To Delivering High Quality Images in Medical Monitors.

toto

Delivering High Delivering High Quality ImagesQuality Images

in Medical Monitorsin Medical Monitors

Your Instructor & Sencore contact

Scott Reardon

1-800-SENCOREwww.sencore.com

Calibrating Medical MonitorsCalibrating Medical Monitors

• Founded 1951

• Located in Sioux Falls, SD

• Leading manufacturer of innovative electronic test & measurement equipment:

– Computer Monitor - high resolution computer monitor repair and calibration

– Digital Video - MPEG-2/ATSC, MPEG-2/DVB, CCIR601 Serial/Parallel, QAM, and HDTV

– High Definition TV - demonstration, testing and calibration applications

– Professional A/V - high-end video display calibration and sound system analysis, & Home Theater

– Medical - Safety Testers, Compliance Testers, and Analyzers, including SPO2 and Patient Simulators

– Cable - RF distribution including CATV, MATV, satellite antenna systems & QAM applications

– Broadcast - broadcast and studio settings, especially digital video and MPEG-2 applications

About SencoreAbout Sencore


Medical Monitors Need Calibration!Medical Monitors Need Calibration!• Critical diagnoses depends on correct

image display.

• Incorrect white balance causes false color imaging.

• Room lighting affects perceived color, brightness, and contrast.

• Overdriven CRTs cause picture distortion.

• Misadjusted User controls distort contrast and brightness ratios.

• Small picture flaws are magnified on high resolution displays.

• CRTs age - recheck calibration every 6 months

IntroductionIntroduction


• Displays are aligned on location after installation and moving is completed

• Picture is optimized for room lighting conditions • Reduce Doctor’s complaints about the display:

That it doesn't look right. The red of the tissue doesn't look right. The display is too dark or the display is too light. The picture is blurry.

• Assure the doctor that the image quality is correct

• Produces full range of accurate colors & grayscale

• Telemedicine - displays need to be calibrated to match what is happening on site

Why Perform Video Calibration?Why Perform Video Calibration?

IntroductionIntroduction


F1

F2

400-800V

Neg. bias

6.3v

25KV

Approx. 20-40% of HV

Video Review - Producing an ImageVideo Review - Producing an Image

• CRT gun (F1/F2, K, G1, G2) produces an electron beam

• Electron beam is accelerated by high voltage

• Phosphor screen gives off light when struck by electron beam

CRT OperationCRT Operation

Calibrating Medical MonitorsCalibrating Medical Monitors Video Review - Producing an ImageVideo Review - Producing an Image

• Changing bias (G1/K voltage) changes electron beam intensity

• Higher bias = less beam current; less bias = more beam current

• Less beam current = less light output; more beam current = more light output

High bias Medium bias Low bias

No light output

Medium light output

Maximum light output

How do video displays make B&W?How do video displays make B&W?

Calibrating Medical MonitorsCalibrating Medical Monitors Video Review - Producing an ImageVideo Review - Producing an Image

• White - all phosphor producing light

• Black - no phosphor producing light

• Contrast ratio - difference between black level and white level

• Ambient light affects contrast ratio


• Level of video signal corresponds to CRT gun conduction & brightness

• Brightness (luminance) = average DC level

• Contrast (difference between bright/dark portions) = peak-to-peak amplitude

• Video signal levels specified in IRE

Video Review - Producing an ImageVideo Review - Producing an Image


• Color CRTs use three electron guns

• Each gun is fed a separate video signal (RGB)

• Red + Green + Blue = white

How do video displays make color?How do video displays make color?

Video Review - Producing ColorVideo Review - Producing Color

Calibrating Medical MonitorsCalibrating Medical Monitors Video Review - Producing ColorVideo Review - Producing Color

• Changing bias of all three guns equally produces shades of gray

• Neutral gray is call White Balance

• Good color tracking maintains “neutral” gray at all luminance levels

High bias

Medium bias

Low bias

Medium light output

Maximum light output

No light output

Calibrating Medical MonitorsCalibrating Medical Monitors Video Review - Video SignalsVideo Review - Video Signals

Tuner Video IF VideoDetecter

RF Input

VideoBuffer

VideoDetecter

Y/CFilter

LumaDelay

Composite InputChroma/Luma

combined

Chroma

S-VideoChroma/Luma

Seperate

LumaAmps

ChromaAmps

ColorDemodular

Adder CRTColorMatrix

YUVSwitch

Y Pr Pb

RGBAMP

R G B


• Composite video contains luminance, chrominance, and sync on the same signal line.

