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Principles of Cathode-RayTube and Liquid Crystal

Display Devices1

The effectiveness of digital diagnostic imaging modalities that rely on display devicesfor interpretation, review, or consultation is determined by the performance of indi-vidual system components, from the image acquisition device through image pro-cessing and transmission, until the image is displayed and a diagnosis is determined.When human observers are in charge of or involved in the diagnostic decision-makingprocess, the quality of the display system has important effects on overall systemperformance and the ability of the observer to make an accurate diagnosis.

Current display offerings for diagnostic radiology systems are based on two com-peting technologies, the cathode-ray tube (CRT) and the active-matrix liquid crystaldisplay (AMLCD). The CRT is a half-century-old mature technology that is based onthe excitation of cathodoluminescent phosphors by focused energetic electron beams.Light is generated in an emissive structure, where it diffuses in a controlled manneruntil it emerges toward the viewer, forming the displayed image. The AMLCD, whichis based on active-matrix liquid crystal (LC) modulators, is the result of 25 years ofengineering advances.

Display image quality can be defined as the relationship between the informationcontained in the image and the information conveyed to the observer through a lu-minance field. When all available information is transferred, the display system isconsidered to provide full fidelity. For applications with high information content(eg, diagnostic radiography), this situation is never found in reality. Display systemsalways degrade the information content of the image because of limitations in manyareas. On the other hand, when the information conveyed matches the limitations ofthe visual system of the observer, the display system can be defined as a high-fidelitysystem, even when it fails to convey image information that is beyond human visualcapabilities (1).

This chapter introduces the design principles and components of modern mono-chrome medical devices used for the display of digital radiographs. The chapter beginsby reviewing basic technologic features of CRT and AMLCD devices, with an emphasison aspects that affect the quality of the displayed image. The chapter then describeskey trade-offs in display design and their relationship to image quality. Finally, the

Advances in Digital Radiography: RSNA Categorical Course in Diagnostic Radiology Physics 2003; pp 91–102.

1From the Medical Imaging and Computer Applications Branch, Office of Science and Technology, Center for Devicesand Radiological Health, Food and Drug Administration, 12720 Twinbrook Pkwy, HFZ-142, Rockville, MD 20857 (e-mail:agb@cdrh.fda.gov).

The mention of commercial products herein is not to be construed as either an actual or implied endorsement of suchproducts by the U.S. Department of Health and Human Services. This chapter is a contribution of the Food and DrugAdministration and is not subject to copyright.

Aldo Badano, PhD

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chapter reviews some of the long list of display speci-fications that need to be considered in comparingdevices for specific applications.

BASIC COMPONENTS OF MEDICAL CRTs

CRT technology has matured for more than 100 years—from the discovery of luminescent “phosphor” mate-rials in 1630 by Vincenzo Cascariolo, an Italian shoe-maker and alchemist, until the recent development ofcomplex electron optics for beam focusing. The firstcommercial CRTs were produced in the early 1920s,but not until the 1940s did massive numbers of unitsbecome available as consumer products (2).

The CRT is a cathodoluminescent display: light isgenerated by exciting a luminescent material with en-ergetic electrons. An electron gun located in the backof the device emits an energetic beam that strikes aphosphor screen within a small spot steered in a rasterscan by magnetic deflection coils. Finally, a cathodo-luminescent phosphor converts electron energy intolight. The beam current is modulated to cause vary-ing brightness. The electron beam travels in a vacu-um region contained by a glass bulb made out ofthick glass to reduce the mechanical stress. A 74-cm-diagonal bulb with a relatively flat faceplate requiresglass thickness of about 13 mm. Figure 1 shows themain components of a typical CRT.

Electron Beam

Electrons are generated by a resistive heater thatpromotes thermal emission from low-surface-poten-tial materials with temperatures of about 600°C. Thestability of the emission is ensured in modern high-performance CRTs by using dispenser cathodes thatconsist of a porous pellet impregnated with emissiveoxide material. Dispenser cathodes can achieve highercurrent density with longer lifetime and better stabil-ity than conventional oxide cathodes because of thereplenishment of the oxide material. In addition, dis-penser cathodes have better aging characteristics, withonly about 1% loss in emission for every 1,000 hours.

The electron beam generated at the cathode is accel-erated and focused by a series of electrostatic Einzel-type lenses that form the electron gun. The beam isthen directed at a particular spot in the emissive screenby a deflection yoke situated at the exit of the electrongun. The yoke is an external device designed to fitclosely the shape of the glass bulb. It generates mag-netic fields responsible for the beam deflection. Theelectron beam then traverses the remaining field-freevacuum space until it hits the front screen. For large-screen CRTs, the degradation of the focus at largeangle deflections is controlled with dynamic focusingprovided by special yoke designs.

Emissive Structure

A key component of the CRT that markedly affectsits image quality is the emissive structure, which con-sists of all of the elements responsible for the genera-tion and delivery of light. Emissive structures varygreatly according to the type of CRT. In general, theyconsist of a conductive coating (normally a thin alu-minum overcoat), a cathodoluminescent phosphor(3), a black-matrix layer, a glass faceplate, and some-times an antireflective coating. A smooth, continuous,and highly reflective submicrometer layer of alumi-num is overlaid on top of the phosphor to conductthe incoming electron current and maximize lightoutput toward the viewer.

