EP0777850A4

EP0777850A4 - Integral field lens illumination for video inspection

Info

Publication number: EP0777850A4
Application number: EP95921336A
Authority: EP
Inventors: Terry L Graves; Howard Fein
Original assignee: Pressco Technology Inc
Current assignee: Pressco Technology Inc
Priority date: 1994-08-26
Filing date: 1995-05-22
Publication date: 1999-02-03
Also published as: EP0777850A1; JPH10505155A; WO1996007076A1; AU2644095A

Abstract

A system for illumination of discrete specimens in an optical inspection system (A) include an image acquisition system (C) and corresponding illumination system (B). A beam splitter (16) intercepts light from a light source (10) and communicates a portion of it to field lens (20). A field lens (20), in turn, communicates light to a specimen (24) disposed in a viewing area (22). Light reflected from the specimen (24) is communicated back through the field lenses (20), through the beam splitter (16) and ultimately to a video camera (34) from which a digitized image is obtained. In another embodiment, light reflected from the specimen (24) and propagated through the beam splitter (16) has communicated to a second beam splitter (30). Separated light therefrom is communicated to first and second video cameras (34, 36), thus allowing for imaging of separate portions of the specimen (24).

Description

INTEGRAL FIELD LENS ILLUMINATION FOR VIDEO INSPECTION

Background of the Invention

This application pertains to the art of video inspection, and more particularly to specialized illumination and image capture therefor.

The invention is particularly applicable to inspection of discrete specimens, particularly those having specific areas of particular interest. The application will be described with particular reference thereto, although it will appreciated if the invention has broader application as in any video inspection environment for which acquisition of detailed images is desirable.

High-speed, automated inspection of mass-produced articles is rapidly becoming an essential part of industrial production. As increasing reliance is placed on automated video inspection, increasing capability and resolution is also desired.

A first generation of improvement to the video inspection system was associated with improvements and basic inspection algorithms. A subsequent generation of improvement was directed toward improving the actual image captured. Such systems employed such components as a solid-state LED array, optionally coupled with a diffuser, to obtain more homogeneous lighting. Of course, the more uniform an illumination field, the more accurate a resultant, captured image would be.

Further improved inspections came with a recognization of a basic problem. That is, in most instances a light source must be typically located on the same axis as a camera lens. Thus, a camera lens may form its own artifact on the resultant image when such image is wholly or partially reflective in nature. This problem may be addressed by implementation of a very narrow lensed camera, such as a pinhole camera. However, while this may be an improvement, it does not totally eliminate the lens itself as an artifact.

Another concern with video inspection systems is competing objectives of analysis. More particularly, an overall analysis of an image is often desirable. However, detailed analysis of a sub-portion of the specimen is also advantageous. This latter advantage is particularly necessary for specimens having a high stress area or the like. Such competing goals often require multiple inspection stations. At a first station, an over all specimen image may be captured and analyzed. Subsequent stations may allow for focusing attention on a particular, specialized area. Such systems require duplication of illumination systems and image acquisition systems. They also require more space on a fabrication line to accommodate multiple inspection stations. Finally, the results of an analysis of both stations must be harmonized and synchronized.

The subject system addresses the above concerns, and others, and provides an efficient mechanism for completing detailed video inspections with minimum artifacts, while allowing concurrent overall inspection and detailed inspection of a single specimen using a single station. In accordance with the subject invention, there is provided a video inspection illumination system which employs a beam splitter. Light from a light source is directed to this beam splitter. A portion of the light from the light source is then directed to a discrete part or specimen in a viewing area. The light source is advantageously provided with a diffuser to render its resultant light more homogeneous. Light reflected from the specimen is communicated back through the beam splitter to a camera arrangement for image acquisition. In accordance with a more limited aspect of the present invention, a second beam splitter is provided to capture light reflected from the specimen after passing through the first beam splitter. This beam splitter, in turn, communicates a first light portion to a first camera and a second light portion to a second camera. One of these cameras provides sufficient magnification to allow for selected, detailed analysis of a subportion of the entire specimen.

In accordance with the yet more limited aspect of the present invention, a comparison is made with resultant, captured images to allow for selective rejection of a specimens not achieving preselected criteria.

An advantage of the present invention is the provision of an inspection system which allows for capture and analysis of highly detailed images. Yet another advantage of the present invention is the provision of an inspection system allows for capturing and analysis of multiple images of a single specimen in a single workstation.

Another advantage of the present invention is the provision of a detailed inspection system which requires fewer parts and provides enhanced specimen throughput.