• S-Video (Y/C) contains luminance and sync on one signal line, with chrominance on a second signal line.

Composite Video vs. S-Video (Y/C)

Video Review - Signal TypesVideo Review - Signal Types


Advantages of “S-video” connection over Advantages of “S-video” connection over composite (RCA) connector:composite (RCA) connector:

• Separates luminance information from color information to increase B&W resolution

• Fewer artifacts with modern video sources like DSS and DVD



Advantage of Component vs. “S-video”:Advantage of Component vs. “S-video”:

• Increased color resolution



HDTV uses component rather HDTV uses component rather than RGB:than RGB:

• Requires less bandwidth

• More content can be stored on a DVD or

broadcast.

Advantage of RGB over component:Advantage of RGB over component:

• RGB signals correspond directly to the CRT guns / light sources

• Provides best possible picture quality for analog CRT displays


Calibrating Medical MonitorsCalibrating Medical Monitors Video Review - Signal TypesVideo Review - Signal Types

Advantage of DVI vs. Component:

• DVI is digital equivalent of analog RGB

• DVI better for fixed pixel (non-CRT) displays connected to digital sources (DVD, digital cable set top converter, video games)

• Removes unnecessary analog-digital-analog conversion - enables pure digital signal to display

• DVI has bandwidth to address each pixel in a digital TV display individually - enabling the highest possible picture quality

• Component better for analog (CRT) displays - analog has theoretically better color and luminance range

Calibrating Medical MonitorsCalibrating Medical Monitors Video Review - FormatsVideo Review - Formats


• Measure of maximum number of light to dark transitions produced horizontally or vertically

• Horizontal resolution usually expressed in pixels

• Vertical resolution expressed in pixels or lines

Horizontal Pixels1

Ver

tica

l P

ixel

s (l

ines

)3

Horizontal Frequency

Vertical Sync

Horizontal Sync

2

Ver

tica

l F

req

uen

cy

4

Video Review - ResolutionVideo Review - Resolution


• Light is electromagnetic energy within a narrow range of frequencies

• Each different wavelength of light energy (if seen alone) is perceived by the human eye/brain as a different, fully saturated, color

Light & Color TheoryLight & Color Theory


• If light from the sun is equally reflected from a surface, the eye/brain sees the surface as white.

• If a surface absorbs all light energy and reflects none, the surface appears black.



• If more than one wavelength is reflected, the eye/brain performs a mixing of all light energy present and perceives a single color which is a result of the mixture.



• Eye is tri-stimulus - sees light using red, green, & blue receptors

• Each receptor has a different response to the light spectrum• CIE Standard Observer Response Curve shows average

response of each receptor across light spectrum

Human VisionHuman Vision


Three characteristics define Three characteristics define how our eye/brain sees light:how our eye/brain sees light:



• Hue: Dominant wavelength, color or tint of the color.



• Saturation: Degree of purity from light of other wavelengths

(zero saturation = white; equal energy of all wavelengths)



• Brightness: Perceived light energy level



Light Measurement Units:Light Measurement Units:

Luminance (brightness):• Footlambert: U.S. unit of luminance (radiated

light), 1 lumen per square foot• Nit (cd/m2): S.I. unit of luminance (radiated

light), 1 candela per square meter

Light & Color MeasurementLight & Color Measurement

Illuminance: ambient light that illuminates the display• contributes to luminance that is observed from the image display• reduces the contrast in the image• need to block from measurement


Light Measurement Units:Light Measurement Units:

Color (hue and saturation):• CIE chromaticity coordinates (x,y): From

1931 CIE Chromaticity Diagram (Kelly Chart)

Light & Color MeasurementLight & Color Measurement

What color?


• Graphically depicts the relationship between hue and saturation.