Cathodoluminescent phosphors are deposited ontoa glass faceplate panel as a powder layer by using asedimentation technique. The choice of phosphor isan important element to consider when comparingdifferent monitors. Typically, two alternatives exist:single-component phosphors (eg, P45) and blendedphosphors (eg, P104). These two phosphors differ inmany aspects. First, the luminous efficiency of a P104screen (ie, the percentage of luminance comparedwith that of a standard phosphor [P4] under specifiedconditions of beam current, high voltage, and face-plate transmission) is about 54% higher than that of aP45 screen. This implies that for the same tube con-figuration, the maximum luminance of a CRT with

Figure 1. Components of atypical medical CRT. The glassbulb contains the electron gun,while the deflection coils fitsnugly outside the neck of thetube. Drawing at right showselements involved in the gen-eration and transport of light.Energetic electrons excite thecathodoluminescent phosphor,generating light that scattersmultiple times before it emergesto form the image on the screen.AR = antireflective. (Adapted,with permission, from refer-ence 40.)

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P104 is higher than that obtained with a P45 phos-phor. On the other hand, P104 phosphor is a mixtureof grains of different color, which causes a granularappearance and affects the perceived image noise.P45 is a single-component phosphor and has re-duced granularity relative to P104 (Fig 2). The differ-ence in the luminance noise levels of P45 and P104phosphors has been documented by Muka et al (4)with measurements of noise power spectra.

These figures come into play when one considersthe long-term use of the display and the aging causedby coulomb loading, a term used to represent theamount of electron energy deposited in the phos-phor grains. Phosphors degrade over time because ofmaterial changes in regions of high electron bom-bardment and high current density. The correspond-ing decrease in brightness that occurs needs to becorrected for over the useful lifetime of the monitor

by increasing the beam current. The output lumi-nance of a P104 phosphor screen decreases fasterthan the output of a P45 screen with a given currentdensity. In other words, the maximum luminance ofa CRT with a P45 phosphor is more stable and needsless adjustment of the electron beam current duringthe lifetime of the monitor.

The glass faceplate of a medical CRT may absorb asmuch as 70% of the direct light to improve contrastand may have a rough surface on the vacuum side toreduce specular reflections. The absorbing faceplatestrongly reduces veiling glare caused by optical scat-tering (5) and also reduces reflections of ambientlight. If we assume a transmission of 30%, the diffusereflections are reduced to no more than 9%, resultingin improved black levels. Meanwhile, the displaybrightness decreases only to 30% (Fig 3). Good-qual-ity medical monitors have a thin-film surface coating,which provides three benefits: (a) conduction (toeliminate static charge and reduce dust collection),(b) abrasion resistance, and (c) antireflective proper-ties. Antireflective coatings have also been shown toreduce veiling glare in CRTs (6). These coatings gener-ally have many thin-film layers and are often lami-nated as added glass thickness to the display surface.Current multilayer designs are effective in reducingthe specular component of the display reflectance,without sacrificing brightness or adding unwantedcolor shifts.

Color CRTs

Although ubiquitous for desktop applications, colorCRTs typically have a lower display image qualitythan monochrome CRTs with similar electron opticsdesign. Color CRTs differ markedly in their emissivestructure. In any of the current main design alterna-tives (shadow mask or aperture grille), the emissivestructure contains a black layer known as the “blackmatrix,” which separates the red-green-blue phosphordots that form an arrangement of color dots or stripesfor luminance and chromatic contrast. In addition toincreasing the degradation in contrast by veiling glare,the light- and electron-scattering processes that takeplace within the emissive structure degrade color satu-ration. Color purity is obtained by increasing opticalabsorption in the emissive structure and by reducingelectronic glare by using low-backscattering materialsas mask coatings (7–9).

Incorrect beam landing is also a major concern incolor CRTs because of the presence of the mask or grille.If this occurs, that is, if the center of the electron beamdoes not fall in line with the center of the phosphor dot,color purity is degraded. In these designs, alignment ofthe electron beam with the openings of mask or aper-ture grille is paramount, and color electron guns oftencompensate for aberrations and space-charge effects thatwould degrade the convergence of the beam.

a. b.Figure 2. Photographs of (a) P104 and (b) P45 CRT screensshowing the difference in noise appearance caused by granu-larity. (Adapted, with permission, from reference 41.)

Figure 3. Transmission through the faceplate of medical moni-tors is typically 0.2–0.5 to reduce reflections from ambient lights.Assuming a transmission of 0.3, diffuse reflections are reducedto 9%, resulting in improved black levels. The display brightnessis only reduced to 30%. Absorption also reduces veiling glare bydampening the scattering within the faceplate. (Reprinted, withpermission, from reference 41.)

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IMAGE QUALITY TOPICS IN MEDICAL CRTs

The CRT Pixel

Many image quality aspects of CRTs are deter-mined by the way the pixel luminance is generatedthrough electron bombardment of a cathodolumi-nescent screen. The position of the electron beam acrossthe display screen is controlled by the horizontal andvertical deflection amplifiers. When an image is dis-played, the scanning electron beam is required tomodulate its intensity according to the gray-scale val-ues representing the image. If there are large changesin image values (which will be translated into largechanges in beam current and luminance output), theelectronics should be capable of modulating the beamwith a time constant smaller than the time neededfor the beam to excite the phosphor at that pixel lo-cation. Therefore, the bandwidth requirements ofsignal amplifiers depend on the pixel array size.

The electron spot size is defined typically as thewidth at 50% of the maximum. At low luminance,CRT spot sizes vary from 0.15 to 0.20 mm. The largebeam current needed to generate higher luminancecauses a larger spot size (0.15–0.30 mm) because ofthe divergence of the beam caused by electrostatic re-pulsion. The width at 5% of the maximum is typicallyabout twice the width at 50%. The spot size is notconstant across the screen but increases at the edgesrelative to the center. To achieve uniform spot sizes, adynamic focus adjustment with the use of deflectioninformation can greatly improve the resolution uni-formity of the monitor. Table 1 shows typical spotsizes for different screen configurations.