Further advantages will become apparent to one of ordinary skill in the art upon reading and understanding the subject, detailed description.

Brief Description of the Drawings

The invention may take form in certain parts, and arrangements of parts, as will be provided in the specification and illustrated in drawings which form a part hereof and wherein: FIGURE 1 illustrates the overall inspection illumination and image capture system of the subject invention;

FIGURE 2 illustrates, in detail, the illumination subportion and ray diagram of the system of FIGURE 1; and

FIGURE 3 illustrates a programmable light source as suitably implemented in connection with the systems of

FIGURES 1 and 2.

Detailed Description of the Preferred Embodiment Turning now to the drawings wherein the figures are for the purpose of illustrating the preferred embodiment only, and not for the purpose of limiting the same, FIGURE 1 illustrates an automated video inspection system A which includes an illumination subsystem B and an image acquisition subsystem C The disclosed system facilitates enhanced defect detection and optical inspection which is particularly suited for discrete parts, as is used in the container industry. As will be appreciated from the descriptions which follow, the system provides for convergent illumination from a light source which may be either programmable or non-programmable. The system also provides divergent illumination from either of these light sources.

A suitable light source is formed by a solid- state illumination source, such as a pulsed array of light emitting diodes. However any source of sufficient illumination levels may be implemented. Strobed light, which includes the noted LED array, inert gas strobes, and the like, maintaining an advantage of "freezing*¹ moving specimens so that an image may be obtained. Continuous light sources require the addition of a commonly-available electronic shutter mechanism to "freeze" a captured image. A multi-spectrum light source is also advantageously provided in certain applications for the reasons noted below.

The system allows for optically magnifying an image portion while utilizing normal camera optics to acquire discrete images. Resultant information on defects generated is made available for subsequent analysis and comparison.

In the structure of FIGURE 1, a light source 10 is used to generate light generally in a direction dl. In the preferred embodiment, the light source 10 is advantageously programmable so as to selectively generate light, as will be described in detail below. Light from the light source 10 is passed through a diffuser 12 to increase homogeneity thereof.

The diffuser 12 is advantageously comprised of a translucent or transparent material, such as glass or plastic. The surface is optically "rough" in such a way that incident light will transmit through the medium and be modified so as to be uniformly distributed over some area as diffused, transmissively scattered light. The particular properties of the diffusion are highly application specific. Selection may be made by choice of translucence, transparency, and roughness.

Light from the light source 10 which has passed through the diffuser 12 propagates along direction d, to a beam splitter 16. The beam splitter 16 functions as a light splitter insofar as a portion of the light will pass through, generally maintaining the direction d,. other of the light will be reflected at an angle of reflection ₂ generally equal to the angle incidence ø, between the incident light direction d, and the generally planar beam splitter 16. The portion of the incident light reflected from the beam splitter 16 travels generally along direction

The beam splitter 16 suitably is comprised of optically surfaced piano-glass plates with metalized coatings. Such beam splitters are commercially available. The property of the beam splitter 16 is generally to both transmit and reflect incident light with equal intensity, but with unequal splitting. For example, 60%/40% splitting is suitably used. Such a splitter may be found with Product No. G72.502 from Edmund Scientific Corp. of Barrington, New Jersey. As with the diffuser, it must be appreciated that the particulars of a particular light splitter are highly application specific.

Light which has been reflected from the beam splitter 16 is propagated in direction d₃ as noted above. This light is then communicated to a field lens 20. In the preferred embodiment, the field lens is comprised of a plano-convex singlet geometry. With this architecture, light which is incident to the field lens 20 is propagated to an illumination or viewing area 22 in a convergent fashion. Thus, convergent illumination is provided to an object to be imaged, illustrated by specimen 24.

It will be noted that, in the preferred embodiment, a series of similar specimens like that provided at 24 are each propagated to the viewing area 22 via a conveyor means 26, such as a moving belt or the like.

The convergent illumination on an imaged specimen is utilized, as shown above. With this illumination, the object or specimen is disposed in space at a distance less than a prime focal distance from a vertex or piano surface of the field lens 20. Thus, illumination is incident as a distributed, shaped, convergent source. When a diffuse array illumination source is implemented, it appears as an infinitely distant continuum. A homogeneous nature of the diffuse source is maintained in the specimen illumination as a result of a uniform illuminating field. It must be appreciated that in the above-described arrangement, a specimen is to be imaged is located inside a prime focal distance of the field lens 20. This results in a desirable structuring of specimen illumination.

