[0001] This invention relates generallyto digital imag-ing devices and specifically to techniques and circuitsfor the filtering of defective picture elements. The termpixels is used belowto mean picture elements.[0002] Digital imaging devices are becoming increas-ingly popular ina varietyof applications, including digitalcameras, fingerprint recognition, digital scanners andcopiers, and the like. Typical prior art digital imaging de-vices are based on charge coupled device (CCD) tech-nology. CCD devices havean array ofCCD cells, eachcell comprisinga pixel. Each CCD pixel outputsa volt-age signal in proportion tothe intensiyof light impinginguponthe cell. This analog voltage signalcan be convert-ed toa digital signal for further processing, digital filter-ing, storage and the like. As is well known in the art,atwo dimensional digital image can be constructed fromthe voltage signals output froma Mo-dimensional arrayofCCD cells, commonly referred to asa sensor array.[0003] CCD arrays havea shortcoming in that CCDfabrication requiresa special process that is not com-patible with standard CMOS processes. Thus,theCCDarray cannot be easily integrated with other logic cir-cuits, such as CCD control logic, analog to digital con-verters and the like. Additionally, in operation,a CCDarray requires multiple high voltage supplies from 5V to12V and CCD arrays tend to consumea large amountof power in use.[0004] An alternative to CCD arrays is using an arrayformed of CMOS cells.A CMOS sensor array can befabricated using standard CMOS processing and thuscan be integrated ontoa single chip with other circuits,such as array control logic, analog to digital converters(A/D's), digital signal processing (DSP) cores and thelike. CMOS arrays provide the additional advantage ofoperating witha single low supply voltage such as 3.3Vor5V, and consuming less power thana comparableCCD array. Finally,a CMOS array can be fabricated ata lower cost thana similar CCD array.[0005] One common problem with both CCD and withCMOS imaging devices is that of point defects whichcause "spot noise" on the image, such as white spotsona dark portion ofthe imageora dark spotona whiteportion of the image. In CMOS imaging devices, whitespots are due to pixels (i.e. CMOS cells) with excessiveleakage current. Dark spots are due to either particlescovering the pixel ora defect in the pixel electronicscausing the pixel not to turn on. Spot noise seriouslylimits the yield of CMOS imaging devices, resulting inincreased costs.[0006] One methodtoremove spot noise electronical-ly has been proposed by Younse et at. in U.S. Patent4,805,023. The Younse et at. implementation requiresexpensive EPROM memoryand involvesa complicatedhardware system, further increasing the cost ofthe im-aging device. Furthermore, the solution proposed byYounse et at. cannot remove temperature dependent10203040502temperatures.[0007] European Patent Application No0 496 573 dis-closesa picture element defect removing circuit whichselectsa picture element and its adjacent picture ele-ments in anM x N block wherethe minimum value ofMandN is 2, determines the maximumand minimum val-ues amongthe adjacent picture elements and whether,from the maximum and minimum values, two arbitrarythreshold values and the value ofthe selected pictureelement,a change is required in the valueofthe select-ed picture element. The picture element defect remov-ing circuit performs several compare-and-select opera-tions in determininga valueforuse as the selected pic-f5ture element and is correspondingly complex.[0008] Therefore,a need exists fora relatively inex-pensive filter that can quickly and reliably filter out theeffects of defective pixels, including spot noise, from animage signal.[0009] The invention is as defined in claims1 or 24.It is provideda filterfor filtering an image which includesa pluraliy of picture elements each represented bya Iu-minance ora chrominance value, including:25means includinga three-picture-element windowfor selectinga picture element and its two adjacentpicture elements ina picture element data stream,meansfor determining whetherthe selected pictureelement lies beMeenthetwo adjacent picture ele-ments in value,meansfor determining whetherthe selected pictureelement differs in value from both ofthe Mo adja-cent picture elements by more thana set amount,means for replacing the selected picture element,35in value, bya value dependenton thetwo adjacentpicture elements, when, in value, the selected pic-ture element does not lie beMeentheMo adjacentpicture elements and differs from both of the twoadjacent picture elements by more than the setamount andmeans for repeating the procedure forthenew se-lected picture element.[0010] One embodiment ofthe invention isa digital45imaging device comprisinga substrate,a sensor arrayformed on the substrate, the array generating an elec-trical signal corresponding tothe amountof light imping-ing uponthe array, and imaging logic formedonthe sub-strate, coupled tothe sensor array and receiving theelectrical signal. The imaging logic includes an analogto digital converter receiving the electrical signal andoutputting digital pixel values anda defective pixel filterreceivingthe digital pixel values and detecting defectivepixelson the basis of variations beMeena selected pixel55value and its neighboring pixel values.[0011] Another embodiment ofthe invention isa dig-ital imager comprisinga lens mechanism,a sensor arraypositioned withina focal plane of said lens mechanism,