Factors Affecting CRT Contrast

One important performance issue associated withimage quality in display systems is the ability of theCRT device to achieve a large value for the small-spotcontrast ratio (10). This capability ensures that darkareas of the screen with subtle image features are notaffected by brighter areas elsewhere in the screen. Thesmall-spot contrast of CRTs is dominated by veilingglare and ambient light reflections (discussed later inthe “Display Reflectance” section).

Veiling glare in display devices is commonly associ-ated with the multiple light-scattering processes thattake place in the emissive structures of CRTs, causinga contrast reduction most marked in low-luminanceregions surrounded by bright areas. Although veilingglare is typically associated with optical scattering orlight diffusion, other sources of veiling glare (lightleakage and electron backscattering) are less knownand also merit a detailed description (Fig 4).

To reduce veiling glare, high-performance mono-chrome and color CRTs typically have an absorptivefaceplate that reduces the brightness. The contributionto veiling glare caused by light-transport processes

within the emissive structure depends on the relativelocation of dark and bright regions in an image.Therefore, its effect is determined by the spatial lumi-nance distribution of each image scene. Conversely,the other two mechanisms that contribute to glare(light leakage and electron backscattering) cause abackground signal that is approximately uniformthroughout the entire display surface.

The reflectivity of aluminum backing films used inCRTs is typically greater than 90%. The transmittedlight will scatter off the walls of the bulb and mayeventually come back and exit through the faceplate,adding a uniform undesired background to the image.This light leakage has been recognized and used aspart of an experimental method to determine the alu-minum layer thickness (11) and also for adjusting thedisplay curve according to illuminance measurementsmade inside the CRT bulb. When the light intensitytransmitted through the aluminum film amounts to10%, a uniform bright field will be contaminated byan additional constant luminance of 5 × 10−4 timesthe bright field intensity. This figure assumes that 90%of the light is absorbed after all scattering events in thewalls of the tube. Typically, coatings for the inside sur-faces of CRT bulbs are carbon-based absorptive mate-rials, although metallic coatings containing copper orsilver are also used in certain applications. If a smalldark spot is placed in the center of an image at a lumi-nance level of 1% of the bright field, its physical con-trast will decrease from 99 to 94. In addition, the thinaluminum coating that covers the phosphor layer mayhave small cracks or holes that will allow more lightgenerated in the phosphor to escape toward the vac-uum cell, further decreasing the contrast.

In addition to the optical component, an impor-tant component of glare is caused by electron back-scattering. The reduction in contrast caused by back-scattered electrons has been studied for fluorescentscreens (12) and for scanning electron microscopes(13). The contrast loss caused by electronic backscat-tering in color tubes has been reported to be as muchas 98% of the total glare degradation (7,9). The con-trast ratio of 10 × 10-cm black squares can be in-

Table 1Pixel Sizes for Two CRT Screen Sizes

Pixel Size (mm)

No. of Array 300 × 400-mm 270 × 330-mmMegapixels Size Screen Screen

1 900 ×.1,100 0.35 0.302 1,200 ×.1,600 0.25 0.215 2,000 ×.2,700 0.15 0.13

Note.—The calculation assumes a 50% overlap betweenadjacent Gaussian-like spots.

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creased by a factor of about 10 through careful selec-tion of coating material and thickness. The absenceof a shadow mask in monochrome tubes results in alower backscattered fraction because all of the elec-trons hit the aluminum conductive coating andphosphor layer.

AMLCDs

The worldwide market for displays has evolved rap-idly during the past several years. Panel sizes and reso-lution have increased considerably, while prices havedecreased.

As opposed to the CRT emissive technology, AMLCDsare light-modulating devices that form the image onthe screen by controlling the transparency of indi-

vidual display pixels. This technology, in its basic ar-rangement, suffers from marked variations in lumi-nance and contrast depending on the viewing angle.However, during the past 10 years, LCD designswith a more uniform luminance and contrast profilewithin a larger viewing angle cone have been intro-duced for radiology. AMLCDs consist of a stack of lay-ers, each serving a particular purpose. Figure 5 showsa cross section of a typical medical AMLCD consistingof a backlight, polarizer and color filters, back andfront plates, and LC cells (14,15).

The LC Cell

Liquid crystal is an intermediate state of matter(16) that exhibits properties typical of solids (ie, acrystalline structure with a highly ordered molecular

Figure 5. Cross section of anAMLCD with in-plane switch-ing pixel design. LC moleculesrotate under the influence ofthe electric field but always re-main in the display plane. Thisarrangement improves theviewing angle performancebut sacrifices brightness,which can be compensated forwith a more powerful back-light. The TFT is the switchingelement for the addressing ofeach pixel. a-Si:H = hydroge-nated amorphous silicon, ITO =indium tin oxide, E = electricfield. (Adapted, with permis-sion, from reference 40.)

Figure 4. Schematic repre-sentation of the three sourcesof veiling glare in CRTs: lightdiffusion, light leakage, andelectron backscattering. AR =antireflective. (Reprinted, withpermission, from reference41.)