The specimen illumination may also be directed through any other modifying optical elements. Such modifying elements are suitably beam splitters, mirrors, lenses, or the like. Such optical elements may be disposed either prior to or following the field lens, as defined from the light source 10, to the specimen 24. In an alternative embodiment, a specimen may also be located outside of a prime focal distance. This would result in a continuously divergent illumination field with similar characteristics as described above for the continuously convergent illumination field. As with any inspection system, the objective is the generation and capture of a specimen image for purposes of analysis. This is accomplished by arranging a system geometry such that an imaging device, that is, camera and optics, essentially looks through the field lens at a specimen.

Returning again to FIGURE 1, light which is incident upon the specimen 24 after having been passed through field lens 20 is reflected therefrom as illustrated generally along direction d₃. This light passes again through the field lens 20. Once the light traveling along direction d₃ passes a second time through field lens 20, it commences converging. Relative orientation of the specimen and imaging device to the field lens is a controllable factor. The mutual relationship relative to field lens dictates an image magnification factor which is directly dependent on distances of the imaging device and the specimen from the field lens.

There are certain imaging limitations inherent with singlet field lenses. These may result in image degradation. The subject system addresses these concerns by ensuring that all imaging through the field lens 20 is completed in a paraxial fashion. With this, a specimen area which is to be imaged is provided relatively close to an optical axis O of the field lens 20. With this, an area to be imaged is provided relatively close to an optical axis of the field lens, at which point aberration and distortion are minimal. This relative orientation allows for high quality images to be made through the singlet field lens 20 while rendering insignificant damaging effects due to its inherent geometric and chromatic aberrations.

Once reflected light is passed through the field lens 20, it is propagated again to the beam splitter 16. Again, a portion of the light will be reflected and a portion transmitted according to the optical characteristics of the beam splitter 16 as noted above. The transmitted portion is indicated generally along direction d₄.

In the preferred embodiment, light propagating generally along direction d₄ is communicated to a second beam splitter 30 which functions as an image splitter. A portion of light is reflected along direction d₅, while a portion continues along direction d₄. Again, the relative apportionment is dictated by choosing the properties of a the image splitter 30. In additional to the foregoing, application to certain specimens may be served by implementation of multiple spectrum light. See, for example, U.S. Patent Application Serial No. 07/990,009, entitled VIDEO INSPECTION SYSTEM EMPLOYING MULTIPLE SPECTRUM LED ILLUMINATION, commonly assigned to the subject application, the contents of which are incorporated herein by reference. In such an embodiment, the splitter 30 may also be formed of a color separator.

The portion of light directly passed through image splitter 30 is communicated to a main image camera 34. In the preferred embodiment, the main image 34 functions to capture an image of an entire specimen. A reflected portion traveling generally along direction d₅ is communicated to a second camera 36 which functions as a magnified camera in the preferred embodiment. The magnified camera 36 is used to explore a special area of interest ("AOI") on a discrete specimen. When the system is used in inspection of specimens, such as riveted or tabbed can lids, the magnified camera 36 may be advantageously used to enhance a main rivet picture. With this architecture, the field lens provides for pre- enlargement of an image prior to using integral camera optics disposed within magnified camera 36 to enlarge a generated image. With such an arrangement, spurious reflection of lens surfaces may be negated and more incident light is provide to the part by implementation of the specialized diffuser 12, as noted above. The resultant affect is a minimization to negligible of these effects by employing broad-band anti-reflective coatings (46 on the plano-convex lens) on lenses 38 and 40 of cameras 34 and 36, respectively.

Each of the cameras 34 and 36 are suitably comprised of solid-state, charge-coupled devices ("CCDs") . Digitized images captured therefrom are communicated to any suitable image analyzing unit 42 which are commercially available and within the understanding of one of ordinary skill in the art. In the preferred embodiment, the main image camera 34 has a typical field of view ("FOV") in the range of 3 to 5 inches. Again, such is highly application specific and is provided merely as an example of a preferred embodiment. The magnified camera 36 is provided with a focus to a specific region of interest on the specimen. In the preferred embodiment, its field of view is suitably 0.25".

Captured images, one analyzed, facilitate selective rejection of unacceptable specimens as dictated by preselected criteria by selective enable ent of a rejection mechanism, such as illustrated by an error blow- off unit 44. When activated, a unit such as that 44 will remove a defective specimen from the conveyor means 26.

Turning now to FIGURE 2, a ray trace diagram of the illumination subsystem B of FIGURE 1 is provided. The numbering provided in connection with claim 1 has been maintained herein. In addition, the anti-reflective coating noted above will be noted to be provided generally at 46. A light controller 48 is utilized to accomplished controllable lighting as detailed below.