• Diagram shows the pure spectral colors around the curved border

• The results of mixing any of these spectral colors are shown at the base and center of the diagram.

CIE Chromaticity DiagramCIE Chromaticity Diagram

Color MeasurementColor Measurement


• By combining the proper mix of RGB

• All three guns are turned on to make white

How do video displays make white?How do video displays make white?



• C.I.E. coordinate x = 0.313 y = 0.329 (D) is the white color which was chosen as the standard white reference for all video display systems. (D6500)



• All three guns are turned off to make black.

• The same point on the chart represents white and black

How do video displays make black?How do video displays make black?



• Using RGB light sources

• Any three colors not lying on a straight line with one another are color primaries.

• The points shown are the NTSC specified CRT phosphor color primaries for US televisions.

• The connecting triangle encloses the range of colors able to be produced by a CRT using these color phosphors.

How do video displays make color?How do video displays make color?



• Color Temperature is sometimes also used to specify different near-white colors.

• Color Temperature is referenced to color of carbon when heated to different temperatures (measured in °Kelvin -- °Celsius plus 273).


Color TemperatureColor Temperature

Calibrating Medical MonitorsCalibrating Medical Monitors Color MeasurementColor Measurement

• Different “whites”correspond to different color temperatures• White sheet of paper looks different under different light• CRT radiates light so color temperature not affected by lighting

conditions (but brightness and contrast are)

Calibrating Medical MonitorsCalibrating Medical Monitors Color MeasurementColor Measurement

• There is an approximate correlation between color temperature and CIE chromaticity

• Accurate calibration is done using CIE coordinates


• Colors which are created by heating carbon form a continuous line across the CIE Chromaticity Diagram - known as the black body curve.

• Only colors exactly on the curve are specified by original absolute color temperature.



• Size

• Centering

• Pincushion

• Trapezoid

• Rotation

• Linearity

1. Calibrate Display Geometry1. Calibrate Display Geometry

Calibration OverviewCalibration Overview

Calibrating Medical MonitorsCalibrating Medical Monitors Calibration OverviewCalibration Overview

2. Calibrate Display Convergence2. Calibrate Display Convergence

• Precisely overlay red, green, and blue images at all points on display.

• Eliminate all color fringing effects.


3. Calibrate Display White Balance3. Calibrate Display White Balance• Use color analyzer

to measure “color” of white in center white window at high and low luminance levels

Calibration OverviewCalibration Overview

Calibrating Medical MonitorsCalibrating Medical Monitors Calibration OverviewCalibration Overview

• Brightness (Black Level)

• Contrast (White Level)

• Color (Saturation)

• Tint (Hue)

• Sharpness

4. Calibrate Display User Controls4. Calibrate Display User Controls


Introducing the Introducing the VP400 & 401 VP400 & 401 ‘VideoPro’‘VideoPro’

Video GeneratorVideo Generator

The VP400 & 401 ‘VideoPro’ Multimedia video generators deliver the HDTV, NTSC, RGB, DVI & RF Video Signals You Need for

Accurate Service & Alignments of Monitors & Direct View Displays.


CompositeS-

Video

Component/RGB VESA/HDTV

Battery operated(8 hours)


NTSC RF(VP401)

DVI(VP401)


SMPTE Bar

Check and adjust color level and hue controls. Check color demodulator accuracy.



Color Bar

Check for primary (red, green, blue) and secondary (cyan, magenta, yellow) colors to confirm proper chroma processing.



Focus

Check for proper static & dynamic focus operation at screen center & edges.



Crosshatch (4:3)

Check and adjust convergence and linearity.



Anamorphic

Check accuracy of widescreen stretch feature (from 4:3 to 16:9).



Overscan

Check and adjust display geometry, including picture centering, size, trapezoid (keystone) correction, pincushion (bow) correction, and linearity.


electrical center

5% overscan(green line)

10% overscan(red line)


Staircase

Check video amplifier linearity and gray scale tracking. Adjust G2 (screen) control to proper setting.



Horizontal Staircase

Check gray scale tracking of CRT projectors with red and blue overdrive on sides of screen.



Pluge (Picture Line-Up Generator Equipment)

Check for proper black level (brightness) and white level (contrast) setup. Includes alternating black/blacker-than-black section.

