Abstract:
An automated optical inspection system with improved accuracy achieved by reducing the negative effects of extraneous light. The system includes a light trap behind a two-way mirror.

Description:
This invention relates generally to automated optical inspection and more specifically to equipment for improved inspection accuracy. 
   Automated optical inspection (AOI) is used in many fields, particularly in the manufacture of electronic assemblies, such as printed circuit boards. Such a system generally includes a light source, a camera that makes an image of the article being inspected and a computer connected to the camera to process the image. 
   The computer detects the presence or absence of features in the image. A circuit board might be declared as good if the computer detects all the components that should be on the printed circuit board in their expected places. Conversely, a board might be declared as faulty if the computer either fails to detect some of the components or identifies objects that should not be present on a properly assembled printed circuit board. 
   In an automated optical inspection system, the relative position of the light source and the camera is usually selected to ensure that the components on the boards are highlighted in the image. For example, when an AOI system is inspecting for the presence of parts packaged in plastic packages, the light source and the camera are often separated by a relatively large angle. Lighting the parts from the same angle they are inspected from often provides insufficient visual contrast between the part and the printed circuit board, making it difficult to determine whether the part is present. 
   On the other hand, when the part being inspected is a shiny metal pad, directing light onto the component from directly above the circuit board and also inspecting from that angle can produce an image with a large contrast between the pad and the circuit board, making inspection more reliable. It is difficult to provide a system with both an illumination and an inspection angle that are the same because that requires having the light source and the camera occupy the same space. In prior art systems, such an illumination angle has been achieved with a device known as a beam splitter. 
     FIG. 1  illustrates a prior art AOI system  100  configured for top lighting, here shown inspecting a printed circuit board  102 . Components  104  are on the surface of the board. Camera  112  is mounted above the board. One of skill in the art will appreciate that the illustrated components are mounted in a housing or other convenient support structure. Further, AOI system  100  would have a conveyor or similar components to move circuit board  102  relative to camera  112 . However, the exact details are not illustrated for simplicity and are not important for the invention. 
   Light source  106  provides the source of illumination. Light source  106  emits light in the direction shown by arrow  1 . Arrow  1  is at a right angle to the optical axis of camera  112 . To align the light from source  106  with the optical axis of camera  112 , beam splitter  108  is used. 
   Beam splitter  108  is a conventional part. A suitable device might, for example, be purchased from Edmond&#39;s Scientific company. Beam splitter  108  includes a two-way mirror  110 . Mirror  110  is mounted at approximately a 45 degree angle relative to arrow  1 . Mirror  110  reflects the light from source  106  towards printed circuit board  102 . After reflection, the source light travels in the direction indicated by arrow  2 . 
   Incident light is reflected from the component  104  under inspection. Some part of the incident light is reflected upwards towards camera  112 , as shown by arrow  3 . Mirror  110  is a two-way mirror, meaning it has a reflection coefficient that differs based on the incident angle of the light. It has a high reflection coefficient for light impinging on the mirror from direction  1 , but a very low coefficient of reflection for light impinging from direction  3 . Mirror  110  largely passes, rather than reflecting, light impinging from direction  3 . 
   Thus, light reflected from component  104  being inspected, passes to camera  112 . In this way, the incident light arrives at the component  104  from a direction that is along the optical axis of camera  112 . 
   We have discovered a drawback of using a beam splitter to align the light source with the optical axis of the camera. Two way mirror  110  can not be made perfectly reflective. Some portion of the incident light is transmitted through mirror  110 , traveling in the direction of arrow A. While the inside of beam splitter  108  is black, it is sufficiently reflective that some of the light traveling in direction A will reflect from the wall of beam splitter  108  and be reflected back towards mirror  110 , traveling in the direction of arrow B. 
