Method and device for non-contact three dimensional object surface imaging

Abstract

A slit m is projected onto an object surface in which reference point X.sub.1 is in a horizontal axis x closest to in focus point P. One image of a field of view area F is acquired after reflection of light comprising said reference point X.sub.1. Position Z.sub.1 of the object in a vertical axis z is determined. Images of respective field of view areas F are acquired after reflection of light having reference points X.sub.2, X.sub.3 . . . X.sub.n by simultaneously moving the object along axis z to maintain reference points X.sub.2, X.sub.3 . . . X.sub.n closest to in focus point P. Positions Z.sub.2, Z.sub.3 . . . Z.sub.n in which images were acquired are determined. The in focus point P along horizontal axis x is determined for each image. A correction differential Δ.sub.1, Δ.sub.2 . . . Δ.sub.n between in focus point P and reference points X.sub.1, X.sub.2 . . . X.sub.n is calculated.

Claims

1. A method for non-contact three dimensional object surface imaging, the method comprising: projecting light onto a target area of an object surface in which there is a reference point X.sub.1 in at least a first axis x of the object surface and an in focus point P.sub.1 which may be offset from the reference point X.sub.1, the projecting being such that the in focus point P.sub.1 is placed as close as possible to said reference point X.sub.1 by adjusting the position of the object along a second axis z so as to determine the position Z.sub.1 of the object in said second axis; acquiring a first image of a field of view area F after reflection of the projected light by the object surface comprising said reference point X.sub.1 the first image containing the position of the first reference point X.sub.1 in the first axis x; determining the position Z.sub.1 of the object in said second axis z and associating the position Z.sub.1 with the first image so that the first image contain the position of the position Z.sub.1 in the second axis z; moving the object along the first axis x and the second axis z; projecting light onto the object surface on successive target areas in which there are other reference points X.sub.2, X.sub.3 . . . X.sub.n of the object surface in said first axis x; acquiring a number of subsequent additional sequential images of the respective field of view areas F in the same way after reflection of the projected light by the object surface comprising said other reference points X.sub.2, X.sub.3 . . . X.sub.n by simultaneously varying the position of the object along the first axis x and the second axis z in order to maintain the other reference points X.sub.2, X.sub.3 . . . X.sub.n as close as possible to the respective in focus points P.sub.2, P.sub.3, . . . P.sub.n within said field of view area F; the number of subsequent additional sequential images containing the position of other reference points X.sub.2, X.sub.3, . . . X.sub.n in the first axis x and other positions of the object Z.sub.2, Z.sub.3, . . . Z.sub.n in the second axis z; and determining the positions Z.sub.2, Z.sub.3 . . . Z.sub.n of the object for the corresponding target areas in which there are the other reference points X.sub.2, X.sub.3 . . . X.sub.n; the number of subsequent additional sequential images containing the other positions of the object Z.sub.2, Z.sub.3, . . . Z.sub.n in the second axis z; wherein said operations of projecting light onto a target area of an object surface are carried out by projecting a pattern of light with a light distribution along at least said first axis x; and wherein the method further comprises: determining the position of the in focus points P.sub.2, P.sub.3, . . . P.sub.n along the first axis x for each of the acquired images; calculating a corresponding correction differential Δ.sub.1, Δ.sub.2 . . . Δ.sub.n as a distance along the first axis x between said in focus points P.sub.2, P.sub.3, . . . P.sub.n and the reference point X.sub.1 and the other reference points X.sub.2, X.sub.3 . . . X.sub.n; and obtaining a representation of the object surface, said representation involving at least parameters relating to the reference point X.sub.1 and the other reference points X.sub.2, X.sub.3 . . . X.sub.n in the first axis x with said correction differential Δ.sub.1, Δ.sub.2 . . . Δ.sub.n and the position Z.sub.1, Z.sub.2, . . . Z.sub.n of the object in the second axis z in the form of (X.sub.1+Δ.sub.1, Z.sub.1), (X.sub.2+Δ.sub.2, Z.sub.2) . . . (X.sub.n+Δ.sub.n, Z.sub.n) wherein each (X.sub.i+Δ.sub.i, Z.sub.i) corresponds to each of said acquired images.

2. The method of claim 1, wherein the pattern of light projected onto the object surface is a measuring slit m projected onto the focal plane of a microscope objective lens along the first axis x.

