind, usually called spatial phase-stepping, the different interferograms are generated simultaneously. This can be arranged in several different ways, e.g. by employing light with different polarisation angles. The instrumentation then is more extensive (e.g., three cameras must be used instead of one) but the form of the surface may change, which is necessary in many important applications.
In each of the references EP-A-506 296, EP-A-0 506 297 and the paper “Three-color laser-diode interferometer”, published in the journal Applied Optics, Vol. 30, No. 25, 1991, a method of extending the measurement range within which the structure of an object can be unambiguously determined is presented. This method is based on the employment of so-called synthetic wavelengths. Two monochromatic light rays with closely spaced wavelengths &lgr;
1
, &lgr;
2
, are generated by a laser diode and are directed onto the measured object. These light rays cooperate to generate a light ray with a so-called synthetic wavelength &Lgr;
12
−1
=&lgr;
1
−1
−&lgr;
2
−1
, which is much longer than the individual wavelengths &lgr;
1
, &lgr;
2
and therefore allows an unambiguous determination of larger structures of the measured object. In practice, however, two separate interferograms are detected, one for each of the wavelengths &lgr;
1
and &lgr;
2
, from which information about the structure of the measured object can be extracted. These two interferograms are detected through temporal phase-stepping, and thus this method exhibits the aforementioned disadvantages. The two wavelengths &lgr;
1
and &lgr;
2
also must be very closely spaced, typically within 0.5 nm, and therefore the differences between the two measured interferograms are small which results in poor measurement accuracy. In practice therefore, in order to obtain an enhanced analysis of the structure of the measured object, one or several additional synthetic wavelengths, generated by combinations of light rays of different wavelengths and shorter than the first synthetic wavelength &Lgr;
12
are employed. The choice of synthetic wavelengths is determined by the wavelengths which are available from laser diodes of multimode type.
Against this background the object of the present invention is to overcome the disadvantages associated with the known technology and particularly with both the known phase-stepping solutions, since these solutions are otherwise very advantageous.
In accordance with the invention, this object is achieved by a method as described in the introduction by the emitted waves comprising waves of three well-defined wavelengths &ngr;
1
, &ngr;
2
and &ngr;
3
, where &ngr;
1
≠&ngr;
2
≠&ngr;
3
, which wavelengths are chosen so that they substant
Bruce David V.
Burns Doane , Swecker, Mathis LLP
Jerker Delsing
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