   This reflected light, upon reaching mirror  110 , is reflected in the direction of arrow C, which directs the light to camera  112 . Thus, in addition to receiving light which represents the image of the board being inspected, camera  112  also receives extraneous light that has passed through mirror  110  and been reflected. We recognized that this extraneous light creates a “shadow image” of light source  106  in the image passed to computer  116  for processing. In some instances, the shadow image has been bright enough to “confuse” the computer  116  into reporting the presence of an unwanted component on printed circuit board  102  or reporting that a needed component is not in its intended location. 
   SUMMARY OF THE INVENTION 
   With the foregoing background in mind, it is an object of the invention to provide an AOI system with improved accuracy. 
   The foregoing and other objects are achieved through the use of a beam splitter with a light trap to reduce the impact of a shadow image. 
   According to a preferred embodiment, the light trap is mounted behind a two-way mirror in a beam splitter. 
   Also according to the preferred embodiment, the light trap includes an angled surface an light absorbing material. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood by reference to the following more detailed description and accompanying drawings in which 
       FIG. 1  is a sketch illustrating a prior art top-lighted AOI system; 
       FIG. 2  is a sketch illustrating a top-lighted AOI system employing the invention; 
       FIGS. 3A ,  3 B,  3 C and  3 D are sketches illustrating alternative implementations of the light trap of  FIG. 2 ; 
       FIG. 4A  is a sketch illustrating an alternative implementation of the light trap of  FIG. 2 ; and 
       FIG. 4B  is a sketch illustrating the light trap of  FIG. 4A  in cross section along the line B—B. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2  shows a top lighted AOI system  200 . AOI system resembles the prior art AOI system of  FIG. 1 , but includes light trap  210  attached to beam splitter  108 . Light trap  210  receives extraneous light passing through mirror  110  and substantially reduces any reflection back towards mirror  110 . In this way, the shadow image problem of the prior art is substantially eliminated. 
   In a preferred embodiment, beam splitter  108  is purchased with multiple ports to which light sources, cameras or other devices might be attached. When not in use, the ports are covered with plugs. Thus, one simple way to attach light trap  210  to beam splitter  108  is to simply remove the plug and attach light trap  210  with screws or other fasteners that are compatible with the design of beam splitter  108 . 
   Turning to  FIG. 3 , details of light trap  210  are shown.  FIGS. 3A  to  3 C shown alternative implementations of a light trap. In  FIG. 3A , a light trap  210 A is shown to include a cylinder  310  with a cone  312  inside. There is open space between the inner walls of cylinder  310  and outer surface of cone  312 , forming a “chamber”  313 . 
   Extraneous light enters the cylinder  310  in direction A. Cone  312  presents an angled surface that reflects the extraneous light in a direction away from direction A. As shown in the  FIG. 3A , the reflected light is directed toward the interior walls of cylinder  310 . The light continues to be reflected between the exterior surface of cone  312  and the interior walls of cylinder  310 , staying within chamber  313 . At each reflection, the light is attenuated. After a sufficient number of reflections, the light is dissipated. 
   Preferably the outer surface of cone  312  and the inner walls of cylinder  310  are made of a light absorptive material. The material should also provide very little back scattering of the light. In this way, the amount of light leaving light trap  210  is so small that it has no practical effect on AOI system  200 . 
   A suitable material for constructing light trap  210  is black anodized aluminum. This material is preferred because of its absorptive properties and because it has sufficient strength to be machined. However, other suitable materials might be employed. Including cloth or paper covering other materials to achieve the required strength to be formed into the desired shapes. 
     FIG. 2B  shows an alternative embodiment of light trap  210 B. Light trap  210 B includes an outer cylinder  314  and an inner cylinder  316  and a cone  318 . Cylinders  314  and  316  are concentric, creating a space or “chamber” between the inner walls of cylinder  314  and the outer walls of cylinder  316 . Inner cylinder  316  has a slit  320  formed around it, creating an opening into space  322 . 