3. The method of claim 1, wherein said step of obtaining a representation of the object surface comprises building a curve (X.sub.1+Δ.sub.1, Z.sub.1), (X.sub.2+Δ.sub.2, Z.sub.2) . . . (X.sub.n+Δ.sub.n, Z.sub.n) involving parameters relating to the reference point X.sub.1 and the other reference points (X.sub.2, X.sub.3 . . . X.sub.n) in the first axis x with said correction differential Δ.sub.1, Δ.sub.2 . . . Δ.sub.n and the position Z.sub.1, Z.sub.2, . . . Z.sub.n of the object in the second axis z such that a curve giving the profile of the object surface being measured is determined.

4. The method of claim 3, wherein the step of obtaining a representation of the object surface comprises carrying out a raster scan consisting in projecting the measuring slit m at different positions Y.sub.1, Y.sub.2, . . . Y.sub.m of a third axis y in order to obtain a three dimensional map (X.sub.i1+Δ.sub.i1, Y.sub.i, Z.sub.i1), (X.sub.i2+Δ.sub.i2, Y.sub.i, Z.sub.i2) . . . (X.sub.in+Δ.sub.in, Y.sub.i, Z.sub.in), for i: 1, 2 . . . m, of the object surface being measured, said map involving parameters relating to the reference points X.sub.i1, X.sub.i2, . . . X.sub.in of said three dimensional map in the first axis x with the corresponding correction differentials Δ.sub.i1, Δ.sub.i2, . . . Δ.sub.in, at different positions Y.sub.1, Y.sub.2, . . . Y.sub.m of the third axis y, and corresponding positions Z.sub.i1, Z.sub.i2, . . . Z.sub.in of the object in the second axis z.

5. The method of claim 3, wherein the step of obtaining a representation of the object surface comprises carrying out an angular scan consisting in revolving the object around an axis passing through one point of the object such that the measurement slit m is projected at different angular positions θ.sub.1, θ.sub.2, . . . θ.sub.m of a fourth axis θ in order to obtain a three dimensional map (X.sub.i1+Δ.sub.i1, θ.sub.i, Z.sub.i1), (X.sub.i2+Δ.sub.i2, θ.sub.i, Z.sub.i2), . . . (X.sub.in+Δ.sub.in, θ.sub.i, Z.sub.in), for i: 1, 2, . . . m, of the object surface being measured, said map involving parameters relating to the reference points X.sub.i1, X.sub.i2, . . . X.sub.in of said three dimensional map in the first axis x with corresponding correction differentials Δ.sub.i1, Δ.sub.i2, . . . Δ.sub.in, at different angular positions θ.sub.1, θ.sub.2, . . . θ.sub.m of the fourth axis θ, and corresponding positions Z.sub.i1, Z.sub.i2, . . . Z.sub.in of the object in the second axis z.

6. The method of claim 2, wherein it further includes a centering step for positioning the object to be measured in a way that the measuring slit m is projected onto the apex position of the object, said centering step comprising: projecting at least two side slits onto the focal plane of the microscope objective lens, the side slits being substantially parallel to the measuring slit m and spaced apart therefrom a given distance along a third axis y such that the measuring slit m is between the side slits; and varying the position of the object along the third axis y until the in focus points P along two side slits at both sides of the measuring slit m and equally spaced at both sides from the measuring slit m are found to be in the same coordinate in the first axis x.

7. The method of claim 1, wherein the simultaneously varying the position of the object along the first axis x and the second axis z moving of the second axis z to maintain the in focus point P as close as possible to the other reference points X.sub.2, X.sub.3 . . . X.sub.n is carried out by using a depth-of-focus algorithm based on structured illumination.

8. The method of claim 1, further comprising the step of determining said reference point and the other reference points X.sub.1, X.sub.2, X.sub.3 . . . X.sub.n for each field of view area F that are located at the same predetermined position in said field of view areas F.

9. The method of claim 1, wherein the in focus points P are determined as the points in the field of view having the maximum of the axial response, said axial response representing how the signal of a depth-of-focus algorithm based on structured illumination changes along the first axis x.

10. The method of claim 1, wherein it further comprises the step of converting the correction differential Δ.sub.1, Δ.sub.2 . . . Δ.sub.n from pixel units into length units.

11. The method of claim 10, wherein said correction differential Δ.sub.1, Δ.sub.2 . . . Δ.sub.n is equal or less than 1 μm.