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arrangement) as well as properties associated withliquids (ie, viscosity). LC materials are typically longorganic molecules with multiple unsaturated bonds.Because of the corresponding charge delocalization,molecules are electrically polarized, forming strongdipoles that tend to orient themselves along a mainaxis (called the director), forming a unique spatialconfiguration determined by elasticity, viscosity, anddeformation constants. This three-dimensional ar-rangement of the LC molecules leads to anisotropy, acharacteristic defined as the dependence of the mate-rial properties on the direction along which theproperty is measured.

To modulate light transmission through the LC, theorientation of the LC molecules must be controlled.When LC molecules encounter a textured surface, theyalign parallel to the grooves. The fundamental discov-ery that led to display applications of LCs (14,15) isthat the orientation of the director can be altered byan external electric field. When the director is twisted,light polarization also twists as it passes through thecell because of the birefringence of the LC layer. Thewavelength dependence of this effect leads to slightlycolored panels when a broad-spectrum backlight isused.

With the help of polarizer films that allow trans-mission of light when the polarization vector andthe axis of the film are aligned, LC cells can be de-signed to transmit or block light, depending on theorientation of the director, which is controlled by theapplied pixel voltage. When the twist in the LC direc-tor and the configuration of top and bottom polarizerfilms are such that light is fully transmitted with noapplied voltage, the arrangement is called “normallywhite.” When light is blocked in these conditions, a“normally black” design results. In addition to thetop and bottom substrates, LCD pixel structures re-quire alignment layers, polarizer films, and electrodes.The gap between the substrates (on the order of a fewmicrometers) is maintained by spherical glass beadsthat act as spacers.

Because of the multitude of elements that lightneeds to go through before generating an image inthe front screen, LCDs are intrinsically inefficient de-vices. Typically, only 3%–5% of the total light gener-ated by the backlight is seen at the front face of colorLCDs. This fraction is higher for monochrome devices(on the order of 8%–15%) because of the lack of ab-sorption in the color filters. Given the transmissionof typical LC stacks, a highly efficient backlight is re-quired. A backlight consists of one or many multiple-phosphor lamps, a reflector, and a diffuser. The criti-cal design parameters are compactness, uniformity,efficiency, and lifetime. In LCD applications, bothbehind-the-panel (brighter) designs and on-edge(more uniform and thinner) designs have been de-veloped. Another way to increase efficiency of the LC

transmission is by employing high-quality polarizerfilms. These are typically iodine-doped polymerfilms stretched in one direction. Finally, a majorcause of luminance loss in color LCDs is the red-green-blue filters used to obtain full color. Typically,each pixel is divided into three subpixels with a red,green, or blue filter on top. The color filters can beformed by patterning and dying resin deposits, bypigment impregnation, or by printing.

The Active Matrix

The control of the pixel luminance is achieved bycontrolling the voltage at each individual pixel. Thehigh-resolution displays used in diagnostic radiol-ogy, with large numbers of rows and columns (highpixel density), require active addressing methodswith a matrix or array of nearly ideal switches (fasttransition from the “black” state to full pixel trans-mission) to allow faster and more accurate control ofthe pixel luminance. The term “active” refers to theability to control each pixel in the array, as opposedto passive addressing, in which pixels are controlleda row or column at a time. In AMLCDs, the active el-ement is usually a thin-film transistor (TFT). Themost commonly used TFT technology for AMLCDs ishydrogenated amorphous silicon (a-Si:H) because ofits high mobility (0.6–1.5 cm2/V⋅s) and the reliabil-ity of the manufacturing process with large area sub-strates (17). A fundamental characteristic of the TFTdesign is the on-off current ratio; low-leakage cur-rents, which affect the “off” state, are needed forhigh-definition display systems (18).

The TFTs are usually located on one of the corners ofthe display pixel. Because an opaque coating shieldsthe TFT circuitry from the high illumination pro-duced by the backlight, light is not transmitted overthe area where the TFT is deposited. In addition, cer-tain pixel areas can have low light transmission (eg,metal electrodes). The fraction of the total pixel areathat allows transmission of light is called the “apertureratio.” In consumer product displays, the aperture ratiocan be as small as 50%, while in high-performance dis-plays, it can be as high as 80%. The aperture ratio af-fects the display power requirements and the controlof the luminance levels. For instance, in a 10.4-inchSVGA display, the power can be reduced by a factorof 0.57 because of an enhanced aperture ratio. Ahigher aperture ratio also increases the achievabledisplay contrast performance by reducing the nonac-tive regions or gaps of the display pixel.

IMAGE QUALITY TOPICS IN MEDICAL AMLCDs

The image quality of medical AMLCDs is affected bycertain factors not addressed in the evaluation ofCRTs. These factors include the gray-scale resolutionand the angular variation in luminance and contrast.

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Gray-Scale Resolution

Display systems for radiology comprise a display de-vice and a display controller. The specifications givenfor a system are valid only for that particular combina-tion. The accuracy of the gray-scale presentation is af-fected in part by the quality of the display controller.The digital-to-analog converter in the display controllerdetermines the ability to finely modify the shape of theluminance response. Conventional controllers with 8-bitdigital-to-analog converters have limited control overthe display gray-scale function. In medical AMLCDs, thegray-scale resolution is affected also by the intrinsicproperties of LC pixels, which are often limited to an8-bit scale in the luminance output. In this case, adeeper monitor gray scale can be achieved by subpixelmodulation or by temporal modulation. Subpixelmodulation uses the subpixel regions of AMLCDs, origi-nally designed for color applications, to generate alookup table that provides additional gray-scale resolu-tion (19). In temporally modulated AMLCDs, the actualpixel luminance is the combined luminance of two dis-tinct luminance levels in two or more consecutiveframes. Because the frame rate is high, human observerscannot discriminate between these two levels and there-fore experience an average pixel luminance.

