SYSTEM AND METHOD OF IMAGING IN AN INTERFEROMETRIC IMAGING SYSTEM

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of US Provisional Application number 60/868,734, filed 06 DEC 2006, which is incorporated in its entirety by this reference.

TECHNICAL FIELD

[0002] This invention relates generally to the interferometric imaging field, and more specifically to a new and useful system and method of imaging in an interferometric imaging system in the interferometric imaging field.

BACKGROUND

[0003] Interferometers have long been used for metrologic measurements of surfaces, such as optical surfaces that are required to be flat or spherical. The interferometers are used in these situations to measure the degree to which the surface is flat or spherical. Interferometric imaging systems can produce, in essence, a topographical map of such surfaces, with "lines" of equal height every half wavelength of the light used to illuminate the surface. Such systems work very well to measure surfaces that deviate by only a few wavelengths of light over the field of view from flatness or spherocity. However, until the advent of multiwavelength interferometric (MWLI) imaging, surfaces with height variations of many wavelengths were difficult or impossible to image and measure. While the resolution in depth of interferometric images can be a very small part of the wavelength of light, the spatial resolution of images is limited to the number of pixels in the cameras used

to capture the interferometric images over the field of view of the camera. Commonly available high-resolution cameras with 4000 by 4000 pixels give too little resolution for many requirements. Also, the square format of such cameras leads to many wasted pixels when oblong objects are measured. Thus, there is a need in the interferometric imaging field to create a new and useful system and method of imaging in an interferometric imaging system. This invention provides such a new and useful system and method.

BRIEF DESCRIPTION OF THE FIGURES

[0004] FIGURE 1 is a schematic view of the interferometry system of the first preferred embodiment.

[0005] FIGURE 2 is an exploded view of the optical subsystem of the first preferred embodiment of the invention.

[0006] FIGURE 3 is an exploded view of the optical subsystem of a variation of the first preferred embodiment of the invention.

[0007] FIGURE 4 is an example of how two images might be used for a super- resolution calculation.

[0008] FIGURE 5 is a flowchart representation of a second preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The following description of the preferred embodiments of the invention is not intended to limit the invention to these preferred embodiments, but rather to enable any person skilled in the art to make and use this invention. [0010] As shown in FIGURES 1-3, a beam splitter 115 is normally used to combine light scattered from the surface of an object 95 in an interferometric

imaging system 100 with a reference beam and illuminate a detection unit 120 discards half the light from each beam. In the preferred embodiment of the invention, the discarded light is used to produce a second image, either to create a super-resolution image, or to create an image having a longer dimension than can be produced from a single detection unit 120. The preferred embodiment of an interferometry system 100 with multiple detection units 120 is shown in FIGURE 1. An exploded view of the optical subsystem of the interferometry system with the detection units arranged for creating super-resolution images are shown in FIGURE 2, and an exploded view of the optical subsystem of the interferometry system with the detection units arranged for creating stitched images, for example, images of oblong objects, is shown in FIGURE 3.

[0011] As shown in FIGURES 1-3, a detector subsystem 100 for an interferometric imaging system includes a parabolic mirror 105 with a focus point 106, a light source 110 located at an optically significant distance from the focus point 106 of the parabolic mirror 105 and oriented such that light is reflected off of the parabolic mirror 105 onto an object 95, a primary beam splitter 115 that receives light reflected from the object 95 onto the parabolic mirror 105 and produces a first beam output and a second beam output, a first detection unit 120 that receives the first beam output of the beam splitter 115, a second detection unit 120 the receives the second beam output of the beam splitter 115, and a processor 125 that processes images from the detection units 120.

[0012] The parabolic mirror 105 functions to reflect the light from the light source 110 onto the object. The parabolic mirror 105 is preferably oriented off of the optical axis 104 such that the light from the light source 110 is reflected onto the object 95, and the object 95 preferably does not intersect the optical axis 104 of the parabolic mirror 105. The parabolic mirror preferably reflects the specularly

