WO2008112021A2

WO2008112021A2 - System for illuminating and imaging an object with a specular reflecting surface

Info

Publication number: WO2008112021A2
Application number: PCT/US2007/081560
Authority: WO
Inventors: Alex Klooster; Douglas Davidson; Jon Upham
Original assignee: Coherix, Inc.
Priority date: 2006-10-16
Filing date: 2007-10-16
Publication date: 2008-09-18
Also published as: WO2008112021A3; WO2008112021A9

Abstract

A system for illuminating and imaging an object with a specular reflecting surface, including a reflector and an imaging-receiving device. The reflector provides substantially uniform diffusivity and substantially uniform reflectance of the light from an illumination source, and defines an aperture. The image-receiving device is located with a particular position behind the aperture of the reflector, and the aperture is sized with a particular dimension, such that the narcissus effect of the system is reduced to a single point and the object with the specular reflecting surface can be imaged with high accuracy.

Description

SYSTEM FOR ILLUMINATING AND IMAGING AN OBJECT WITH A SPECULAR REFLECTING SURFACE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of US Provisional Application number 60/829,671, filed 16 October 2006, which is incorporated in its entirety by this reference.

TECHNICAL FIELD

[0002] This invention relates generally to the optical imaging field, and more specifically to an improved system for illuminating and imaging of an object with a specular reflecting surface.

BACKGROUND

[0003] Specular reflection is the perfect, mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction. Diffuse hemispherical illumination is often required to obtain ideal machine vision imagery from an object with a specular reflecting surface. When a camera lens interrupts the hemispherical illuminator surface, specular objects that are tilted at an angle less than D/(2*L) with respect to the camera axis will not be properly imaged. As shown in FIGURE 1, the effective illumination area for a horizontal reflecting surface to be zero inside a diameter equal to (D-A)/2. Note that in FIGURE 1, the illuminator has a hole, which is an aperture stop that is bigger than the normal aperture stop shown. The effective illumination for an object surface 150 that reflects an image of the camera aperture is zero because of the "narcissus effect" which occurs when the image-receiving device 102 is receiving a virtual image of itself and sees a black hole.

[0004] Thus, there is a need in the optical imaging field to create an improved system for illuminating and imagine an object with a specular reflecting surface. This invention provides such an improved system.

BRIEF DESCRIPTION OF THE FIGURES

[0005] FIGURE 1 is a cross-section view of a system that produces a significant narcissus effect.

[0006] FIGURE 2 is a schematic drawing of the system of the preferred embodiment of the invention.

[0007] FIGURE 3 is a cross-section view of the system of the preferred embodiment of the invention that reduces the narcissus effect to a single point. [0008] FIGURE 4 is a cross-section view of the system of the preferred embodiment of the invention that shows the virtual light source.

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 FIGURE 2, the first preferred embodiment of the system

200 for illuminating and imaging an object with a specular reflecting surface 232 includes: a reflector 222 and an image-receiving device 202. The reflector 222 includes a diffusive and reflective surface 226 that provides substantially uniform diffusivity and substantially uniform reflectance of the light from an illumination source 228. The reflector 222 also defines an aperture 216. The image-receiving device 202 is located behind the aperture 216 of the reflector 222, such that the narcissus effect of the system 200 is reduced to a single point. [0011] The image-receiving device 202 function to capture an image or video of an object with a specularly reflective surface 232. The image-receiving device 202 is preferably a high resolution solid state coupled charge device (CCD) with a particular width and height, but may alternatively be an optical camera, a fiber optic system, or any other suitable image-receiving device 202.

[0012] The reflector 222 functions as a base for a diffusive and reflective surface 226 that reflects the light from an illumination source 228 onto a specularly reflective surface 232 and functions as an aperture 216 for the image-receiving device 202. The reflector 222 is preferably shaped to provide substantially uniform illumination, more preferably in the shape of a hemisphere, but alternatively may be any suitable shape.

