outline characteristics and performance evaluation of lcds

1

Characteristics and PerformanceEvaluation of LCDs used for

Medical Image Display

Kenneth A. Fetterly, Ph.D.Mayo Clinic, Rochester, MN

AAPM 2007

Outline

I. ResourcesII. Technology overviewIII. Evaluation toolsIV. Fundamental characteristicsV. Matching the display to the taskVI. Assessing and maintaining quality

What’s not included

Cathode ray tube monitorsLCD with analog driver cardsMammography specific requirements

QC procedures vendor provided

Along the way…

“Medical Imaging Grade” (MIG) DisplaysProfessional DisplaysConsumer DisplaysColor and Grayscale LCDs

2

Along the way…

Assessment MethodsQuantitative

PhotometerColorimeterScientific CCD Camera

QualitativeTest PatternsTrained Eye

I. ResourcesAAPM TG18: Assessment of Display Performance

for Medical Imaging Systems (Online Report no.03)

NEMA DICOM, Part 14: Grayscale StandardDisplay Function

VESA: Flat Panel Display MeasurementsDIN V 6868-57: Image Quality Assurance in X-Ray

Diagnostics, Acceptance Testing for ImageDisplay Devices.

Peer review journals

II. LCD Technology: Optical Stack

Backl igh t

Rear Polarizer

Diffuser

LCD Panel

Color Filter

Front Polarize r

Image provided by J. Kofler, Ph.D.

Optical Theory


3

Element Structure


LCD Technology Types

Twisted Nemic (TN)Parallel Vertical Alignment (PVA)

Super PVA (S-PVA)

In-plane Switching (IPS)Super IPS (S-IPS)Horizontal IPS (H-IPS)Dual-domain IPS (DD-IPS)

Pixel Sub-Elements

Photos provided by Ehsan Samei, Ph.D.

III. Evaluation Tools

PhotometerLuminance

ColorimeterColor and luminance

CCD Camera2D Photometer

ProceduresAAPM TG18Vendor providedSite specific

Test PatternsCalibration SW

Vendor3rd party

4

Display Luminance

DefinitionDensity of luminous intensity emitted bya light source in any particular directionLight output, or brightness

UnitsCandela per square meter (cd/m2)Sometimes referred to as a “nit”

Photometer

Measures luminanceCandela per square meter (cd/m2)

Practical range0.1 to 1000 cd/m2 (5%)

Narrow acceptance angle1 to 5 degrees

Photometer

Measures luminanceUnits- cd/m2

Colorimetry

International Commission on Illumination (CIE)CIE 1931 (x, y, z)

Used for display specification

CIE 1976 (u’, v’)Useful for color matching and quality control

Color Temperature (K)3212

4'

+−=

xy

xu

3212

9'

+−=

xy

yv

5

1931 CIE Chromaticity Diagram

www.vision.ee.ethz.ch, accessed June, 2007.

1976 CIE Chromaticity Diagram

Photometers and Colorimeters CCD CameraHamamatsu Orca ER

Linear response, 12 bit GrayscaleMatrix 1344x1024Peltier cooled to 20° C below ambientRMS noise: 8 electronsDynamic Range 2250:1C-mount lenses

Nikon 180 mm, Fujitsu 4 mm (wide FOV)

Manual pixel value to luminance calibration

6

CCD Camera CCD Camera

Acts as a 2D photometerRequires calibration

Used to measureLuminance response

ResolutionNoiseViewing angle effects

IV. Fundamental CharacteristicsResolutionNoiseDead Pixels and ArtifactsLuminance propertiesAmbient light reflectionViewing angleColor toneHVS perception

Resolution: Pixel Matrix

3.10.1602560 x 20485 MP

2.40.2102048 x 15363 MP

1.90.2701600 x 12002 MP

Nyquist Limit(mm -1)

Pixel Size(mm)Matrix Size

7

Resolution: Optical Blur

0

500

1000

1500

2000

2500

3000

-0.1275

0.0000

0.1275

-0.1275

0.0000

0.1275

Pix

elV

alue

Horizontal Distance (mm)

Vertical Distance(mm)

p

Frequency (mm -1)

0 1 2 3 4

MT

F

0.0

0.2

0.4

0.6

0.8

1.0MTFCamera

MTFPixelMTFLCD

LCD Nyquis t Frequency

Visual Noise

Fundamental difference in quality of LCDs.Can affect diagnostic performance for

subtle findings.Not all displays provided by medical display

manufacturers are good, not all consumeror professional displays are bad.

Visual NoiseLow Visual Noise High Visual Noise

Dead Pixels and Artifacts

Size and numberWhole or partial pixelsDust and other contaminationMIG displays have specifications

LocationLocation in display influences importance

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Dead Pixels and Artifacts Luminance Properties

The luminance luminance vs. digital drivinglevel response function of LCDs is highlyvariable.