   Extraneous light enters light trap  210 B traveling in the direction A. Cone  318  has an outer surface shaped to reflect light traveling in direction A towards slit  320 . In a preferred embodiment, the outer surface of cone  318  is parabolic. In this way, extraneous light is diverted into chamber  322 . Though not explicitly shown in  FIG. 3B , space  322  is sealed with a wall at the end facing beam splitter  108 . In this way, extraneous light becomes trapped in chamber  322  and is not reflected back towards mirror  110 . In the preferred embodiment, the inner wall of cylinder  314 , the outer wall of cylinder  316  and the outer surface of cone  318  are made of light absorptive material, such as black anodized aluminum. In this way, extraneous light dissipates as it is reflected from the walls in chamber  322 . 
   Turning now to  FIG. 3C , a further embodiment of light trap  210 C is shown. In the embodiment of  FIG. 3C , light trap  210 C includes a cylinder  350  and a cone  352 . Cylinder  350  and cone  352  are positioned to leave a chamber  358  between the outer surface of cone  352  and the inner surfaces of cylinder  350 . 
   In this embodiment, the base of cone  352  faces the source of extraneous light and acts to gather the extraneous light. The extraneous light passes through opening  354  in cone  352 . Extraneous light is reflected from surface  356 . Surface  356  is angled to reflect the light away from opening  354 . In this way, the extraneous light is diverted into chamber  358 . Preferably, the inner surfaces of chamber  358  are made of a light absorptive material. The extraneous light is dissipated within chamber  358  without any noticeable reflection back towards mirror  110  and does not interfere with the operation of AOI system  200 . 
   In the embodiment of  FIG. 3D , a cone  362  is positioned within cylinder  360 . The top of the cone has an opening therein, exposing interior walls. The opening within cone  362  forms a cavity  364 . Angled walls  366  within cavity  364  ensure that extraneous light is reflected inwards into the cavity. It is preferable the that angled walls  366  are made of light absorbing material such that light reflected into cavity  364  is quickly dissipated. 
     FIG. 4  shows an alternative embodiment of the light trap. Here, light trap  410  is shown with a plurality of cavities  416  for absorbing extraneous light. The cavities  416  are formed from a plurality of pyramids  414  on a flat surface  412 . The outer surfaces of the pyramids  414  are preferably a light absorbing material. Any light reflected from the outer surfaces is diverted into the cavities  416  formed between adjacent pyramids and dissipated. 
   With the light trap installed, the image formed by camera  112  has drastically improved contrast, sharpness and definition because there is less extraneous light to interfere with the light reflected from the object being inspected. 
   Having described one embodiment, numerous alternative embodiments or variations might be made. For example, in the preferred embodiment, the components of the light trap were made with black anodized aluminum. Other non-reflective materials might be suitable, such as flat black or painted plastic or other flat black surfaces. 
   Also, the illustrations show direct paths taken by light. It should be appreciated that light can be bent in space by mirrors or in optical fibers or optical wave-guides. Though straight-line paths are shown for the light, it is possible that the light might not travel in a straight line if mirrors or other light bending devices are used. However, the components might still be “facing” each other in an optical sense despite the fact that the path of the light has been diverted. 
   As another example, it should be noted that the light trap of the invention included an angled surface that diverted extraneous light into a cavity. For example, the outer surfaces of cones  312  ( FIG. 3A ) and  318  ( FIG. 3B ) and angled surface  356  ( FIG. 3C ) are all angled relative to the direction A in which the extraneous light impinges. These angled surfaces divert light so that it does not reflect back towards mirror  110  where it might interfere with the operation of AOI system  200 . In the above-described embodiments, the angled surfaces are made of absorptive material. However, it is not necessary that the angled surfaces be absorptive. Even if the angled surfaces are not absorptive, the extraneous light would dissipate in the chambers  313  (FIG.  3 A),  322  ( FIG. 3B ) or  358  (FIG.  3 C). 
   It should also be appreciated that other shapes of chambers could be formed. And, the angled surface might be given any of a very large number of shapes and still perform the function of directing the incident light away from the opening that leads back to the beam splitter  108 . 
   Therefore, the invention should be limited only by the spirit and scope of the appended claims.