Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2002-10-22
2004-08-24
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S241300
Reexamination Certificate
active
06781699
ABSTRACT:
TECHNICAL FIELD
Interferometric measuring systems with optical probes as can be arranged for the practice of our invention provide for measuring localized surface features, geometric surface forms, and overall dimensions. The invention is particularly applicable to the measurement of cylindrical, conical, and flat surfaces whose roughness approaches tolerances for geometric form as well as to the measurement of test pieces having multiple surfaces requiring individual or comparative measurements.
BACKGROUND
Tolerances for many precision manufactured components continue to go beyond the capabilities of conventional contact measuring techniques. Optical measuring techniques, particularly those using interferometric mechanisms, provide for measuring with much greater precision. However, the roughness of the surfaces under test often exceeds one-half of the wavelengths used in conventional interferometers (i.e., wavelengths in the visible or near-infrared range). Surface features larger than one-half the measuring wavelength cannot be unambiguously measured with conventional interferometers. Longer wavelengths can be used, but lasers for producing such longer wavelengths are less common and more expensive than those available for producing wavelengths in the visible or near-infrared range.
Manufactured components that include multiple surfaces can require measurements of their individual surface forms (e.g., roundness and straightness) as well as measurements of relationships between their surfaces (e.g., runout and perpendicularity). Measuring each of the surfaces individually with setups or recalibrations between the different measurements is time consuming and can make comparisons difficult.
SUMMARY OF THE INVENTION
Our interferometer in one or more of its preferred embodiments provides for measuring multiple surfaces with a compound optical probe. Sub-test beams emitted from the probe separately measure the multiple surfaces. A confocal optical system distinguishes the measurements between the surfaces. Each of the sub-test beams can be composed of two fundamental wavelengths of light from different interferometers. Combined, the two interferometers greatly increase the dynamic range of measurement for measuring rough surfaces with conventional lasers.
An exemplary interferometer for measuring multiple surfaces of a test piece in accordance with our invention includes a test arm and a reference arm that convey test and reference beams along different but ultimately interconnected paths. A beamsplitter within the test arm separates the test beam into first and second sub-test beams. A focusing optic of the confocal optical system within the test arm focuses the first and second sub-test beams to different points of focus. A compound probe also within the test arm conveys the first and second sub-test beams to the different points of focus.
Preferably, each of the sub-test beams is intended for measuring a different surface of the test piece at normal incidence. As such, the principal axes of the sub-test beams are oriented normal to their incident test surfaces, which can be oriented in different directions. Directional optics within the probe direct the sub-test beams to their points of focus at their intended orientation. Additional sub-test beams can be split from the test beam within the test arm for measuring more than two surfaces of the test piece, each being directed to a point of focus at normal incidence to a different test surface.
The test surfaces are preferably measured individually in succession. An actuator relatively moves the probe with respect to the test piece between two or more measuring positions. In a preferred embodiment, the actuator is movable between two positions for measuring two surfaces of a test piece. At a first of the positions, the point of focus of the first sub-test beam is positioned on the first surface of the test piece and the point of focus of the second sub-test beam is positioned off both the first and second surfaces of the test piece. At a second of the positions, the point of focus of the second sub-test beam is positioned on the second surface of the test piece and the point of focus of the first sub-test beam is positioned off both the first and second surfaces of the test piece. Similarly, at a third or higher measuring position, the additional points of focus are positioned in turn on other of the test piece surfaces while the remaining points of focus are positioned off of all the test surfaces.
A detection system detects an interference signal between the reference beam and the first sub-test beam when the probe is located at the first position and detects an interference signal between the reference beam and the second or higher sub-test beam when the probe is located at the second or higher position. The detection system is preferably arranged in conjunction with a confocal optical system that excludes from detection light that is not focused on one of the test surfaces. An imaging optic of the confocal optical system can be used to refocus the sub-test beams conjugate to their points of focus of the focusing optic. A limited aperture size near the focus of the imaging optic limits a depth of focus through which light is effectively collected by a detector at the end of the confocal optical system. If any of the test surfaces are located out of focus (e.g., by as few as 10 to 100 microns), little of the reflected light reaches the detector. The aperture size can be limited by locating a stop near the conjugate focal point or by locating a detector of limited dimension near the same point of focus.
For measuring rough surfaces or surfaces with significant discontinuities, such as surfaces with an average roughness approaching one-half of wavelengths in the near-infrared range, our invention provides laser sources that produce two beams having different fundamental wavelengths of light. Beamsplitters divide each of the different wavelength beams into test and reference beams. Another beamsplitter combines the two different wavelength test beams into a common test beam composed of the two different wavelengths. It is the common test beam that is divided into the multiple sub-test beams, resulting in each of the sub-test beams being composed of the two wavelengths.
Each of the different wavelength reference beams preferably propagates along respective reference delay lines of the reference arm for controlling the optical path lengths traversed by the two reference beams. Preferably, the two reference delay lines have adjustable optical path lengths to equate optical path lengths between the test and reference arms of the interferometer. The optical path lengths of the test and reference arms can also be equated by incorporating similar path-length adjustments within the test arms.
The detection system preferably includes first and second arrays of detectors for separately detecting interference between each of the two pairs of test and reference beams. The detectors within each of the first and second arrays are preferably relatively phase shifted for simultaneously detecting a plurality of phase-shifted measurements within each of the first and second pairs of test and reference beams. The simultaneous phase-shifted measurements allow for discerning more accurate phase differences between the test and reference beams at each fundamental wavelength.
Although accurate, the two individual wavelength measurements produce ambiguous results for surface discontinuities greater than one-half the fundamental wavelengths. Our invention, however, provides a controller that combines information from the first and second arrays of detectors to produce aggregate interference measurements having a sensitivity equated to an effective wavelength significantly longer than either of the two different fundamental wavelengths. The aggregate measurements are useful for measuring surfaces with a roughness exceeding one-half the two fundamental wavelengths.
The actuator is preferably a part of a relative motion system between the probe and the test
Dunn Thomas J.
Kulawiec Andrew W.
Tronolone Mark J.
Connolly Patrick J
Corning-Tropel
Ryan Thomas
Turner Samuel A.
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