Consistent presentation of images requirescalibration to a standard based on thecontrast response of the human visualsystem (HVS).

Grayscale Calibration

DICOM Part 14 adopted the Barten model ofthe HVS response to luminance contrast asthe Grayscale Standard Display Function(GSDF).

Based on Just Noticeable Difference (JND)index.For a given background luminance, specifies the

change is luminance that is required forperception of a spatially varying sine wave.

Barten Model

Lightbox

Medical LCD

Consumer LCD

9

Luminance vs. DDL JND Index vs. DDL

Luminance Properties

400 to 5001.6 to 2.0

0.5 to 1.0100 to 200

Consumer

600 to 6502.3 to 2.6

0.5 to 1.0400 to 600

MedicalImaging

JNDsTotalper DDL

Lum (cd/m2)MinimumMaximum

Calibrating to the GSDF

Use a look up table (LUT) to map inputDDL (p-value) to desired luminance.

Display systems are 8 bit.256 shades of gray8 bit LUT would result in loss of

grayscale detail

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Spatial Dithering

Each LCD pixel has 3 subpixels.Can be used to create intermediate

luminance values.Results in (256 x 3) – 2 = 766 gray

levels for grayscale displayPotentially more unique gray levels for

color displays because colors are indifferent proportion.

JND/DDL vs. DDL

TG-18 QC Test Pattern

Is “QUALITY CONTROL”equally visible?

Is the grayscaleramp continuous?

Influence of Ambient Light

ReflectionFunction of the “optical stack,” including

protective panels, etc.Diffuse: adds to display luminanceSpecular: creates high luminance artifact

HVS AdaptationEye adapts to environment, not just display“Disability glare” model may be useful

11

Coefficient of Diffuse Reflection

Reflected Lum inance (cd/m 2)

<0.3

< 1

< 1.5

ReadingRoom

(0 to 50 lux)

0.006

0.02

0.03

DiffuseReflection *

(cd/m 2 per lux)

1.2

4

4 to 6

TypicalOffice

(200 lux)

6LCD

20CRT

20 to 30X-rayfilm

OperatingRoo m

(1000 lux)Surf ace

*Chawla and Samei, Med Phys, 2006

Influence of Ambient Light

Optimum level: 25 to 40 luxMcEntee et. al., SPIE 6164, 2006

7 JND lost compared to dark environmentof 400 to 600 available

Viewing Angle

Non-Lambertian surfaceLuminance and contrast (and color) change as

a function of viewing angle.

Lambertian non-Lambertian

Visual Influence of Viewing Angle

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Luminance vs. Viewing Angle

DDL = 255 DDL = 0

0

200

400

600

800

Lum

inan

ce(c

d/m

2)

Horizo

ntal

Vertical

0

2

4

6

8

10

12

14

16

Lum

inan

ce(c

d/m

2)

Horizo

ntal

Vertical

Axes Range: -40º to 40º

JND Contrast vs. Viewing Angle

DDL = 240 to 255 DDL = 0 to 15

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Per

ceiv

edC

ontr

ast(

JND

/DD

L)

Horizo

ntal

Vertical

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Per

ceiv

edC

ontr

ast(

JND

/DD

L)

Horizo

ntal

Vertical

Axes Range: -40º to 40º

Color Tone

Color tone differences between displayscan affect perception of contrastCan be distracting in a multi-monitor

workstationIncludes grayscale and color displays

Grayscale- changes in backlight color toneColor- tone controlled by calibration

Color Tone and Perception

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Color Just Noticeable Difference V. Matching the Display to the Task

Image vs. Display Matrix SizeImage vs. Display NoiseColor vs. GrayscaleViewing DistanceAmbient Light ConditionsDiagnostic or Case Review

Image vs. Display Matrix Size

Display of every image pixel is desirable.Especially MR, CT, US, …

For CR/DR, zoom allows full resolution

Need to consider screen layout.1 MR image per display- 1024x768 display

6 MR images per display- 1200x1600 display12 MR images per display- 1536x2048 display

Image vs. Display Noise

Image noise should dominate display noise.Avoid adding systematic degradation to theimaging chain.