Turning now to FIGURE 3, functionality of the programmable light source for light source 10 as implemented in the preferred embodiment will be disclosed. As illustrated, the light source 10 is comprised of a programmable light source 50. Often times specimens are circular in nature, such as is found by the container industry. Thus, a circular arrangement for the light source 10 is provided by the illustration. A suitable light source for the preferred embodiment is comprised of a plane of solid-state light devices, such as light emitting diodes ("LEDs") . Such planar rays of LEDs have been well described in the prior art and will not be repeated herein. Arranging LEDs in a series of concentric, controllable areas allows for selective control of illumination. As evidenced by FIGURE 3, the LEDs are arranged in separate controllable zones, zones 1-4. Each of these zones is selectively controllable. Thus, intensity, enablement, or duration of light elements of the particular zone may be provided. This allows for selective control of specimen illumination to eliminate at artifacts, such as hot spots, which is highly contingent on a specific part to be inspected.

As an alternate embodiment, a programmable light source 10 may also be comprised of an inert gas or xenon strobe which may also be programmable by employing a series of co-axial, individually activated tube portions.

This invention has been described with reference to a preferred embodiment. It is intended that any variations be included insofar as they come within the scope of the following claims or the equivalents thereof.

Claims

Having thus described the invention, it is now claimed:

1. A video inspection illumination system comprising: a beam splitter; light source adapted to direct light to the beam splitter; a field lens adapted to receive a portion of light from the light source via the beam splitter and communicate it to a viewing area and to receive light after reflection from an associated specimen disposed in the viewing area; and means for securing the light source relative to the viewing area such that substantially all light passes from the light source to the viewing area through the beam splitter and the field lens.

2. The video inspection illumination system of claim 1 wherein the beam splitter is comprised of a substantially planar, partially reflective mirror.

3. The video inspection illumination system of claim 2 further comprising: a secondary beam splitter adapted to receive light from the beam splitter after reflection from an associated specimen disposed in the viewing area.

4. The video inspection illumination system of claim 2 further comprising at least one camera for acquiring light from the beam splitter after reflection from an associated specimen disposed in the viewing area.

5. The video inspection illumination system of claim 3 further comprising a camera adapted for acquiring a portion of light from the secondary beam splitter after reflection from an associated specimen disposed in the viewing area.

6. The video inspection illumination system of claim 5 further comprising a secondary camera adapted for acquiring a second portion of light from the secondary beam splitter after reflection from an associated specimen disposed in the viewing area, the second portion of light being distinct from the portion of light.

7. The video inspection illumination system of claim 6 wherein the secondary beam splitter is comprised of a second, substantially planar, partially reflective mirror. 8. The video inspection illumination system of claim 7 further comprising: a processor; means for communicating digitized picture information from each camera to the processor; the processor including means for determining acceptability of an associated specimen from the digitized picture information; and means for selectively rejecting an associated specimen in accordance with acceptability of an associated specimen as determined by the processor.

9. A video inspection method comprising the steps of: selectively generating light from a programmable light source; communicating generated light through a diffuser to form uniform light; communicating diffused light to a beam splitter; communicating a portion of the uniform light from the beam splitter to a field lens so as to form focussed light; communicating at least a portion of the focussed light to an inspection area; communicating at least a portion of the focussed light, after reflection from a specimen disposed in the inspection area, back through the field lens as reflected light; communicating the reflected light from the field lens to at least one video camera; and digitizing the reflected light communicated to the video camera to form a digital image thereof.

10. The method of claim 9 wherein the step of communicating uniform light to a beam splitter includes the step of communicating uniform light to the beam splitter comprised of a substantially planar, partially reflective mirror.

11. The method of claim 10 further comprising the step of communicating the reflected light to a second beam splitter prior to communicating the reflected light to the video camera such that a first sub-portion of the reflected light is communicated to the video camera.

12. The method of claim 11 further comprising the step of communicating a second sub-portion of the reflected light from the second beam splitter to a second video camera. 13. The method of claim 12 wherein the step of communicating the reflected light to the second beam splitter includes the step of communicating the reflected light to the second beam splitter comprised of a substantially planar, partially reflective mirror.

14. The method of claim 13 further comprising the steps of: communicating digital images of the specimen formed by each video camera after exposure thereof to reflected light communicated thereto to a digital processor; and determining, via the digital processor, acceptability of the specimen in accordance with the digital images communicated thereto.

