Image analysis – Applications – Manufacturing or product inspection
Reexamination Certificate
2000-06-23
2004-07-06
Mehta, Bhavesh M. (Department: 2625)
Image analysis
Applications
Manufacturing or product inspection
C382S305000, C348S126000
Reexamination Certificate
active
06760471
ABSTRACT:
STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable.
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
FIELD OF THE INVENTION
This invention relates generally to automated optical inspection (AOI) systems and more particularly to illumination systems used in AOI systems.
BACKGROUND OF THE INVENTION
As is known in the art, an automated optical inspection (AOI) system typically includes an illumination system which projects or otherwise provides light to illuminate a specimen being inspected and a camera which captures an image of the specimen and converts it to electrical signals. One or more frame grabbers transfer the electrical signals representing the image to a computer or other processing device for further processing.
It is difficult, however, to produce uniform illumination over the field of view of a camera. A processing device which is unaware of the specific nonuniformities in lighting cannot differentiate between those patterns of light in the image which are due to the appearance of the specimen and those patterns of light in the image which are due to irregularities in the illumination system.
For example, an image of a uniformly illuminated plain uniform grey surface ideally has pixel values which are all equal. In practice, however, the pixel values will not be equal unless the camera is ideal and the illumination is perfectly uniform. The computer or other processing device receives a “true” image only if the effects of nonuniform illumination can somehow be cancelled out. Nonuniformity of illumination is thus a source of noise in AOI systems.
There are a number of reasons for nonuniform illumination in AOI Systems. A typical illumination system includes one or more fluorescent lamps. On problem with fluorescent and other types of lamps is that intensity of the lamp varies along the length of the lamp. The intensity also varies with the amount of current provided to the lamp, the age of the lamp, specific variations in the manufacture of the lamp including its shape, and the temperature at which the lamp operates. It is a daunting prospect to attempt to regulate the light at one point, let alone regulate the light at all points on the object. Thus, suffice it to say that for a variety of reasons, it is relatively difficult to produce uniform illumination over the field of view of a camera.
One technique to overcome this illumination problem is to use lamps which provide relatively uniform intensity. Such lamps, however, are relatively expensive. More importantly perhaps such lamps still have variations in intensity although the variations are less severe than the intensity variations in relatively low cost lamps.
It would, therefore, be desirable to provide a system for compensating nonuniformity in an image due to variations in illumination characteristics. It would also be desirable to provide an AOI system for printed circuit boards which compensates for variations in illumination characteristics. It would be further desirable to provide a system which performs compensation in real time.
SUMMARY OF THE INVENTION
In accordance with the present invention, an automated optical inspection (AOI) system includes an image acquisition system for capturing an image and for providing a digital signal to a compensation circuit which compensates the pixel values to provide compensated or corrected pixel values. The compensation circuit provides the compensated pixel values in real time to a video frame memory and to a direct memory access (DMA) channel. The DMA channel transfers the compensated pixel values to a storage device which is coupled to or provided as part of a processor.
With this particular arrangement, an AOI system which corrects errors in a digital image due to illumination is provided. The system corrects the errors on a pixel by pixel basis as signals representing the video frame are being transferred from the image acquisition system to a frame memory. By storing compensation values in the compensation circuit and coupling the compensation circuit to a DMA channel, it is possible to compensate the pixel values in real time as the pixel values are transmitted from the image acquisition system to the frame memory. Compensating the pixels for irregularities in real time makes it possible to continue processing video information obtained by the image acquisition system at the same rate as without correction. In one embodiment, the image acquisition system includes a digital camera and in another embodiment the image acquisition system includes an analog camera having a video digitizer coupled thereto.
The details of the compensation memory architecture vary with the nature of the aberrations expected during inspection. For example, if a pure linear correction with no offset errors is expected, only a scale factor memory is needed. On the other hand, if offset errors are expected, then an offset memory is also required (e.g. to eliminate the effects of unexcludable stray light). If a nonlinear correction is needed, additional scale memories and associated arithmetic circuits to perform scale factor computations may be needed. In a preferred embodiment, only offset and scale factor are linearly corrected. It should be appreciated, however, that the concept of performing real time correction can be extended beyond simple linear calculations or reduced to simpler linear multiples without offset correction.
In preferred embodiments, the system uses a plurality of lighting modes and it may be desirable to provide the compensation memory from a plurality of separate banks of memories with each bank storing a set of correction or compensation coefficients. In this case, each lighting mode use a separate bank and thus a separate set of compensation coefficients. When multiple memory banks are used, a bank switch coupled between processor and the compensation memory allows the processor to select a particular one or ones of the banks of compensation memory. The appropriate memory bank is selected by the processor prior to the time a frame is acquired. In particular, the processor selects particular values in the memory to be used according to which lighting mode will be used.
In one embodiment, the compensation circuit includes a compensation memory coupled to an adder circuit and a multiplier circuit. The image acquisition circuit provides a pixel value (in the form of a digital signal) to a first port of the adder circuit and the compensation memory provides an offset value to a second port of the adder circuit. The adder circuit combines the two values fed thereto and provides a partially compensated pixel value to the input of the multiplication circuit. The multiplication circuit further compensates the pixel value prior to providing the fully compensated pixel value to the video frame memory and the DMA channel.
A further advantage of the present invention is that the cost of illumination hardware can be reduced. As mentioned above, relatively low cost illuminators tend to have worse nonuniformities than relatively expensive illuminators. The present invention, however, makes it possible to tolerate greater irregularity in the illumination, and therefore makes it possible to employ relatively low cost illuminators while still achieving an accuracy which is equivalent to that obtained with relatively high cost illuminators.
In accordance with a further aspect of the present invention, an AOI system includes an image acquisition system having an output port coupled to a first input port of an adder circuit. A second input of the adder circuit is coupled to an offset memory and an output of the adder circuit is coupled to a first input of a multiplier circuit. A second input of the multiplier circuit is coupled to an offset memory and an output of the multiplier circuit has an output coupled to a video frame memory and a DMA channel. The DMA channel is coupled to a processor memory. The processor memory may be provided, for example, either as a main memory in a so-called “motherboard” or as a processor on a frame grabber module which includes
Chawan Sheela
Daly Christopher S.
Daly, Crowley & Mofford
Mehta Bhavesh M.
Teradyne Legal Dept.
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