Abstract:
A system for capturing color images using monochrome image sensors is herein disclosed. Differences in monochrome pixel intensity are correlated with color using known reflection/transmission ratios of a beam splitter.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority under 35 U.S.C. §119(e)(1) to U.S. Provisional Patent Application Ser. No. 60/758,522, filed Jan. 12, 2006, entitled “Color Imaging Using Monochrome Imagers”, and bearing Attorney Docket No. A126.190.101. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Standard, prior art imaging systems (cameras) often capture color images using a single imaging device with a color filter array of RGB (red/green/blue) filters. However, this arrangement causes a significant loss of spatial resolution. Other prior art imaging systems use an assembly of three imaging devices, each of which has its own respective red, green, or blue filter. Undifferentiated light is provided to the imaging devices by a triple beam splitter that may include a dichroic optical filter. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0003]      FIG. 1  is a schematic view of a camera having two monochrome imaging devices.  
         [0004]      FIG. 2  is a graph showing one relationship between the reflection/transmission ratio of a beamsplitter as a function of the wavelength of light transmitted or reflected by the beamsplitter.  
         [0005]      FIG. 3  schematically illustrates an example offset determination associated with the imaging devices of  FIG. 1 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0006]     In the following detailed description of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof.  
         [0007]     In one embodiment, two substantially identical monochrome imagers  10 ,  12  are spatially aligned and affixed to the two output paths of a beam splitter  14 . The pixels of the imagers  10 ,  12  are aligned across the beam splitter  14  in the X and Y directions (relative to the width and height of each imager  10 ,  12 ) and with respect to rotation to less than a fraction of a pixel error or thereabout. In one embodiment, the pixels of imagers  10 ,  12  are aligned to within about 1/10 of a pixel error. The beam splitter  14  has a specifically defined or known reflection/transmission ratio as a function of wavelength that imposes a difference in the pixel intensities reaching the two imagers, despite the fact that there is only a single pixel intensity value input to the beam splitter  14 . Because the beam splitter  14  reflection/transmission ratio will unequally transmit light incident upon the beamsplitter  14  based on the light&#39;s wavelength, one of imagers  10  or  12  will receive more light than the remaining imager  10  or  12 . Calibration is performed to determine the difference in imager response for each wavelength that is created by the beamsplitter.  
         [0008]     In another embodiment, two substantially identical monochrome imagers  10 ,  12  are only generally aligned and secured to the two output paths of a beam splitter  14 . In this embodiment, perfect physical alignment of the two imagers  10 ,  12  is not contemplated. Instead, patterns of known geometric properties are imaged by each imager  10 ,  12  and the pixel arrays of each imager  10 ,  12  are mapped, the one to the other through a comparison of the images captured by the respective imagers  10 ,  12 . It is to be understood that where significant pixel-to-pixel alignment occurs, correction algorithms may be used to ensure that any offset between the respective pixel arrays is considered and appropriate correction is made. By way of example, in one embodiment pixel intensity values may be integrated or otherwise aggregated over a range of pixels to obtain an average or composite pixel intensity value that may be related to similar pixel intensity values from the opposing imager  10  or  12  during a calibration process and/or during actual use of the system.  
         [0009]      FIG. 3  illustrates schematically the calculation of an offset value as between the two imagers  10 ,  12 .  FIG. 3  shows an overlay of two images  20  and  20 ′ of a test pattern captured from imagers  10 ,  12 , respectively. As can be seen, there is an offset, defined by terms ΔX and ΔY, between the images  20  and  20 ′. This offset is recorded during calibration and used to align the images during use.  
         [0010]     In one embodiment, the imagers  10 ,  12  are area scan imagers such as, by way of example only, a CCD or CMOS device. In another embodiment, the imagers  10 ,  12  might be a line scan imaging device or a TDI imaging device.  
         [0011]     Note that in the embodiment illustrated in  FIG. 1 , the imagers  10 ,  12  are affixed directly to the beamsplitter  14  using an optically neutral adhesive  13 . In some embodiments, a framework (not shown) may be utilized to securely hold the imagers  10 ,  12  in the required relationship to the beamsplitter  14  in a mechanical fashion. In other embodiments, a pellicle beamsplitter (not shown) may be used in lieu of the solid beam splitter shown in  FIG. 1 . As will be appreciated, any type of suitable beam splitter may be used. By way of example only, prismatic (with or without metallic or dielectric optical coatings) and thin-film beam splitters may be used in various embodiments.  
         [0012]     A generic curve that schematically illustrates reflection/transmission ratios (or coefficients) as a function of wavelength is shown in  FIG. 2 . In some embodiments, one or more optical or electronic filters are employed to limit data to the range of wavelengths (Δ) where there is a one-to-one relationship between the ratio or coefficient and the wavelength of the light incident on the beam splitter  14 . In other embodiments, light sources used in conjunction with the camera are limited to outputting light within a given range of wavelengths using suitable filters and the like. In yet other embodiments, suitable optical filters are used on both the light sources and the imagers  10 ,  12 .  
         [0013]     The reflection/transmission ratio of the beam splitter is preferably specified such that for any given λ, there exists only one particular ratio of intensities, i.e. the relationship between wavelength and the reflection/transmission ratio is a one-to-one function. Discontinuities, minima or maxima in the reflection/transmission ratio v. wavelength curve may introduce indeterminacy in that a single reflection/transmission ratio may apply to more than one wavelength. Reflection/transmission ratio v. wavelength curves of this nature may still be used however, where image processing software may account for these discontinuities. In one embodiment, indeterminacy is resolved by looking to the colors of pixels adjacent and/or near the indeterminate pixel(s) and selecting a wavelength or color for the indeterminate pixel(s) that comports with the established reflection/transmission ratio v. wavelength relationship and which is closest in color to the surrounding pixels.  
         [0014]     During operation, a camera such as that illustrated in  FIG. 1  having two imagers  10 ,  12  coupled to a beamsplitter  14  receives a single light signal. This signal is in most instances light that is reflected from an object being imaged. In one embodiment, for each aligned pixel pair of the two imagers, the respective pixel intensities of the aligned pixel pair are averaged as shown by the formula: 
 