Factors Affecting AMLCD Contrast

Because of the thin faceplate that AMLCDs com-monly have, these devices do not suffer from veiling

glare (20). However, in addition to ambient light re-flections, the small-spot contrast of medical AMLCDsis affected by cross talk and by the angular variationsof luminance with off-axis viewing angles.

Cross talk.—The effect of cross talk is seen as achange in the pixel luminance of the displayed imagein a region in which there is a considerable variationin gray level across the vertical or horizontal direc-tion. Cross talk is a general term used to describe twophenomena that degrade display contrast. On onehand, optical cross talk is generally a short-range ef-fect with a characteristic distance of less than 10 dis-play pixels and is negligible at longer pixel-to-pixeldistances. On the other hand, electronic cross talkhas complex spatial characteristics (21,22) that de-pend strongly on orientation (vertical vs horizontalaccording to the panel wiring scheme).

In AMLCDs, the voltage applied across the LC cellthrough the pixel electrodes defines the pixel lumi-nance. Electronic cross talk is associated with unwantedmodification of the pixel voltage effectively applied tothe LC cell caused by incomplete pixel charging, byleakage currents in the TFT, and by parasitic capacitivecoupling. Accordingly, display cross talk is more im-portant in large panels with high spatial and gray-scale resolution (23,24). Methods used for reducingelectronic cross talk employ modified driving tech-niques to bracket the desired voltage at each indi-vidual pixel in the active-matrix array.

Figure 6 shows small-spot contrast ratio measure-ments for a variety of medical CRTs and AMLCDs.For a spot size of 10 mm, the measured contrast ratiofor CRTs is lower than 150, while medical AMLCDscan achieve ratios of 800 because of the lack of veil-ing glare and controlled cross talk (Table 2).

Non-Lambertian emission.—Like most emissive dis-plays (25), CRTs emit light in such a way that the angu-lar luminous intensity approximately follows a cosinedistribution according to Lambert’s law. Consequently,

Table 2Small-Spot Contrast Ratio Measured for a 10-mm DarkSpot

Small-SpotDisplay Device Contrast Ratio

Medical AMLCD (Planar C3) (22,42) 750Medical CRT (Clinton DS2000) (10) 152Color AMLCD (Silicon Graphics SW1600) (10) 145Medical CRT (Siemens Simomed) (10) 141Medical CRT (Image Systems M24L) (43) 89Color CRT (Sony Trinitron Ultrascan) (10) 48Color CRT (Hitachi Megascan) (44) 25

Note.—Ratios were measured with a collimated luminanceprobe, according to methods described in references 5 and10. Measurements were performed with a circular spot forCRTs and with a square spot for AMLCDs.

Figure 6. Measurements of contrast ratio (CR) for CRTs andAMLCDs as a function of the dark spot size. The smallest spotsize that can be measured with this method is between 4 and 6mm, depending on the distance to the emission surface.

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the display luminance remains constant across all view-ing directions, which is typical of Lambertian emittersurfaces. That is not the case for AMLCDs. The lumi-nance and contrast of AMLCDs are a strong function ofthe viewing direction. In some AMLCDs, at large off-axis angles, the variations can be severe enough tocause an inversion of the gray scale, a condition that isgenerally unacceptable in diagnostic display devices.Figure 7 shows measured contrast variations for a med-ical AMLCD. In this example, the contrast reduction ex-perienced along the diagonal viewing directions is themost severe (a factor of 0.1 at 45°) (26).

The viewing angle problem in medical imaging canbe appreciated when one views different areas of alarge display screen (which can reach more than 30 cmon a side). In this scenario, the more severe changes inthe luminance presentation curve and available con-trast associated with different viewing directions arelikely to happen between the center and the corners ofthe screen (Figs 8, 9). The second aspect of this prob-lem arises when more than one individual is lookingat the same image displayed on the same screen. Inthis case, the variations can be much larger becausethe angles that are involved and the departure fromthe on-axis calibration are also larger.

Several solutions have been developed to compen-sate for angular variations in AMLCD display lumi-nance. The approaches come from a recognition thatthe anisotropy of the light modulation is the dominat-ing factor in defining the viewing angle characteristicsof the device. When light is emitted at different angleswith respect to the display surface normal, it has differ-ent effective path lengths through the LC. This condi-tion is most severe for intermediate gray levels, whereLC molecules in conventional modes are oriented ob-liquely with respect to the display surface. The solu-tions reviewed here belong to three different classes:compensation films, multiple domains, and modifiedLC alignments. In current AMLCD designs, many ifnot all of these classes are combined and employedin the same device. The benefits of using one of thesesolutions are compounded with the addition ofother solutions, but each solution has drawbacks.

Special birefringent films have been designed tocompensate for the anisotropy introduced by the LCalignment with respect to the different directions oflight transmitted by the LC cell (27,28). Because thefilms are static, in the sense of not dynamically adjust-able with the pixel gray level, the compensation is op-timal only for a single luminance level.

The viewing angle of AMLCDs can be markedlyimproved by dividing the pixel area into multiplesubpixel domains having different LC director orien-tations. Because each domain has an asymmetric re-sponse with respect to the viewing direction, the neteffect is an average emission that tends to reduce theluminance variations with angle (29,30). The main

challenge associated with this technology is the domainstability associated with a highly changing directororientation across the small subpixel dimensions. Inaddition, the fabrication cost increases considerablywith the number of domains because of the increasein processing steps.