15. A video inspection illumination system comprising: a beam splitter; a controllable light source adapted to controllably direct light to the beam splitter; a diffuser adapted to receive light from the light source so as to generate uniform light therefrom; a field lens adapted to receive a portion of uniform light from the beam splitter and communicate it to a viewing area and to receive light after reflection from an associated specimen disposed in the viewing area; and means for securing the light source relative to the viewing area such that substantially all passes from the light source to the viewing area through the diffuser, the beam splitter and the field lens.

16. The video inspection illumination system of claim 15 wherein the field lens is comprised of a plano¬ convex singlet oriented relative to viewing area such that light communicated therethrough to the viewing area as a convergent beam.

17. The video inspection illumination system of claim 16 further comprising means adapted to orient the associated specimen relative to the viewing area at a distance less than a prime focal distance from a vertex or piano surface of the field lens, whereby illumination of each associated specimen is provided from a distributed, shaped and convergent beam.

18. A video inspection system comprising: a beam splitter; a controllable light source adapted to controllably direct light to the beam splitter; a diffuser adapted to receive light from the light source so as to generate uniform light therefrom; a field lens adapted to receive a portion of uniform light from the beam splitter and communicate it to a viewing area; means for securing the light source relative to the viewing area such that substantially all passes from the light source to the viewing area through the diffuser, the beam splitter and the field lens; at least one camera for acquiring light after reflection from an associated specimen disposed in the viewing area.

19. The video inspection system of claim 18 further comprising means for securing the field lens relative to the camera and the viewing area such that light reflected from the associated specimen is again passed through the field lens prior to passing to the camera.

20. The video inspection system of claim 19 further comprising means for securing the beam splitter relative to the camera and the field lens such that light reflected from the associated specimen is again passed through the beam splitter prior to passing to the camera. 21. The video inspection system of claim 20 further comprising: a secondary beam splitter adapted to receive light from the beam splitter after reflection from an associated specimen disposed in the viewing area, prior to communicating a portion of the received light to the camera.

22. The video inspection system of claim 21 further comprising a secondary camera adapted for acquiring a second portion of light from the secondary beam splitter after reflection from an associated specimen disposed in the viewing area, the second portion of light being distinct from the portion of light.

23. The video inspection system of claim 22 wherein the field lens is comprised of a plano-convex singlet oriented relative to viewing area such that light communicated therethrough to the viewing area as a convergent beam.

24. The video inspection system of claim 23 further comprising means adapted to orient the associated specimen relative to the viewing area at a distance less than a prime focal distance from a vertex or piano surface of the field lens, whereby illumination of each associated specimen is provided from as distributed, shaped and convergent beam.

25. The video inspection system of claim 24 further comprising means for supplying a control signal to the controllable light source so as selectively control illumination of subportions of each associated specimen disposed in the viewing area.

26. The video inspection system of claim 25 further comprising: means for communicating digital images of the specimen formed by each video camera after exposure thereof to reflected light communicated thereto to a digital processor, the digital processor including means for determining acceptability of the specimen in accordance with the digital images communicated thereto.

27. The video inspection system of claim 18 further comprising means for supplying a control signal to the controllable light source so as selectively control a light level of light communicated to the viewing area.

28. The video inspection system of claim 27 further comprising: means for communicating a digital image of the specimen formed by the video camera after exposure thereof to reflected light communicated thereto to a digital processor, the digital processor including means for determining acceptability of the specimen in accordance with the digital image.

29. A multi-spectral video inspection illumination system comprising: a beam splitter; a multi-spectral light source adapted to direct light to the beam splitter; a field lens adapted to receive a portion of light from the light source via the beam splitter and communicate it to a viewing area and to receive light after reflection from an associated specimen disposed in the viewing area; means for securing the multi-spectral light source relative to the viewing area such that substantially all multi-spectral light passes from the light source to the viewing area through the beam splitter and the field lens; the field lens being further adapted to receive the multi-spectral light after reflection from the associated specimen; first and second video cameras adapted for forming a digitized image from light communicated to lenses respectively associated with each video camera; and a spectral separator communicating light of a first selected wavelength to a first video camera and light of a second selected wavelength to a second video camera such that first and second digitized images are formed therefrom.

30. The multi-spectral video inspection system of claim 29 further comprising a comparator for comparing the first and second digitized images to data representative of acceptability of the associated specimen.

31. The multi-spectral video inspection system of claim 30 further comprising means for selectively rejecting the specimen in accordance with an output of the comparator.