 I   XY =( I   1XY   +I   2XY )/2 
 
 where I 1XY  is the measured pixel intensity of a first pixel of the aligned pixel pair and I 2XY  is the measured pixel intensity of the remaining pixel of the aligned pixel pair. 
 
         [0015]     Thereafter, a coefficient that is in one embodiment defined by the difference of the respective pixel intensities I 1XY , I 2XY  divided by the average pixel intensity I XY  is calculated and plotted against known wavelength values as part of a calibration process. This calibration process relates wavelength to pixel intensity as follows: 
 
λ XY   =f (( I   1XY−   I   2XY )/ I   XY ) 
 
         [0016]     Note that where other coefficients, calibration procedures, or fitting methods or algorithms are used, this function may appear in a different form, but it is to be kept in mind that the basic relationship between wavelength and pixel intensity will be substantially the same for any given beamsplitter  14 . One method of calibrating wavelength with respect to pixel intensity is to limit incident light input to the camera to a particular wavelength or narrow range of wavelengths and then measure pixel intensity in the pixel pairs of the respective imagers  10 ,  12 . Another method of calibrating wavelength with respect to pixel intensity is to use a standard light source and direct the camera to capture an image of a color target having a reflectance band that is substantially at or distributed around a known wavelength and then measure pixel intensity in the pixel pairs of the respective imagers  10 ,  12 .  
         [0017]     In one embodiment, a camera incorporating imagers  10 ,  12  and a beam splitter  14  may be used to inspect substrates at a high rate of speed as described in co-pending U.S. patent application entitled “Camera Module for an Optical Inspection System and Related Method of Use”, Ser. No. 11/179,019 filed on Jul. 11, 2005, hereby incorporated by reference. Successive images of individual fields of view of a substrate may be captured by the respective imagers  10 ,  12  as described in the incorporated patent application. Because each field of view of the substrate is captured using alternating imagers  10  or  12 , the monochrome image capture rate of a camera incorporating two imagers  10 ,  12  may approach twice the image capture rate of the imagers  10 ,  12  individually. Thereafter, color images of all or only selected portions of the substrate are captured using the imagers  10 ,  12  in combination with one another as described herein. Monochrome and color images may then be used to inspect the substrate for defects. Accordingly, both high speed monochrome image capture and color image capture may be accomplished using the same apparatus.  
       CONCLUSION  
       [0018]     While various examples were provided above, the present invention is not limited to the specifics of the examples. In one basic embodiment, the present invention is characterized by the output (pixel intensity) from two monochrome (black and white) imaging devices being averaged (I XY ) and used to calculate a coefficient (λ XY ) that is calibrated against the actual wavelength of the light presented to the two monochrome imaging devices. Since beam splitters can and often do have a wavelength dependent operating characteristics, it is important to use a beam splitter that exhibits a one-to-one relationship between reflection and transmission or which can manipulated in some manner to exhibit a one-to-one relationship between reflection and transmission. It is to be kept in mind that the relationship between reflection and transmission for a given beamsplitter may not be linear, but over at least a given range of wavelengths, the relationship must be such that for each coefficient (λ XY ), there is only one wavelength value. In addition to providing color information from monochrome imagers, this invention may increase the usable dynamic range of sensor over a single imager since one imager will always be more sensitive to a particular wavelength while the other is less sensitive.  
         [0019]     Although specific embodiments of the present invention have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the invention. It is manifestly intended that this invention be limited only by the following claims and equivalents thereof.  
         [0020]     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the present invention.