[0013] The diffusive and reflective surface 226 functions to reflect and randomly diffuse light from an illumination source 228. For example, if the illumination source 228 is a coherent light source, the diffusive and reflective surface 226 functions to transform the light from the coherent light source into a light that is diffused incoherently in all directions. The diffusivity and reflectivity of the surface 226 are preferably both highly efficient. The surface 226 is preferably substantially uniform in the diffusivity, but may alternatively have any diffusivity. While the reflectivity of the surface 226 is preferably high such to improve efficiency, the surface 226 may allow a some portion of the light to pass through the surface 226, perhaps to illuminate other portions of the system, or to provide an ambient light source for another application. [0014] The aperture of a typical camera or image-receiving device functions to limit the angle of light (also called the cone of light) reaching the image plane of the image capture device 202. The aperture 216 of the preferred embodiment functions, however, to reduce the narcissus effect of the system 200 to a single point. This single point preferably corresponds to one pixel in the image receiving device 202, however, it may correspond to a group of pixels in the image receiving device 202, or correspond to an area on the object surface 150, 350, where the area is preferably smaller than the size of the aperture 216. In one variation, the aperture 216 preferably has a diameter that is significantly less than the width and height of the image-receiving device 202. In another variation, the aperture 216 acts as an aperture stop for the optical system and, more preferably, functions as the smallest aperture stop within the imaging system 200. The aperture 216 preferably has a circular shape and a particular size. A smaller aperture will reduce the distance A, and thus improve the effective illumination of the object 150,350, however, most image-receiving devices 202, such as cameras, are less efficient with a smaller aperture size. Preferably, the aperture is sized as small as possible to still allow the full resolution and sensitivity of the image-receiving device 202. [0015] In the preferred embodiment, the imaging system 200 also includes a lens subsystem 212. The lens subsystem 212 functions to focus the light passing through the aperture 216 onto the image plane of the image-receiving device 202. The lens subsystem 212 is preferably located outside the reflector shell, more preferably directly behind the defined aperture 216. Alternatively, a virtual lens system may be implemented with signal processing, such that the light received at the image plane of the image-receiving device 202 may be focused by calculations in a post processing of the image received by the image-receiving device 202, in lieu of or in addition to a physical lens system. However, any suitable lens subsystem 212 may be used. If the lens subsystem 212 is positioned outside the reflector 222, the aperture 216 in the reflector 222 may function as the effective aperture stop for the imaging system 200.

[0016] In the preferred embodiment, the imaging system 200 also includes an illumination source 228. The illumination source 228 functions to illuminate the object. The illumination source 228 preferably includes a ring of light sources aimed toward the reflector 222. The light sources are preferably high-intensity LEDs, but may alternatively include any suitable number of suitable light sources. [0017] As shown in FIGURES 3-4, the preferred embodiment of a system 300 for imaging a specularly reflective surface 332 includes an image-receiving device 302, a reflector 322 defining an aperture 316, and a lens subsystem 312. Except as noted below, the image-receiving device 302, the specularly reflective surface 332, the reflector 322 the diffusive surface 326, and aperture 316 of the second preferred embodiment are similar to the image-receiving device 202, the specularly reflective surface 232, the reflector 222, the diffusive surface 226, and aperture 216 of the first preferred embodiment.

[0018] As shown in FIGURE 3, with a small enough aperture 316, the effective object illumination 350 is only affected for an area the size of the aperture 316 (Dimension A), and as shown in FIGURES 3-4, with a small enough aperture, the only area where there is no effective object illumination 350 is at the point perpendicular to the center of the aperture 316. A mirror or other specularly reflecting surface 332 at the object plane will produce an out of focus image of the aperture 316 that has a blur size equal to the dimension of the aperture 316 and has finite luminance except for a single point at the center of the image (due to the "narcissus effect"). As shown in FIGURES 3-4, for a single point at the center of the object plane no light from the illumination source 328 is visible because the virtual light source for this point is the image-receiving device itself, which has zero luminance. For small aperture sizes, many lens subsystems 312 will produce sharp images of the object when the aperture stop is located in front of the first (or only) lens element of a lens subsystem 312. Lens subsystems 312 which have been optimized for an external aperture location can also be employed. Preferably, by incorporating the lens aperture stop 312 in the reflector 322, dead zones in the angular illumination field are significantly reduced and/or eliminated. [0019] 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.

Claims

CLAIMS We Claim:

1. A system for illuminating and imaging an object with a specular reflecting surface, comprising:

• a reflector that provides substantially uniform diffusivity and substantially uniform reflectance of the light from an illumination source; wherein the diffusive and reflective surface defines an aperture; and

• an image-receiving device located behind the aperture of the reflector; wherein the narcissus effect of the system is reduced to a single point.

2. The system of claim l, further comprising a lens subsystem to focus light onto the image-receiving device.

3. The system of claim 2, wherein the lens subsystem is located between the reflector and the image-receiving device.

4. The system of claim 2, wherein the lens subsystem is a processor, wherein the processor calculates a focus from the received image.

5. The system of claim 1, wherein the reflector has a concave hemispherical shape and a diffusive and reflective surface.

6. The system of claim 5, wherein the diffusive and reflective surface provides substantially uniform diffusivity.

7- The system of claim 6, wherein the diffusive and reflective surface provides substantially uniform reflectance of the light from the illumination source.

8. The system of claim i, wherein the aperture is sized to enable full resolution of the image-receiving device.

9. The imaging system of claim 1, wherein the aperture acts as an aperture stop for the system.

10. A system for illuminating and imaging an object with a specular reflecting surface, comprising:

• a reflector having a concave hemispherical shape and a diffusive and reflective surface that provides substantially uniform diffusivity and substantially uniform reflectance of the light from the illumination source; wherein the diffusive and reflective surface defines an aperture with an aperture diameter; and

• an image-receiving device located behind the aperture of the reflector; wherein the image-receiving device defines a image width and height; and wherein the aperture diameter is significantly less than the image width and height.

11. The system of claim 11, further comprising a lens subsystem located between the reflector and the image-receiving device and adapted to focus light onto the image-receiving device.