Characterization of crosstalk in high-resolution active-matrix liquid crystal displays for medical imaging

Characterizationofcrosstalkinhigh-resolutionactive-matrixliquidcrystaldisplaysformedicalimagingAldo Badano andJerzyKanickiDept.ofElectrical Engineering andComputerScience,Universityof Michigan, AnnArb or, MI.ABSTRACTInactive-matrix liquid crystaldisplays withlargepixelarrayandlarge numberofgraylevels, theluminance ofapixeldep endsontheluminance ofrestofimage.Thisartifactknownascrosstalk iscausedbyparasiticphenomenaintheactive-matrixarray.Whenterpretinghighinformationcontentmedicalimageswithsubtlefeaturesandstructured background,crosstalk cana ectimage delityanddiagnostic p erformance.Conventionalmetho ds rely on the measurement of the luminance change of small square targets lo cated across the screen whenchanging the background intensity.We present a metho d that describ es b oth the magnitude and the spatial extentofthecrosstalk artifact.Themetho disbasedonformulation ofaresp onsefunctionthatcorresp onds todi erential contribution ofavertical orhorizontal line to theluminance ofa small centered target.Theresp onsefunction is de ned for a given screen p osition, along the horizontal or vertical display axis.Our measurements showthatsubtledi erencesbetweenvertical andhorizontal crosstalkcanbedetectedwiththeprop osedmetho d,andthat most of the inuence is con ned to a 40-pixel region ab out the target.The results obtained with the prop osedcharacterization metho dallowforthemo deling ofcrosstalk e ectstodetermine itsimpactonavarietyofvisualtasks.KeywordList:small-sp ot contrast, electronic crosstalk, medical display, active-matrix liquid crystal displa.1.INTRODUCTIONContrast measurements are an essential part of the assessment of display image quality.The contrast p erformanceis typically determined by measuring the maximum and minimum luminance generated by the display using sp eci ctest patterns for a given exp erimental condition that should sp ecify ambient illumination levels.1Both maximum andminimum luminance levels are de ned b efore the measurementby the initial display calibration pro cedure.Althoughwidely used particularly in active-matrix liquid crystal display (AM-LCD) sp eci cations, the full- eld contrast ratioof display devices determined withuniform bright and dark elds do esnot provide a complete characterization ofthe contrast resp onse.All displaycontrast metrics are a function of the target size and, in AM-LCDs, are a ectedby electronic crosstalk.Electronic crosstalk is of particular imp ortance in high-resolution at-panel displays with alarge number of graylevels.2Theundesiredscene-dep endentartifactisasso ciatedwithanunwtedmo di cation ofthevoltage e ectivelyapplied to the liquid crystal cell.The changes in pixel voltage translate into changes in light transmission throughtheliquidcrystal,a ectingdesiredpixelluminance.Sourcesofcrosstalkincludeincompletecharging,leakage and photo-generated currents in the thin- lm transistor, and parasitic capacitive coupling.Display crosstalkismoreimp ortantforlargesizepanelshavinghigherresolutionandgrayscale.Severalauthorsvestudiedthecrosstalk artifact in large active-matrix arrays and have prop osed mo di ed driving techniques that comp ensate thesignaldistortion.3{5Othershavefo cusedtheirworkonthestudyofpixeloltagechangesanditse ecttransmission-voltage characteristics of the liquid crystal cell.6Although having di erent origins, crosstalk artifactshave b een also studied for passive matrix organic p olymer light-emitting displays.7In the Flat Panel Display Measurements Standard,1metho ds to quantify display crosstalk are included in thegrayscale artifacts Section, along with other related phenomena such as streaking, ghosting and trailing.Crosstalkis de ned as \unwanted coupling b eteen adjacentor nearby circuits that causes signal prop erties of one elementtob e injected into other elements".The standard suggests a classi cation into short- and long-range crosstalk e ects.Thecompletecharacterization ofelectronic crosstalkinat-paneldisplayswithhighqualityformedical imagingapplications requires the measurement of the small-sp ot luminance of a target in backgrounds of di erentintensity.Inprevious work, wehave studied thedegradation incontrast asso ciated with veiling glare in a catho de-ray tub e(CRT). Measurements of the small-sp ot contrast were used to determine a one-dimensional rotationally symmetricresp onsefunctionthatcharacterizes veiling glare in CRTs.8,9Theelectronic crosstalk artifactissimilar toveiling