Alternating

Black

10 IRE



Needle

Check for Sweep Velocity Modulation (SVM) operation, high voltage regulation, and setting Contrast & Brightness on CRT displays.



Ramp

Visual check of grayscale tracking. Poor performance causes visual color tint at one or more light levels. Checks digitizing linearity of video signal processors. Poor performance causes vertical bands.



Window

Check chromaticity with color analyzer. Adjustable IRE level window with pluge bars to monitor black level.



Regulate

Check high voltage and scan current regulation.


Black / Whitealternating

1 pixel wide


Raster

Check purity & white balance. Includes adjustable 0-100 IRE levels. Color can be gated on/off to produce red, green, blue, cyan, magenta & yellow raster.



Checker

Check for regulation of low voltage supply to video amps and video ringing. Should see sharp vertical lines with no video ringing.



Multiburst

Check for ringing, smearing, and sharp resolution. Increase Sharpness control until all bursts are equal brightness without ghosting.



Sharpness


Align controls that artificially enhance picture transitions (contrast, picture, aperture, scan-velocity modulation). SVM - bottom center vertical lines straight & same width, set for least impact on width of black lines compared to white lines. Contrast/Picture/Aperture - sharp B/W transitions, white edges not blurred or blooming, no ghosting, clear, bright multiburst.

Straight, same width

Clear, same brightness

No blooming


• Different display types produce different light spectrums• Our eyes still see the same result• To provide accurate measurements Colorimeter must use

CIE standard observer filters that replicate eye response• What worked for CRTs doesn’t work for other display types

3 chip DLP Projector

CRT Projector

Direct View CRT

LCD Projector

Color CalibrationColor Calibration


• Colorimeter is a tri-stimulus measurement device• Filters and light sensors to separate light into R,G & B• Output voltages proportional to applied light



CP5000 Color Analyzer

• Accurate method to set white balance (set to industry specs for best video display performance)

• Objective test (measure light produced by display)

• Easy to use and interpret (x-y and RGB bar graphs)

• CP500x accurate for all display types

• NIST traceable

CP5001 ‘ColorPro’Color Analyzer

CP5000 ‘ColorPro’Color Analyzer



• Probe connects to PC’s serial or USB port or Pocket PC

• Select standard preset white references, or input custom setting

• Flexible refresh rate synchronization for testing at a fixed display format or to follow changing formats.

• Luminance measured in units of nits or footlamberts.


ColorPro Analyzer overview

Calibrating Medical MonitorsCalibrating Medical Monitors Color CalibrationColor Calibration

• Delta RGB analog bar graph - quick adjustment of cutoff and drive controls

• Change resolution for fine touchup adjustments

• Selectable color reference




• Chromaticity diagram display provides quick visualization of control interaction

• Center target defines selected white reference color coordinates

• Measurement cursor & color “steering lines” track control adjustments



• Calibration Report printout provides documentation & re-calibration reference

• Pre-and post-calibration performance easily documented

• Standard or extended form tracks display at high & low luminance, or at multiple luminance points (ISF)



Identifies when next

calibration is due

Shows before and after color temperatures

Documentation that display is optimized



Calibration Library- On-line library of digital service menu adjustment codes available to ColorPro owners to simplify white balance calibration without service literature.


Calibrating Medical MonitorsCalibrating Medical Monitors White Balance CalibrationWhite Balance Calibration

• Accurate image colors are only possible when white balance is correct & tracks accurately from black to white.

• White balance is produced at all luminance levels when:

1) RGB sources/CRT guns balanced/biased to produce desired white reference at darkest gray

2) RGB video amplifiers have correct gain & linearity to produce desired white reference at all luminance levels from black to gray to white

• Brightness control adjusts light level of the darkest parts of picture (should be called black level control)

• Contrast control adjusts light level of the brightest picture parts (should be called brightness control).