12. A device for non-contact three dimensional object surface imaging, wherein the device comprises: light projecting equipment for projecting light onto a target area of an object surface in which there are reference points X.sub.1, X.sub.2, X.sub.3 . . . X.sub.n in a first axis x of the object surface closest to the in focus point P, said light projecting equipment being suitable for projecting a pattern of light onto the object surface, the pattern of light comprising at least one measuring slit m to be projected onto the focal plane of a microscope objective lens with a light distribution along the first axis x; image-acquiring apparatus for acquiring one image of field of view areas F comprising said reference points X.sub.1, X.sub.2, X.sub.3 . . . X.sub.n after reflection of the projected light by the object surface; an arrangement for determining the positions Z.sub.1, Z.sub.2, Z.sub.3 . . . Z.sub.n of respective reference points X.sub.1, X.sub.2, X.sub.3 . . . X.sub.n in a second axis z; an arrangement for varying the position of the object along the first axis x; an arrangement for simultaneously varying the position of the object along the first axis x and the second axis z such that the in focus point P is maintained as close as possible to the reference points X.sub.1, X.sub.2, X.sub.3 . . . X.sub.n; finding a corresponding correction differential Δ.sub.1, Δ.sub.2 . . . Δ.sub.n as a distance along the first axis x between said in focus point P and the reference point X.sub.1 and the other reference points X.sub.2, X.sub.3 . . . X.sub.n; the varied position of the object being in the form of (X.sub.1+Δ.sub.1, Z.sub.1), (X.sub.2+Δ.sub.2, Z.sub.2) . . . (X.sub.n+Δ.sub.n, Z.sub.n) wherein each (X.sub.i+Δ.sub.i, Z.sub.i) corresponds to each image acquired by the image-acquiring apparatus, and a centering apparatus for positioning the object to be measured in a way that the measuring slit m is projected onto the apex position of the object, said centering apparatus comprising: an element for projecting at least two side slits onto the focal plane of the microscope objective lens along corresponding side axes x′, the side axes x′ being substantially parallel to the first axis x and spaced apart therefrom a given distance along a third axis y such that the measuring slit m is between the side slits; and an arrangement for varying the position of the object along the third axis y until the in focus points P along two side slits at both sides of the measuring slit m and equally spaced at both sides from the measuring slit m are found to be in the same coordinate in the first axis x.

13. The device of claim 12, wherein the light projecting equipment comprises at least one LED, a diaphragm, collimation optics, a mirror, an optical lens, a beam splitter and a microscope objective lens.

14. The device of claim 12, further including an arrangement for varying the position of the object along the third axis y for carrying out a raster scan in order to obtain a representation of the object surface.

15. The device of claim 12, further including an arrangement for varying the angular position of the object around a fourth axis θ passing through one point of the object for carrying out an angular scan in order to obtain a representation of the object surface.

16. The device of claim 12, wherein the arrangement for determining the positions Z.sub.1, Z.sub.2, Z.sub.3 . . . Z.sub.n of the object in the second axis z comprises a position sensor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Particular embodiments of the present methods and devices will be described in the following by way of non-limiting examples, with reference to the appended drawings, in which:

(2) FIG. 1 is a diagrammatic elevational view of one exemplary embodiment of a device for non-contact measuring the surface of an object,

(3) FIGS. 2a, and 2b are diagrammatic views of the device in FIG. 1 shown in different steps of a method hereof that are performed for non-contact measuring of the object surface; and

(4) FIG. 3 is a diagrammatic top plan view of an enlarged portion of FIG. 2a.

DETAILED DESCRIPTION

(5) The figures illustrate a one exemplary embodiment of a non-contact, high precision, fast measurement device. The non-limiting example of the device that is shown in the FIG. 1 of the drawings has been generally indicated at 100. It is an optical profiler capable of measuring any optical surface. Although it is useful in many applications, the present example relates to a measuring device 100 for non-contact measurement of the surface of a lens. The lens has been generally indicated herein at 300 in the figures.

(6) The measuring device 100 includes light projecting equipment 110 that include one or a series of LEDs 111. Such light projecting equipment 110 in the device 100 shown are suitable for projecting a pattern of structured light m onto a target area of the lens 300 through a diaphragm 112. As shown in FIG. 1 the light projecting equipment 110 further include collimation optics 113, a 45° mirror 114, an optical lens 115, a beam splitter 116 as well as a microscope objective lens 150.