reflecting beams (from the object surface) to a focus point 106, and non-specularly reflected beams (from the object surface) in a collimated beam to the beam splitter 115. Both the collimated and the unfocused light from both the specularly reflected beam and the non-specularly reflected beam enter the beam splitter 115. [0013] The light source 110 functions to emit a light beam and is preferably oriented to reflect the light beam off of a parabolic mirror 105 and onto an object 95. The light is preferably projected from a point 110 displaced slightly from the focus point 106 of the parabolic mirror 105. The light source 110 is preferably located a significant optical distance from the focus point of the parabolic mirror 105, more preferably a distance of half the diameter of the aperture 107 from the focus point. The light source 110 is preferably an optical fiber, but may also be a diode laser, an LED or any other suitable light source. As shown in FIGURE 1, Light scattered non- specularly from the surface of the object 95 is shown as solid lines diverging from a point on the surface, being collimated by the parabolic mirror, and passing as a collimated beam through an aperture 107 and the beam splitter 115 to the lens 121, where it is converged to a point on the image receiver to produce an image of the object surface. The light is projected as a diverging beam on to an off axis parabolic mirror 105. The dashed lines show the path of the illuminating light falling on an object with a surface perpendicular to the optic axis of the parabolic mirror. Since the light is launched from 110 the diverging beam is collimated into a parallel beam falls at a slight angle to the perpendicular on to the surface of the object. The specularly reflected light, as would be obtained if the object were a flat mirror, is shown as a dashed line that converges to a point 106 on the opposite side of the focus point from 110.

[0014] The beam splitter 115 functions to combine the specularly reflected beam and the non-specularly reflected beam reflected from the parabolic mirror 105.

A typical beamsplitter is used to combine or to produce beams of equal intensity and thus transmits half of the beam, and reflects half of the beam. The beam splitter 115 preferably combines the light source 111 with the specularly reflected light to create a reference beam. The beam splitter 115, which may also be referred to as a "beam combiner", also preferably functions to divide the beams approximately equally, and the light normally thrown away from such a beamsplitter 115 is used effectively to produce at least one additional image as is shown later. Note that the beam splitter preferably performs a dual function of combining both the object and reference beams, and splitting the beams into multiple beams so that additional images may be captured from the one object beam entering the optical subsystem, and the extra light beam is used to capture and/or enhance other images, rather than just being discarded. The reference beam generated by the light source 111 is often independently controlled since the reference beam produced by specularly reflected light reflected from the object surface may be heavily attenuated, and the modulation produced when strong and weak beams interfere is much less than when the beams are of equal intensity. As shown in FIGURES 1-3, light from a reference beam source 111 is shown diverging at the same angle as the light diverging from the focal point 106 of the parabolic mirror 105, and the light from the reference beam light source 111 is combined in the beam splitter 115 with light diverging from the focal point 106 of the parabolic mirror 105. In one variation a second beam splitter may be used to split one of the beams from the primary beam splitter 115. The second beam splitter along with two additional detection units 120 may also divide the beam from the primary beam splitter 115 to capture multiple images. The second beam splitter preferably functions only as a beam splitter and preferably does not combine any beams. In another variation, both a second and third beam splitter may be used (along with additional detection units 120) to each split an object beam from the

primary beam splitter 115 and capture additional images, and in this case, the second and third beam splitters preferably function only as beam splitters, and preferably not as beam combiners.

[0015] The detection unit 120 functions to focus the non-specularly reflected beam to an image, and to preferably collimate the specularly reflected beam and enable the specularly reflected beam to function as a reference beam, and to detect an image from the focused non-specularly reflected beam. As shown in FIGURES 1- 4, the dashed lines diverge from and pass through a beamsplitter 115 to a lens 121 in a detection unit 120 that collimates the specularly reflected light on to a sensor 120. In the preferred embodiment of the invention, the detection unit includes a lens 121, and an image sensor 122. The lens functions to focus the non-specularly reflected light from the object surface onto the image sensor 122. Preferably the lens 121 also functions to collimate the specularly reflected light beam into a collimated light beam for use as a reference beam. In the preferred embodiment, the lens 121 also functions to collimate light from an additional light source 111 for use as a reference beam. The image sensor 122 functions to capture an interferometric image of the object surface from an object beam and the reference beam. The image sensor 122 is preferably a charge coupled device, but may alternatively be any suitable image sensor. In one variation the detection unit 120 may be moved or rotated by a mechanical system to adjust the region of the object surface that is visible to the sensor 122. In another variation, as shown in FIGURE 2, the field of view of each detection unit 120 may be of the same region of the object surface. The detection units 120 may be arranged so that the images cover different portions of different pixels of each camera, as is shown in FIGURE 4 by solid lines outlining the pixels of the first image receiver and the dashed lines the pixels of the second. In yet another variation, as shown in

FIGURE 3, each detection unit 120 has a field of view of different but overlapping parts of the surface of the object.

