Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2003-06-23
2004-05-25
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06741361
ABSTRACT:
TECHNICAL FIELD
The processing of data gathered by frequency-scanning interferometers involves converting rates of interferometric variation accompanying variations in beam frequency into such length measures as surface topography or distance.
BACKGROUND
Frequency-scanning interferometers, also referred to as wavelength-scanning interferometers or multi-wavelength interferometers, derive from measures of interference taken at a succession of different beam frequencies (or wavelengths) path length differences between interfering object and reference beams. In contrast to conventional interferometers that compare path length differences between points within the same interference patterns and use additional interference patterns to resolve ambiguities of the intra-pattern comparisons, frequency-scanning interferometers resolve points within interference patterns individually, based upon interferometric (e.g., intensity or phase) fluctuations of corresponding points within different interference patterns produced at different beam frequencies.
As such, a wider range of surface roughness and distances can be unambiguously measured by frequency-scanning interferometers. Conventional interferometers are typically limited to measuring step sizes in the direction of illumination within the fringe spacing of their interference patterns, which correspond to the wavelength of the illumination. The measurement of such step sizes by frequency-scanning interferometers is independent of the nominal wavelength of illumination, depending instead on the average interval between the beam frequencies. The finer the interval, the larger the range of unambiguous measurement. Thus, frequency-scanning interferometers can provide measures of rough or diffuse surfaces at beam frequencies that produce speckle-ridden interference patterns unintelligible to conventional interferometers.
Frequency-scanning interferometers are especially useful for measuring surface profiles of test objects as measures of surface variations taken normal to a reference plane or surface. Recent developments of frequency-scanning interferometry include the use of components such as tunable diode lasers and CCD detector arrays. As a result, compact, accurate, and fast systems have been developed, which have the capability of performing measurements for both imaging and non-imaging applications.
A known type of frequency-scanning interferometer system
10
is depicted in FIG.
1
. While in the overall form of a Twyman-Green interferometer, a tunable laser
12
under the control of a computer
14
produces a measuring beam
16
that can be tuned through a range of different frequencies. Beam conditioning optics
18
expand and collimate the measuring beam
16
. A folding mirror
20
directs the measuring beam
16
to a beamsplitter
22
that divides the measuring beam
16
into a object beam
24
and a reference beam
26
. The object beam
24
retroreflects from a test object
30
, and the reference beam
26
retroreflects from a reference mirror
32
. The beamsplitter
22
recombines the object beam
24
and the reference beam
26
, and imaging optics
34
(such as a lens or group of lenses) focus overlapping images of the test object
30
and the reference mirror
32
onto a detector array
36
(such as a CCD array of elements). The detector array
36
records the interferometric values of an interference pattern produced by path length variations between the object and reference beams.
24
and
26
. Outputs from the detector array
36
are stored and processed in the computer
14
.
The elements of the detector array
36
record local interferometric values subject to the interference between the object and reference beams
24
and
26
. Each of the interferometric values is traceable to a spot on the test object
30
. However, instead of comparing interferometric values between the array elements to determine phase differences between the object and reference beams
24
and
26
throughout an interference pattern as a primary measure of surface variation, a set of additional interference patterns is recorded for a series of different beam frequencies (or wavelengths) of the measuring beam
16
. The tunable laser
12
is stepped through a succession of incrementally varying beam frequencies, and the detector array
36
records the corresponding interference patterns. Data frames recording individual interference patterns numbering
16
or
32
frames are typical.
The local interferometric values vary in a sinusoidal manner with changes in beam frequency, cycling between conditions of constructive and destructive interference. The rate of interferometric variation, e.g., the frequency of intensity variation, is a function of the path length differences between the local portions of the object and reference beams
24
and
26
. Gradual changes in intensity (lower interference frequency variation) occur at small path length differences, and more rapid changes in intensity (higher interference frequency variation) occur at large path length differences.
Discrete Fourier transforms can be used within the computer
14
to identify the interference frequencies of interferometric (e.g., intensity) variation accompanying the incremental changes in the beam frequency of the measuring beam
16
. The computer
14
also converts the interference frequencies of interferometric variation into measures of local path length differences between the object and reference beams
24
and
26
, which can be used to construct a three-dimensional image of the test object
30
as measures of profile variations from a surface of the reference mirror
32
. Since the reference mirror
32
is planar, the determined optical path differences are equivalent to deviations of the object
30
from a plane. The resulting three-dimensional topographical information can be further processed to measure important characteristics of the object
30
(e.g. flatness or parallelism), which are useful for quality control of precision manufactured parts.
Considerable computational time is required for computing the Fourier transforms for each of a number of points sampled from the interference patterns. For example, intensity detector arrays having a matrix of one thousand by one thousand detector elements require up to one million Fourier transforms to be performed. The computing time for each Fourier transform increases with both the number of different interference patterns recorded and the number of Fourier frequency samples tested. The range of detectable interference frequencies is dependent upon the number of recorded interference patterns, and the accuracy with which the interference frequencies can be identified depends upon the number of Fourier frequency samples used. Accordingly, computing time, which is affected by multiple dimensions, can slow measurement procedures, rendering the measurement procedures too slow for “real time” or “inline” inspections.
SUMMARY OF INVENTION
Significant reductions in computational time are made for processing interferometric data produced by frequency-scanning interferometers. Improvements are made to both simplify and streamline processing. Faster measurements and measurements with higher accuracy are possible.
One object of the invention is to provide an improved frequency-scanning interferometry system for distance or range measurement, including such systems that produce 3-D images of the surface profile of a test object, wherein computations of distance or range values are carried out with speed and accuracy. A more general object of the invention is to provide an improved system for deriving distance or range measurements from interferometric data.
The invention can be practiced as a multi-stage process for interpreting interferometric fluctuations of frequency-scanning interferometers. A succession of N interference patterns are produced between object and reference beams at N different beam frequencies within a range of beam frequencies. Interferometric data is recorded for a corresponding area appearing in each of the
Harter Secrest & Emery LLP
LightGage, Inc.
Ryan Thomas B.
Shaw Esq. Brian B.
Turner Samuel A.
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