US 6,445,812 B1 · Illumination compensation system for industrial inspection

Abstract

An illumination compensation system for correcting smooth intensity variations due to illumination changes is based on an assumption that an underlining image reflectance function is approximately a piecewise constant and that an image irradiance function is spatially smooth. The system first takes the logarithm of an image brightness function. Gradient constraints are then computed using a finite difference. Reliable gradient constraints are selected based on a local uniformity test. A process is subsequently applied to estimate the logarithmic irradiance function. A logarithmic irradiance function is subtracted from the logarithmic image brightness function and an exponential operation of the above subtracted image function is taken and an illumination compensated image is outputted from the system.

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Claims21

Dependent claims

A method for compensating for variations of illumination in an automatic visual inspection system, the method comprising the steps of:

receiving image data;
estimating an illumination component based on the image data;
removing the estimated illumination component from the image data to provide illumination-compensated image data;
and
outputting the illumination-compensated image data to the visual inspection system for further processing,
wherein the step of estimating an illumination component comprises the steps of:
computing an image brightness function representing the image data, the image brightness function being the product of an irradiance function and a reflectance function;
and
estimating the irradiance function from the image data;
and
wherein the step of removing the estimated illumination component comprises the steps of:
computing the reflectance function using the estimated irradiance function and the computed image brightness function, the computed reflectance function representing the illumination-compensated image data.

9 more elements

A system for compensating for variations of illumination in an automatic visual inspection system, comprising:

an illumination estimator adapted to estimate an illumination component of received image data;
and
a compensator adapted to remove the estimated illumination component from the image data to provide illumination-compensated image data, wherein the illumination-compensated image data is output to the visual inspection system for further processing,
wherein the illumination estimator comprises:
means for computing an image brightness function representing the image data, the image brightness function being the product of an irradiance function and a reflectance function;
and
means for estimating the irradiance function of the image brightness function from the image data;
and wherein the compensator comprises:
means for computing the reflectance function using the estimated irradiance function and the computed image brightness function, the computed reflectance function representing the illumination-compensated image data.

6 more elements

A program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform method steps for compensating for variations of illumination in an automatic visual inspection system, the method steps comprising:

receiving image data;
estimating an illumination component based on the image data;
removing the estimated illumination component from the image data to provide illumination-compensated image data;
and
outputting the illumination-compensated image data to the visual inspection system for further processing,
wherein the step of estimating an illumination component comprises the steps of:
computing an image brightness function representing the image data, the image brightness function being the product of an irradiance function and a reflectance function;
and
estimating the irradiance function from the image data;
and wherein the step of removing the estimated illumination component comprises the steps of:
computing the reflectance function using the estimated irradiance function and the computed image brightness function, the computed reflectance function representing the illumination-compensated image data.

8 more elements

A program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform method steps for improving automatic visual inspection performance under varying illumination conditions, the method steps comprising:

preprocessing an inspection image to provide an illumination-compensated inspection image, wherein the step of preprocessing the inspection image comprises the steps of estimating an illumination component of the inspection image, and removing the estimated illumination component from the inspection image to provide the illumination-compensated inspection image;
aligning the illumination-compensated image with a corresponding reference image;
and
taking the difference between the illumination-compensated image and the reference image to provide a difference image.

1 more element

Dependencies are inferred from the claim text.

Description4,664 words

Summary660 words

#1. Field of the Invention

#The resent invention relates to improving images for inspection purposes and more particularly to applying an illumination compensation to the images to correct smooth intensity variations due to illumination changes.

#2. Description of the Prior Art

Brief description of the drawings4 figures

#FIG. 1 illustrates a flow chart of the present invention.

#FIG. 2 illustrates a square non-conforming finite element Ei,j that the element basis function is a second-order polynomial with coefficients determined by its neighboring nodal variables.

#FIG. 3 illustrates (a) a reference image, (b) an inspection image, (c) a difference image without illumination compensation, and (d) a difference image with the illumination compensation system applied before image subtraction.

Detailed description3,904 words

#The image reference approach is very popular in industrial inspection due to its generality for different inspection tasks. Unfortunately, this approach is sensitive to illumination variations. The present invention provides an illumination compensation system for correcting smooth intensity variations due to illumination changes. By using the present invention as a pre-processing step in the image reference based inspection or localization, the image inspection or localization is made more robust against spatially smooth illumination changes.

#The flow chart of the illumination compensation system of the present invention is shown in FIG. 1. The image brightness function is inputted to the take LOG operator 10 where the logarithm of the image brightness function is taken. Then in gradient constraints computer 12, the gradient constraints are computed using finite difference. In reliable constraints selector 14, reliable gradient constraints are selected based on a local uniformity test. In surface fitting processor 16, a process is subsequently applied to estimate the logarithmic irradiance function. In image subtractor 18, the logarithmic irradiance function is subtracted from the logarithmic image brightness function. Finally, in take exponential operator 20, the exponential operation of the above subtracted image function is taken and the illumination compensated image is outputted from the system.

#The present invention is based on the assumption that the underlining image reflectance function is approximately a piecewise constant and that the image irradiance function is spatially smooth. Within reliable constraints selector 14, reliable gradient constraints on the smooth irradiance function are computed and selected from the image brightness function by using a local uniformity test. Within surface fitting processor 16, two surface fitting systems are presented to recover the smooth image irradiance function from the elected reliable gradient constraints found in gradient constraints computer 12. One is a polynomial surface fitting system and the other is a spline surface fitting system. The spline surface fitting formulation leads to solving a large linear system, which is accomplished by an efficient preconditioned conjugate gradient system. Once the image irradiance function is estimated, the spatial intensity inhomogeneities can be easily compensated.

Facts

Application

09/235,997Filed 22 Jan 1999

Granted

3 Sep 2002

Term ended

22 Jan 2019

Estimated · how

20 years from earliest known nonprovisional domestic filing. Maintenance-fee lapses, terminal disclaimers and statutory extensions have not been checked. The continuity chain may be incomplete; this is not an enforceability determination. Patent term adjustment is unavailable; no adjustment was added.

Classification8 codes

G06T 7/001
Industrial image inspection: using an image reference approach
G06T 5/94
Dynamic range modification of images or parts thereof: based on local image properties, e.g. for local contrast enhancement
G06V 10/145
Optical characteristics of the device performing the acquisition or on the illumination arrangements: illumination specially adapted for pattern…
G06T 2207/10152
Special mode during image acquisition: varying illumination

Illumination compensation system for industrial inspection

Life of this patent

Illumination compensation system for industrial inspection

Life of this patent

Life of this patent

Abstract

Drawings

Drawings

Claims21

Description4,664 words

Drawings

Description4,664 words

Classification8 codes

Life of this patent