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Image vs. Display NoiseMedical Imaging Consum er

Color vs. Grayscale

Some modalities use color in imagesUS, CT reconstruction, functional MR

What about grayscale images?A grayscale LCD is a color LCD w/o color filters

Grayscale vs. color monitor is irrelevant…as long as it is calibrated, low noise, etc

Grayscale monitors are brighterColor monitors are catching up

Viewing Distance

Affects display size and pixel pitchrequirements.Radiology Reading Room

Distance: 50 cm typicalLCD Size: 21”

Optimal or practical?Operating/Emergency Room

Distance: 50 cm to 3 m.LCD Size: 21” to 40”+

Ambient Light Conditions

Reading Room Recommended25-40 lux (McEntee et. al., 2006)

Standard Office200 to 400 lux

Operating Room800 to 1200 lux

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Display Reflectance

Inherently low for LCDs, but high for somewith protective cover panels

Reflected Luminance

Compromises dynamic range and low-luminance contrast


Recommendations

Keep ambient as low as possible given theclinical circumstancesUse indirect “soft” lightingUse spot lightingAvoid direct illumination of display

Avoid direct illumination of human eye

Diagnostic vs. Case Review

Diagnostic- “Medical Imaging Grade”Grayscale calibration

Low visual noiseAppropriate resolutionLuminance stabilizationFree from artifactsQC Schedule

Diagnostic vs. Case Review

Case ReviewAssumes that Radiology report is available

Grayscale calibration requiredConsider specific clinical use, may requireDiagnostic qualitySame considerations as Diagnostic, butperformance requirements may not be ashigh

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Grades of Displays

VariableVariableHighestImage Quality

NoNoYesQualityGuarantee

NoLimitedYesRemoteMonitoring

NoNoYesLumStablization

3rd partyYesYesCalibration

ConsumerProfessionalMedicalImaging

VI. Assessing and Maintaining Quality

AAPM TG-18Pre-purchase evaluationAcceptance TestingRoutine Quality ControlWarrantyPractical Examples

AAPM TG18 Report

Assessment of Display Performance forMedical Imaging SystemsCRTs and LCDs

Qualitative and Quantitative testsBasic to sophisticatedQuick to resource intensive

Tests to Ensure Quality

QualQualBothResolution

QualBothBothArtifacts

QualBothBothChromaticity

--QuantVeiling Glare

--BothVisual Noise

QualBothBothLum Uniformity

BothBothBothLum Response

--QuantReflection

XXXOn-hours

XXXCleaning

RoutineQC

AcceptanceTest

PurchaseEvaluation

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Routine QC

Remote Luminance MonitoringMonitors luminance via built-in photometer

As good as the built-in photometer

On-site EvaluationTG18- 1 to 3 months

Practical for LCDs- 3 to 6 months

Routine QC

Use clinical display softwareTests entire imaging system, not just displays

AdaptableCustomize QC process to accommodatetechnology and circumstances

Warranty

Understand the manufacturer’s warrantyAcceptance test against specificationsAre replacement displays new or used?Medical Imaging Grade LCDs

Performance based warranty, 3-5 years

Luminance, artifacts, etc.

Consumer DisplaysFunctional warranty, 1 year

Example 1:Evaluating Luminance Response

Grayscale ramp

Low-contrast targets

Grayscale ramp


Grayscale ramp


Qualit ative

5 min5 stepsRouti neQuality Cont rol

10 min18 stepsAcc eptanceTesting

4 Hours256 stepsPre-purchaseEvaluations

TimeRequiredQuantitative

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Example 2:Maintaining Maximum Luminance

Medical Imaging Grade LCDsCalibration is stored on the monitor.

Built-in photometer and feedback circuit tomaintains luminance.Front of screen and/or backlight meters.Luminance maintained to within 5%.GSDF calibration is stable.


“Professional” grade displaysCalibration is stored on the monitor.

No built-in photometer and feedback circuitto maintain luminance.Luminance falloff due to natural backlightdegradation.How should luminance be managed?


The planCalibrate the display to 200 cd/m2

“Overdrive” the luminance by 15%Allow luminance to decay to -15%How does this influence GSDF conformance?

Example 2: Influence of “Over” and“Under-Driving” Luminance

Dig ital Drivin g Level

0 64 128 192 256

Lu

min

ance

(cd

/m2 )

1

10

100

85% Luminance200 cd/m 2 GSDF115% Luminance

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Example 2: Influence of “Over” and“Under-Driving” on Contrast

Digita l Driv ing Level

0 64 128 192 256

Co

ntr

ast

(JN

D/D

DL

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

85% Lumina nce200 cd/m2 GSDF115% Luminance

Example 2: Luminance Changeover Time

100

150

200

250

300

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000

Display On-time (hours)

Max

imu

mL

um

inan

ce(c

d/m

2 )

Summary

Consult the literatureUnderstand how the display will be used

clinicallyUnderstand benefits and limitations of displaysConstruct a quality maintenance programCustomize quality maintenance program based

on clinical use and LCD properties

outline characteristics and performance evaluation of lcds

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