If the LC director remains in the display plane forall gray-scale states, the asymmetry for the differentangles is minimized. This is the basis of the in-planeswitching structure used in many medical imagingAMLCDs. With in-plane switching, the pixel elec-trodes are located on the same bottom glass plate. Theswitching of the LC cell between on and off statescomes from the application of an electric field parallelto the glass plates (31,32). Although this improves theangular constancy of the luminance, it also reducesthe transmission through the LC stack caused by thepresence of interdigitated electrodes, resulting in alower luminance. Another design that improves theviewing angle performance is the vertically aligned LCmode. In a normally black mode, vertically alignedcells provide an almost perfect blockage of light whencrossed polarizers are used. This arrangement can beachieved by oblique electric fields with displacedelectrodes or, more commonly, with pyramid-shapedprotrusions on both substrate plates (33). Figure 10shows the arrangement of LC molecules in an in-plane-switching mode and a vertically aligned mode,along with the more conventional twisted-nematicmode used in low-grade AMLCDs.

DISPLAY REFLECTANCE

The reflections of ambient light from CRT devicescan be represented by the addition of a specular anda diffuse component (Fig 11) with different effectson the quality of the image displayed. More gener-ally, reflections have to include a third component

Figure 7. Contrast ratio measurements for a medical AMLCDat different off-axis angles from the display surface normal. Thecontrast ratio in this case is the ratio of maximum to minimum lu-minance with a 20% region in a midgray background. (Adapted,with permission, from reference 41.)

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called “haze,” which becomes important in flat-panel LC displays.

Because of the nature of CRT emissive structures, alarge fraction of the light that illuminates the deviceis reflected either at the first surface or after multipleinternal scattering. Light that enters the faceplate andstrikes the phosphor layer encounters a structure thatby design is highly reflective. The phosphor structureconsists of small grains in a binder with a reflectivebacking. Similar to radiographic screens, this struc-ture is designed for good light emission with littleself-absorption.

To dampen specular reflections, antireflective struc-tures are used that consist of several thin-film layers de-signed to reduce the reflectance of the front surface byincreasing the light transmission into the faceplate (34).

Figure 9. Angular changes of the luminance in medicalAMLCDs for on-axis luminance output and for 45° along thehorizontal (45°H) and diagonal (45°D) viewing directions.

Figure 8. Effect of viewingangle on luminance calibrationfunctions of AMLCDs for afixed centered observer. Theluminance output seen at rightfor on-axis viewing is distortedat the corners and at the edgeof the display screen. If a le-sion is present in these screenlocations, its detectability willbe different than if it were inthe center. (Adapted, with per-mission, from reference 41.)

Normally, antireflective coatings will also include aconductive layer that dissipates the static charge gen-erated at the front surface and helps maintain a dust-free surface (35). The reflections from antireflectivecoatings can have a color shift when illuminated with abroad-spectrum light source because of the wavelengthdependence of the thin-film response (36). However,by decreasing the reflection of incident light, antire-flective coatings may increase diffuse reflections be-cause more light enters the faceplate. The effectivenessof antireflective coatings is then associated with a com-promise between the specular and diffuse componentsof ambient light reflection.

The diffuse reflection of light adds an unstruc-tured constant luminance to the image, which re-duces the contrast in dark regions. To reduce diffusereflectance, medical CRTs have an absorptive face-plate that attenuates light, which scatters severaltimes in the glass. For a faceplate with a transmit-tance of 50%, the diffuse reflections will be reduced

Figure 10. Relative orientation of the LC molecules with re-spect to the display plane in the twisted-nematic (TN), in-planeswitching (IPS), and vertically aligned (VA) LC modes. Thealignment of the LC director with the display surface improvesthe viewing-angle characteristics of the in-plane switching andvertically aligned modes. E = electric field.

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to no more than 25% because the reflected lightwill travel through the glass twice. More reductionis typically found because of the oblique directionsin which the reflected light may travel and becauseof multiple internal scattering. However, this re-duction comes at the expense of a 50% decrease indisplay brightness. In color monitors, the black-matrix material that is between the phosphor dotsis of considerable benefit in absorbing incidentlight without reducing brightness. In this sense, thedesign of color monitors is advantageous from thestandpoint of both veiling glare and ambient re-flection. However, black-matrix phosphor technol-ogies have not been used to date with high-bright-ness monochrome phosphors.

Ideally, display devices are designed to absorb am-bient light. New flat-panel display devices offer op-portunities for the absorption of ambient light thatare not possible with CRTs. AMLCDs are being builtwith designs that optimize the absorption of ambi-ent light for use in sunlit environments, such as foravionic applications (37). Typical values for thespecular reflection coefficient of display devices varyfrom 0.0019 to 0.042. The coefficients measured forthe monitors without an antireflective coating areconsistent with the value computed according toFresnel equations for a glass-air optical boundary(0.04). Devices with an antireflective coating have asmaller specular reflection coefficient. Measured val-ues of the diffuse reflection coefficients all fallwithin a smaller range (0.018–0.064 cd/m2/lux),with the exception of advanced AMLCD designs foravionic applications.

For flat-panel systems, the surface reflections in-clude a local component. In this case, the completereflection characteristics of the device are best de-scribed by the bidirectional reflection distributionfunction (BRDF). The BRDF, a formalism oftenused in optics (38), is defined for any reflecting ob-ject as the ratio of differential reflected luminance(dLo) to the differential illuminance (dEi) incidenton the surface. Here, we consider the reflectancefrom the display to be shift invariant, or indepen-

dent of position across the screen, therefore ne-glecting all edge-related phenomena. The completeexpression is then given by a six-dimensional func-tion:

where λ is the photon wavelength, and p is the po-larization of the incoming light beam. The BRDF hasunits of inverse steradians (sr−1). The angle of inci-dence of ambient light is defined by (θi, φi), while(θo, φo) represents the angles that define the directionof reflected light.