White Balance/Gray-Scale Tracking OverviewWhite Balance/Gray-Scale Tracking Overview


• Brightness & Contrast controls affect RGB output equally and do not affect the white balance of dark or bright picture portions

• If the display’s gray scale doesn’t track accurately adjusting brightness or contrast controls produces a noticeable color shift due to the luminance level change

Normal

Lower LuminancePoor tracking

Lower LuminanceGood tracking

White Balance/Gray-Scale Tracking Overview White Balance/Gray-Scale Tracking Overview - continued- continued


White Balance - preliminary setup

1. Display the Pluge, Window, or Staircase pattern.

White Balance CalibrationWhite Balance Calibration


Pluge Pattern

alternating 0/7.5 IRE

Black

10 IRE


2. With the contrast control set to approximately 3/4 of full scale, adjust the brightness control for proper display of black in the Pluge or Window pattern (for the Staircase pattern, make the first bar black and the second bar just barely visible).


White Balance - preliminary setup


3. Display the Needle pattern.

4. Adjust the contrast control for maximum brightness with no signs of blooming in the maximum white bar of the gray scale or raster distortion of the needle pulses.

White Balance - preliminary setup - CRT displays


White Balance - preliminary setup - Fixed pixel displays

3. Display the Lo-Hi Tracking pattern.

4. Adjust Contrast control as high as possible without white clipping or compression.

97.5 %100 %(white)

95 %


Color Tracking Adjustment Controls• Cutoff/Bias controls adjust color balance at low luminance levels.• Gain/Drive controls adjust color balance at high luminance levels.

Gain/Drive Controls

Cutoff/Bias Controls


White Balance


1. Determine the white reference/CIE chromaticity coordinates to which the display is to be adjusted.

Note: Most displays should be adjusted to the CIE D65 daylight standard (x = 0.313, y = 0.329). This is the NTSC and ATSC white reference standard.



2. Display a 20 or 30 IRE white window pattern on the monitor (1 lumen - just enough light to get a reading).

Note: If starting from scratch after a CRT or other video component replacement, preset the display’s drive (gain) controls for maximum brightness and the cutoff (bias) controls for minimum brightness.



3. Leaving the cutoff control for the initially strongest color (as viewed on the dim CRT) at its original or preset level, adjust the other two cutoff controls to obtain color balance at the desired white reference.



4. Select a 80-100 IRE level window pattern.

5. Leave the drive control for the initially weakest color (as viewed on the bright CRT) at its original or preset level, and adjust the other two drive controls to again obtain color balance at the desired white reference.



6. Repeat steps 2-5 until the display’s color temperature remains relatively constant (tracks) over the full range of IRE levels or contrast control adjustment.


Calibrating Medical MonitorsCalibrating Medical Monitors Calibrating User ControlsCalibrating User Controls

When the color temperature is calibrated perform final adjustment of the User controls at typical ambient lighting.

CRT Displays

1. Brightness (black Level)

2. Sharpness/Enhancement

3. Contrast (white Level )

4. Color

5. Hue/Tint

Fixed Pixel Displays

1. Brightness & Contrast

2. Sharpness/Enhancement

3. Color

4. Hue/Tint


• Brightness control adjusts light level of the back or darkest luminance portions of the picture.

• If set too dark:

– subtle dark gray details of the scene are lost to black

• If set too bright:

– darkest grays and deep blacks are all a lighter gray, which effectively lowers the contrast ratio of the display and reduces picture quality

Brightness ControlBrightness Control


• Contrast control adjusts light level of the white or high luminance portions of a scene.

• Sometimes labeled picture control - should be labeled white level control

• If set too low:

– picture image is dim, whites become dull & image loses its luster because the contrast ratio is too low.

• If set too high (this is often the factory setting):

– power supply may be overdriven causing raster distortion

– may cause pixel blooming (occurs when the screen pixels are struck so hard that light spreads to the adjacent pixels, defocusing the white image)

Contrast ControlContrast Control


• Artificially enhance picture transitions between black & white

• Recommend turning SVM off

Enhancement Controls -Enhancement Controls - (Sharpness, Picture, Aperture, Scan Velocity Modulation)


Brightness Control - Brightness Control - CRT displays

1. Select VP300 Pluge pattern.

2. Watch second box from the inside (alternating between 7.5 IRE/black & 0 IRE/blacker-than-black).

3. Adjust the control until the 7.5 IRE & 0 IRE levels appear the same and no flashing can be seen. The

outer 10 IRE box should still be slightly visible.