[0016] The processor 125 functions to process the images produced by the detection units 120. The image processing may include super-resolution of multiple images of the same region of the object surface, stitching together multiple images of different regions of the object surface, or some combination of the two. As an example, a detailed feature such as the line sketched in FIGURE 4 representing the edge of a hole, can be imaged with greater accuracy if the images are combined and the resultant image used for the measurements. Accurate combining of more than one interferometric image is well documented in U. S. Patent Application 11/181,664 filed 14 JULY 2005 by inventors Jon Nisper, Mike Mater, Alex Klooster, Zhenhua Huang entitled "A method of combining holograms" which is incorporated in its entirety by this reference. In one variation, the images from each detection unit 120 may be combined and stitched together to form an image that may contain many more pixels than are available from a single detection unit, and more preferably where the object being imaged is an oblong object such as a cylinder head or the cylinder head mating surface of a motor block. In the variation mentioned above where more than two beam splitters may be used to further split the beams from the primary beamsplitter 115 and capture multiple images, the optical processing may be used to create super-resolution images from more than two images, stitch together images, or any combination of super-resolution and image stitching, such as stitching together two sets of two images, and then using the stitched images to create a super-resolution image.

[0017] The preferred embodiment of the invention preferably also includes a plate with an aperture 107. The aperture 107 preferably functions to limit the cone of light that may enter the optical subsystem.

[0018] As shown in FIGURE 5, a method 600 of imaging in an interferometric imaging system includes illuminating a surface of an object in the interferometric imaging system with an object beam S610, combining light from the object beam reflected from the object surface with light from a reference beam in an optical beamsplitter, wherein light reflected from the surface and light from the reference beam are both partially reflected and partially transmitted by the optical beamsplitter S620, imaging the partially reflected object beam and the partially transmitted reference beam on to a first image detector S 630, imaging the partially transmitted object beam and the partially reflected reference beam on to a second image detector S640, and processing a first detected image and a second detected image of the surface of the object S650.

[0019] Step S610, which recites illuminating a surface of an object in the interferometric imaging system with an object beam, functions to. The illumination is preferably performed by an optical fiber that emits light reflected off of a parabolic mirror at a position that is off axis with the optical axis relative to the object. [0020] Step S620, which recites combining light from the object beam reflected from the object surface with light from a reference beam in an optical beamsplitter, functions to combine the light reflected from the surface of the object (the object beam), and the reference beam. In addition to combining the beam in this step, the beam splitter preferably provides the additional functionality of creating a second beam from the discarded light when the beams are combined. Preferably the reference beam is from an external source, but the reference beam may be generated by collimated specularly reflected light, or any other suitable reference beam source. Preferably in this step, the light reflected from the surface and light from the reference beam are both partially reflected and partially transmitted by the optical

beamsplitter to create two identical sets including one object beam and one reference beam.

[0021] Step S630, which recites imaging the partially reflected object beam and the partially transmitted reference beam on to a first image detector functions to capture an interferometric image of the object surface from both the object beam and the reference beam.

[0022] Step S640, which recites imaging the partially transmitted object beam and the partially reflected reference beam on to a second image detector functions to capture a second interferometric image of the object surface from both the object beam and the reference beam. This second image may be of the same object surface region, or a partially overlapping object surface region, or a completely different object surface region.

[0023] Step S650, which recites processing a first detected image and a second detected image of the surface of the object functions to produce an additional image which may be a super-resolution image, an image with a larger field of view than the first image and the second image, a stitched image, or any combination of super- resolution and image stitching. The image stitching is preferably performed by matching height and slope measurements of the surface, more preferably in a region where the images overlap.

[0024] In one further variation of the method 600, a second beam splitter may be used to split one of the beams from the primary beam splitter. The second beam splitter along with two additional detection units may also divide the beam from the primary beam splitter to capture multiple images. In another further variation, both a second and third beam splitter may be used (along with additional detection units) to each split an object beam from the primary beam splitter and capture additional images. The additional non-primary beam splitter(s) preferably function only as

beam splitters, and preferably not as beam combiners. For both variations the optical processing may be used to create super-resolution images, stitch together images, or any combination of super-resolution and image stitching, such as stitching together two sets of two images, and then using the stitched images to create a super- resolution image.

[0025] As a person skilled in the art will recognize from the previous detailed description and from the figures and claims, modifications and changes can be made to the preferred embodiments of the invention without departing from the scope of this invention defined in the following claims.