(7) The target area of the lens 300 contains reference points X.sub.1, X.sub.2, X.sub.3 . . . X.sub.n, one of which, X.sub.1, is shown in the FIGS. 2a and 2b of the drawings. The reference points X.sub.1, X.sub.2, X.sub.3 . . . X.sub.n are in a horizontal axis x of the lens 300. The reference points X.sub.1, X.sub.2, X.sub.3 . . . X.sub.n are those being closest to the in focus point, shown at P in the drawings, that is, those within the depth of field of the optics 150 used to image the surface of the lens 300. The above mentioned pattern of structured light projected by the projecting equipment 110 in the device 100 shown has a light distribution along the horizontal axis x in the form of a measuring slit m, as shown in the FIGS. 2a, 2b and 3 of the drawings.

(8) The measuring slit m is therefore projected onto the focal plane of the microscope objective lenses 150 along the horizontal axis x. An objective lens 150 used was a Super Long Working Distance (SLWD) infinity corrected Nikon 100× objective lens with high numerical aperture (NA) of 0.7 and a working distance of 6.5 mm. The 100×SLWD objective lens has a depth of field of about 1 μm such that only parts of the object being measured within a 1 μm region around the focal plane of the objective lens 150 are substantially in focus.

(9) The measuring device 100 may further include an image-acquiring apparatus 120 which, in the particular example shown, includes a CCD camera 121 with a frame rate of the order of 50 fps. For each of the acquired images the in focus point P along the projected measuring slit m can be obtained by determining the point having the maximum value of the axial response of a depth-of-focus algorithm based on structured illumination. The camera 121 is capable of acquiring images of field of view areas F comprising reference points X.sub.1, X.sub.2, X.sub.3 . . . X.sub.n after reflection of the projected light by the surface of the lens 300.

(10) The image-acquiring apparatus 120 further includes the above microscope lens (which is shared both by the light projecting equipment 110 and the image-acquiring apparatus 120), a mirror 117 and a field lens 118. The beam splitter 116 couples the light projecting equipment 110 with the image-acquiring apparatus 120.

(11) The measuring device 100 further includes an arrangement 130 for varying the position of the lens 300 along the horizontal axis x, that is, for causing the lens 300 to be laterally displaced.

(12) The measuring device 100 further includes an arrangement 140 for displacing the measuring head 160 along the vertical axis z, that is, for causing the measuring head 160 to be vertically displaced along the vertical axis z such that the reference points X.sub.2, X.sub.3 . . . X.sub.n are maintained as close as possible to the in focus point P as shown in the FIGS. 2a, 2b and 3.

(13) The arrangement 130 varying the position of the lens 300 along horizontal axis x may include at least one high accuracy air bearing stage. An exemplar stage used was a 100 mm stroke drive unit model ABL-15010 available from Aerotech.

(14) As for the arrangement 140 for simultaneously varying the position of the lens 300 along the vertical axis z, in the embodiment shown they may include one or more crossed roller bearing stages.

(15) The provision of both arrangements 130, 140 allows the measuring head 160 to be moved vertically along axis z while simultaneously moving the lens 300 horizontally along axis x such that the reference points X.sub.1, X.sub.2, X.sub.3 . . . X.sub.n are maintained as close as possible to the in focus point P according to a closed loop tracking algorithm.

(16) The device 100 may also be provided with an arrangement 170 for determining the positions Z.sub.1, Z.sub.2, Z.sub.3 . . . Z.sub.n of the reference points X.sub.1, X.sub.2, X.sub.3 . . . X.sub.n in the vertical axis z. In addition, an arrangement for determining the positions of the lens 300 in the horizontal axis x may also be provided. All or some of these arrangements for determining the positions of the lens 300 may include high accuracy linear encoders.

(17) It is to be noted that points Z.sub.1, Z.sub.2, Z.sub.3 . . . Z.sub.n in vertical axis z are related to the vertical position of a point of the lens 300 that is being measured by the device 100. In this respect, the arrangement 140 for varying the position of the lens 300 along the vertical axis z cause the measuring head to be vertically displaced along axis z resulting in points Z.sub.1, Z.sub.2, Z.sub.3 . . . Z.sub.n (corresponding to points that are in focus by the above tracking algorithm) of the lens 300 to be varied along said axis z.

(18) The light projecting equipment 110 and the image-acquiring apparatus 120 may both be encased into a measuring head 160 that is provided in the measuring device 100. This measuring head 160 is supported by a granite column (not shown) for higher accuracy measurements.