The precise evaluation of this function is timeconsuming and costly. The BRDF can be measured(a) with a goniometric setup with fixed light sourceand variable detector, (b) with a variable sourceand fixed detector, or (c) with a conoscopic ap-proach in which the directional intensity ismapped into a two-dimensional distribution re-corded by a position-sensitive planar detector. The

Figure 11. Specular and dif-fuse reflections for a CRT. Thethick lines indicate the positionof the electron beam and theluminance that it generatesupon impinging on the phos-phor layer. The specular reflec-tions occur mostly at the frontsurface of the faceplate. Thereflective coating, designed pri-marily to increase the light out-put of the phosphor, also in-creases the diffuse componentof the display reflections.

Figure 12. One-dimensional reflection signature of the AMLCD.The value at 0.1° corresponds to the measurement at 0°, whichis within the specular window. Depending on the specific defini-tions used for the specular reflection, we can identify the “haze”window as expanding from 1° to 10°. The measurements forangles larger than 10° reflect the diffuse component. a.u. = arbi-trary units.

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first two methods suffer from severe dependence ofsource and detector positioning precision that hasto be better than 1°. The third method requires ex-pensive instrumentation. However, the intermedi-ate component of the reflection signature, which iscalled “haze,” can be characterized by taking incre-mental measurements of the reflected luminanceobtained with a small diffuse light source (5-mmdiameter). As the angle between the direction of themeasurement with the light-measuring device andthe specular direction increases, we can record oneslice of the full BRDF, as shown in Figure 12. Notethat in this experiment, the uncertainties in the po-sitioning of the source and meter were sufficient tomask the specular peak (between 0° and 1°).

DISPLAY NOISE

Noise sources in a display device can be cataloged indifferent ways (Table 3). For instance, we can dis-criminate sources of spatial noise (eg, CRT phos-

phor granularity) from sources of temporal noise(image lag or ghosting in AMLCDs). Spatial noisein a display device can obscure small low-contrastimage features. This is analogous to the effect ofx-ray quantum noise in planar radiography and incomputed tomography. The characteristics of thespatial noise in displays can be appreciated by us-ing a magnifier lens to view the light emission pat-tern from a region with uniform midgray bright-ness (Fig 2). In AMLCDs, the most notable featureof the noise characteristic is the subpixel structureof complex design used in medical displays, shownin Figure 13. This periodic structure introduceshigh-frequency components of the noise that havebeen shown to complicate the measurement ofnoise power spectra with conventional methods(39).

Acknowledgment: The author thanks the many col-laborators who have contributed to the material re-viewed in this chapter.

Table 3Fundamental Differences and Sources of Noise in CRT and AMLCD Technologies

Comparison CRT AMLCD

Mechanism Light emitting Light modulatingFront panel Curved or flat FlatArray formation Scanning beam Active-matrix addressingEmission Near-Lambertian Far from LambertianLong-range interactions Veiling glare Cross talkPixel structure Gaussian spot Rectangular (subpixel domains)Sources of noise

Fixed pattern (spatially random) Phosphor noise, scan nonuniformity LC nonuniformity, cell thicknessvariations, spacers

Structured noise Raster Nonactive pixel regionsArtifacts Deflection Cell voltage variation

Landing Backlight nonuniformityElectronics Electronics

Temporal Flicker Flicker and LC ghosting

Figure 13. Pixel structure fora dual-domain AMLCD. Indi-vidual display pixels consist ofsix subpixel regions in a chev-ron arrangement determined bythe dual domain and threecolor stripes. A simplifiedequivalent circuit is presentedat right, showing one TFT ac-tive switch per color subpixel.The domains are separated bya region in which the LC mol-ecule orientation changes dra-matically (disclination) with lowlight transmission.

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22. Martin S, Badano A, Kanicki J. Characterization of a highquality monochrome AM-LCD monitor for digital radiology.Proc SPIE 2002; 4681:293–304.

23. Libsch FR, Lien A. A compensation driving method for re-ducing crosstalk in XGA and higher-resolution TFT-LCDs.Proc Soc Inf Displ 1995; 26:253–256.

24. Libsch FR, Lien A. Understanding crosstalk in high-resolu-tion color thin-film-transistor liquid crystal displays. IBM JRes Dev 1998; 42:467–479.

25. Lee SJ, Badano A, Kanicki J. Monte Carlo modeling of or-ganic polymer light-emitting devices on flexible plastic sub-strates. Proc SPIE 2003; 4800:156–163.

26. Badano A, Flynn MJ, Martin S, Kanicki J. Angular depen-dence of the luminance and contrast in medical monochromeliquid crystal displays. Med Phys 2003; 30:2602–2613.

27. Hoke CD, Mori H, Bos PJ. An ultra-wide-viewing angleSTN-LCD with a negative-birefringence compensation film.Proc Int Displ Res Conf 1997; 17:21–24.

28. Mori H, Bos PJ. Application of a negative birefringencefilm to various LCD modes. Proc Int Displ Res Conf 1997;17:M88–M97.

29. Nam MS, Wu JW, Choi YJ, et al. Wide-viewing-angle TFT-LCD with photo-aligned four-domain TN mode. Proc Soc InfDispl 1997; 28:933–936.

