US 6,639,685 B1 · Image processing method using phase-shifted fringe patterns and curve fitting

Abstract

A contour measurement system determines three dimensional surface information of an object using a projection moiré technique to obtain a single fringe pattern that is analyzed using a phase-shift image processing method. A grating is projected onto the object and the reflected fringe pattern is recorded by a camera and stored digitally as an array of pixels. An estimated fringe spacing is then determined for each pixel. Using the estimated fringe spacings, four phase-shifted fringe patterns are generated by correlation integration and then used in a phase calculation to obtain wrapped phase information for the object. This phase information is then unwrapped and normalized, producing a set of data points representative of the unwrapped phase information. Noise suppression using pattern reconstruction and smoothing can be utilized at this point to improve the signal to noise ratio. Thereafter, updated estimated spacings are determined for each data point using a curve fitting function such as a localized least squares fit. The system then performs one or more additional iterations of the correlation integration, phase calculation, unwrapping, noise suppression, and fringe spacing update steps, each time using the newest updated estimated spacing in the correlation integration. After the final iteration, the unwrapped phase information is demodulated and used to calculate the three dimensional coordinates of points on the object surface. A fractional pixel analysis technique is also disclosed for use in the correlation integration and updated estimated spacing calculations.

Claims42

Dependent claims

An image processing method for extracting information from a fringe pattern produced by an optical measurement of an object, comprising the steps of:

(a) recording a single image of the fringe pattern;
(b) determining one or more estimated fringe spacings using fringe lines contained within the image;
(c) generating a plurality of phase-shifted image data sets using the image and the estimated fringe spacing(s);
(d) generating wrapped phase information using the phase-shifted image data sets;
(e) unwrapping the phase information and generating a set of data points representative of the unwrapped phase information;
(f) performing at least one iteration of the following steps (f1) and (f2):
(f1) determining an updated estimated fringe spacing for each of a number of the data points using a curve fitting function;
and
(f2) repeating steps (c), (d) and (e) using the updated estimated fringe spacings;
and
(g) generating output data representing information contained in the fringe pattern using the unwrapped phase information determined during the final iteration of step (e).

8 more elements

An image processing method for extracting information from a fringe pattern produced by an optical measurement of an object, comprising the steps of:

(a) recording an image of the fringe pattern;
(b) determining one or more estimated fringe spacings using fringe lines contained within the image;
(c) generating a plurality of phase-shifted image data sets using the image and the estimated fringe spacing(s);
(d) generating wrapped phase information using the phase-shifted image data sets;
(e) unwrapping the phase information and generating a set of data points representative of the unwrapped phase information;
(f) performing at least one iteration of the following steps (f1) and (f2):
(f1) determining an updated estimated fringe spacing for each of a number of the data points using a curve fitting function, wherein the curve fitting function comprises a curve fitting function that, for each selected one of the data points, is applied to a localized group of data points, wherein the localized group of data points comprise a plurality of the data points that, along with the selected data point, together correspond to a localized portion of the image;
and
(f2) repeating steps (c), (d) and (e) using the updated estimated fringe spacings;
and
(g) generating output data representing information contained in the fringe pattern using the unwrapped phase information determined during the final iteration of step (e).

8 more elements

An image processing method for extracting information from a fringe pattern produced by an optical measurement of an object, comprising the steps of:

(a) recording an image of the fringe pattern as a set of pixel data points, each of which has an associated intensity value;
(b) determining one or more estimated fringe spacings using fringe lines contained within the image;
(c) generating a plurality of phase-shifted image data sets using the image and the estimated fringe spacing(s), and calculating a number ν of image data sets Im1, Im2, . . . Imv, having equally-spaced phase shifts, with the image data sets being generated by a correlation calculation using the equation:
Imi(x)=Im(x){circle around (×)}hi(x), i=1,2, . . . , v,
where:
Im(x) represents the intensity values of the pixel data points of the image in a direction x that is generally perpendicular to the direction of the fringe lines, and
hi(x) represents a phase window that is a function of the fringe spacing;
(d) generating wrapped phase information using the phase-shifted image data sets;
(e) unwrapping the phase information and generating a set of data points representative of the unwrapped phase information;
(f) performing at least one iteration of the following steps (f1) and (f2):
(f1) determining an updated estimated fringe spacing for each of a number of the data points using a curve fitting function;
and
(f2) repeating steps (c), (d) and (e) using the updated estimated fringe spacings;
and
(g) generating output data representing information contained in the fringe pattern using the unwrapped phase information determined during the final iteration of step (e).

12 more elements

A method for optical measurement of the surface contour of an object, comprising the steps of:

(a) projecting a fringe pattern of light onto an object along a projection axis;
(b) sensing an image of the light reflected from the object at an angle relative to the projection axis;
(c) determining one or more estimated fringe spacings using fringe lines contained within the image;
(d) generating a plurality of phase-shifted image data sets using the image and the estimated fringe spacing(s);
(e) generating wrapped phase information using the phase-shifted image data sets;
(f) unwrapping the phase information and generating a set of data points representative of the unwrapped phase information;
(g) performing at least one iteration of the following steps (g1) and (g2):
(g1) determining an updated estimated fringe spacing for each of a number of the data points using a curve fitting function, wherein the curve fitting function comprises a curve fitting function that, for each selected one of the data points, is applied to a localized group of data points, wherein the localized group of data points comprise a plurality of the data points that, along with the selected data point, together correspond to a localized portion of the image;
and
(g2) repeating steps (d), (e) and (f) using the updated estimated fringe spacings;
and
(h) generating height information for the object using the unwrapped phase information determined during the final iteration of step (f).

9 more elements

Dependencies are inferred from the claim text.

Image processing method using phase-shifted fringe patterns and curve fitting

Life of this patent

Image processing method using phase-shifted fringe patterns and curve fitting

Life of this patent

Abstract

Claims42