(19) The disclosed device 100 can be operated for carrying out the following steps in order to perform a non-contact, high precision, fast measurement of lens 300: projecting a measuring slit m onto a target area of the surface of the lens 300 to be measured in which there is a reference point X.sub.1 in the horizontal axis x that is closest to the in focus point P, as shown in the FIG. 2a of the drawings; acquiring one image corresponding to a field of view area F after reflection of the projected light by the surface of the lens 300 comprising this reference point X.sub.1; determining the vertical position Z.sub.1 of the lens 300 in the vertical axis z; projecting the measuring slit m onto the surface of the lens 300 on successive target areas in which there are respective reference points X.sub.2, X.sub.3 . . . X.sub.n of the lens 300 in said horizontal axis x; acquiring a number of images of the respective field of view areas F after reflection of the projected light by the surface of the lens 300 comprising the reference points X.sub.2, X.sub.3 . . . X.sub.n by simultaneously varying the vertical position Z.sub.2, Z.sub.3 . . . Z.sub.n of the measuring head 160 along the vertical axis z while maintaining the reference points X.sub.2, X.sub.3 . . . X.sub.n as close as possible to the in focus point P, as shown in FIG. 2a or 2b; determining the vertical positions Z.sub.2, Z.sub.3 . . . Z.sub.n for each corresponding positions of the lens 300 in which the images of the field of view areas F comprising the reference points X.sub.2, X.sub.3 . . . X.sub.n have been acquired, determining the position of the in focus point P along the horizontal axis x for each of the acquired images; calculating corresponding correction differentials Δ.sub.1, Δ.sub.2 . . . Δ.sub.n, as shown in FIG. 3 of the drawings, as a distance along horizontal axis x between the in focus points P and the reference points X.sub.1, X.sub.2 . . . X.sub.n (correction differentials Δ.sub.1, Δ.sub.2 . . . Δ.sub.n are then converted from pixel units into length units and it is of the order of up to 1 μm), and obtaining a representation of the surface of the lens 300 through the reference points X.sub.1, X.sub.2, . . . X.sub.n in the horizontal axis x with the correction differential Δ.sub.1, Δ.sub.2, . . . Δ.sub.n and the vertical position Z.sub.1, Z.sub.2, . . . Z.sub.n of the lens 300 in the vertical axis z.

(20) The measuring device 100, when operated according to above mentioned method steps, is capable of providing a 2D profile of the surface of the lens 300. More particularly, the measuring device 100 is capable of providing an accurate profile of the surface of the lens 300 according to the following curve: [(X.sub.1+Δ.sub.1, Z.sub.1), (X.sub.2+Δ.sub.2, Z.sub.2) . . . (X.sub.n+Δ.sub.n, Z.sub.n)].

(21) In addition, when operated according to above mentioned method steps, the measuring device 100 may further be capable of providing a 3D map of the lens 300. More particularly, the measuring device 100 can be capable of providing: an accurate representation of the surface of the lens 300 according to a three dimensional map [(X.sub.i1+Δ.sub.i1, Y.sub.i, Z.sub.i1), (X.sub.i2+Δ.sub.i2, Y.sub.i, Z.sub.i2) . . . (X.sub.in+Δ.sub.in, Y.sub.i, Z.sub.in)], for i: 1, 2 . . . m, of the surface of the lens 300 by carrying out a raster scan in which the measuring slit is projected at different positions Y.sub.1, Y.sub.2, . . . Y.sub.m of a third axis y, perpendicular to the first axis x; and providing a representation of the surface of the lens 300 according to a three dimensional map [(X.sub.i1+Δ.sub.i1, θ.sub.i, Z.sub.i1), (X.sub.i2+Δ.sub.i2, θ.sub.i, Z.sub.i2) . . . (X.sub.in+Δ.sub.in, θ.sub.i, Z.sub.in)], for i: 1, 2, . . . m, of the surface of the lens 300 by carrying out an angular scan including revolving the lens 300 around an axis passing through a center of rotation thereof such that the measurement slit is projected at different angular positions θ.sub.1, θ.sub.2, . . . θ.sub.m of a fourth axis passing through the lens 300.

(22) A suitable curve analysis, such as best fitting and/or nominal shape comparison analysis, software can be used to process 2D profile and 3D map of the lens 300. This software may be part of the device 100 or it may be run from a remotely located computer system.

(23) Although only a number of particular embodiments and examples of the present devices and methods have been disclosed herein, it will be understood by those skilled in the art that other alternative embodiments and/or uses and obvious modifications and equivalents thereof are possible.

(24) The present disclosure covers all possible combinations of the particular embodiments described herein and should not be limited by particular embodiments, but should be determined only by a fair reading of the claims that follow.