30. Chen J, Bos PJ, Bryant DR, et al. Four-domain TN-LCDfabricated by reverse rubbing or double evaporation. ProcSoc Inf Displ 1995; 26:868–868.

31. Masutani Y, Tahata S, Hayashi M, et al. Novel TFT-arraystructure for LCD monitors with in-plane switching mode.Proc Soc Inf Displ 1997; 28:15–18.

32. Wakemoto H, Asada S, Kato N, et al. An advanced in-plane switching mode TFT-LCD. Proc Soc Inf Displ 1997;26:929–932.

33. Ohmuro K, Kataoka S, Sasaki T, et al. Development of su-per-high-image-quality vertical-alignment-mode LCD. ProcSoc Inf Displ 1997; 28:845–848.

34. Tong HS, Prando G. Hygroscopic ion-induced antiglare/antistatic coating for CRT applications. Proc Soc Inf Displ1992; 23:514–517.

35. Ono Y, Ohtani Y, Hiratsuka K, et al. A new antireflectiveand antistatic double-layered coating for CRTs. Proc SocInf Displ 1992; 23:511–514.

36. Born M, Wolf E. Principles of optics. 3rd ed. Oxford,England: Pergamon, 1965.

37. Brinkley R, Xu G, Abileah A, et al. Wide-viewing-angleAMLCD optimized for gray-scale operation. Proc Soc InfDispl 1998; 29:471–474.

38. Elias M, Simonot L, Menu M. Bidirectional reflectance of adiffuse background covered by a partly absorbing layer. OptCommun 2001; 191:1–7.

39. Badano A, Drilling S, Imhoff B, Jennings RJ, Gagne RM,Muka E. Noise in flat-panel displays with sub-pixel struc-ture. Med Phys (in press).

40. Samei E, Siebert JA, Andriole K, et al. General guidelinesfor purchasing and acceptance testing of PACS equipment.RadioGraphics (in press).

41. Badano A. Display systems. RadioGraphics (in press).42. Martin S, Badano A, Kanicki J. High-resolution medical im-

aging AM-LCD: contrast performance evaluation. Proc IntDispl Res Conf 2002; 22:119–122.

43. Flynn MJ, Badano A. Image quality degradation by light scat-tering in display devices. J Digit Imaging 1999; 12:50–59.

References1. Flynn MJ, Kanicki J, Badano A, Eyler WR. High-fidelity

electronic display of digital radiographs. RadioGraphics1999; 19:1653–1669.

2. Keller PA. The cathode-ray tube: technology, history andapplications. New York, NY: Palisades, 1992.

3. Ozawa L. Cathodoluminescence: theory and applications.New York, NY: Kodansha, 1990.

4. Muka E, Mertelmeier T, Slone RM. Impact of phosphor lumi-nance noise on the specification of high-resolution CRT dis-plays for medical imaging. Proc SPIE 1997; 3031:210–221.

5. Badano A, Flynn MJ. A method for measuring veiling glarein high performance display devices. Appl Opt 2000; 39:2059–2066.

6. Badano A, Flynn MJ, Muka E, Compton K, Monsees T.Veiling glare point-spread function of medical imaging moni-tors. Proc SPIE 1999; 3658:458–467.

7. de Vries GC. Contrast-enhancement under low ambient illu-mination. Proc Soc Inf Displ 1995; 26:32–35.

8. van Oekel JJ. Improving the contrast of CRTs under lowambient illumination with a graphite coating. Proc Soc InfDispl 1995; 26:427–430.

9. van Oekel JJ, Severens MJ, Timmermans GMH, et al. Im-proving contrast and color saturation of CRTs by Al2O3shadow mask coating. Proc Soc Inf Displ 1997; 28:436–439.

10. Badano A, Flynn MJ, Kanicki J. Accurate small-spot lumi-nance measurements. Displays 2002; 23:177–182.

11. Ehemann GM, LaPeruta R, Garrity ER. Method of determin-ing the quality of an aluminized, luminescent screen for aCRT. U.S. patent 5,640,019, 1997.

12. Mansell JR, Woodhead AW. Contrast loss in image devicesdue to electrons back-scattered from the fluorescent screen.J Phys D Appl Phys 1983; 16:2269–2278.

13. Hasselbach F, Krauss HR. Backscattered electrons andtheir influence on contrast in the scanning electron micro-scope. Scanning Microsc 1988; 2:1947–1956.

14. Kaneko E. Liquid crystal TV display: principles and applica-tions of liquid crystal displays. Tokyo, Japan: KTK Scientific,1987.

15. Depp SW, Howard WE. Flat-panel displays. Sci Am 1993;3:90–97.

16. Collings PJ. Liquid crystals, nature’s delicate phase of mat-ter. Bristol, England: Hilger, 1990.

17. Chen CY, Kanicki J. High field-effect mobility a-Si:H TFTbased on high deposition-rate PECVD materials. IEEETrans Electr Dev 1996; 17:437–439.

18. Musa S. Active-matrix liquid-crystal displays. Sci Am 1997;277:87–92.

19. Flynn MJ, Compton K, Badano A. Luminance response cali-bration using multiple display channels. Proc SPIE 2001;4319:654–659.

20. Badano A, Flynn MJ. Monte Carlo modeling of the lumi-nance spread function in flat panel displays. Proc Int DisplRes Conf 1997; 17:382–385.

21. Badano A, Kanicki J. Characterization of crosstalk in high-resolution active-matrix liquid crystal displays for medicalimaging. Proc SPIE 2001; 4295:248–253.