NTSC

0 /7.5 IRE

7.5 IRE

10 IRE

7.5 IRE

HDTV

0 /- 4 IRE

0 IRE

10 IRE

0 IRE

Calibrating User ControlsCalibrating User Controls


Brightness Control - Brightness Control - Fixed Pixel displays

1. Select VideoPro’s Lo-Hi Tracking pattern.

2. Increase brightness to see black (-4 IRE) vertical bar, then decrease until bar just disappears.

0 %(black)

- 4IRE(blacker

thanblack)

97.5 %100 %(white)

95 %


Brightness Control - Brightness Control - Fixed Pixel displays

3. Increase the Brightness control if needed to make 2.5% and 5% boxes both visible.

4. Set contrast so 95% and 97.5% boxes are visible.

0 %(black)

- 4IRE(blacker

thanblack)

97.5 %100 %(white)

95 %

5 %

2.5 %


Sharpness/Enhancement Controls - Sharpness/Enhancement Controls - Fixed Pixel & CRT displays

Straight, same width,

no ghosting

1. Select the VideoPro’s Sharpness pattern.

2. Adjust sharpness control while watching the bottom vertical lines. Set for best balance between ghosting and poor focus.• too high - faint ghosting beside line• too low - appears out of focus and lost resolution


Sharpness/Enhancement Controls - Sharpness/Enhancement Controls - Fixed Pixel & CRT displays

3. If the top multiburst section becomes too dull or unclear increase the Sharpness control for the best compromise

Clear, same brightness


1. Select VideoPro’s Needle Pulse pattern.

2. Watch the vertical lines as you adjust the contrast:

•top white bar is same width as 4 gray bars below

•vertical lines are straight.

•If contrast is too high, needle pulses will bend & boxes bloom

3. Recheck Brightness control setting and readjust if needed.

Contrast Control Contrast Control - CRT Displays

same width(no

blooming)Straight(no raster distortion)


Color Level ControlColor Level Control

1. Select SMPTE Color Bars pattern.

2. View pattern with blue filter, paying attention to large top outside blue & white bars, and small bars below them.

3. Adjust Color control to make these bars appear to have the same intensity (brightness) level.

4. Remove blue filter and view yellow & red bars. They should be very colorful without blooming into adjacent bars.

Top Outside Bar

Small Center Bar

Small Center Bar

Large Outside Bar



Tint ControlTint Control

1. Select SMPTE Color Bars pattern.

2. View pattern with blue filter, paying attention to the Cyan/Magenta bars that are located above and below one another.

3. Adjust Tint/Hue control to make these bars appear to have the same intensity (brightness) level.

Magenta

Magenta

Cyan

Cyan



Decoder Adjust

Color, Hue and Decoder/matrixColor, Hue and Decoder/matrix

• Use if display has 2 User Controls and color Service Adjustments (RYR/red gain, RYB/red hue, GYR/green gain, GYB/green hue)

• Set color decoder/matrix circuits for accurate color.


1. View color primary through color filter & adjust chroma gain to match intensity of white bar and surrounding color.

Blue section: set “User Menu” Color & Hue controls.Red & green sections: set “Service controls” (RYR/red gain, RYB/red hue,

GYR/green gain, GYB/green hue)

2. Adjust chroma hue to match intensity of secondary color bars in each color block.

75% white reference

Primary color sections

Secondarycolor bars


Benefits of a Properly Calibrated Video Display:Benefits of a Properly Calibrated Video Display:

Displays a sharper-focused, full resolution image. Shows full detail in both the darkest & brightest parts of all scenes. Is properly matched to the viewing environment. Produces a full range of accurate colors, including flesh tones. Minimizes picture artifacts (distortions) Satisfied user

Calibrating Medical MonitorsCalibrating Medical Monitors DICOM OverviewDICOM Overview

A trauma patient is admitted at an outpatient clinic or rural clinic at a remote location. Several X-rays are taken and sent to a hospital for consultation.

Imagine the following:

Calibrating Medical MonitorsCalibrating Medical Monitors DICOM OverviewDICOM Overview

How can we make sure that what the physician sees at the clinic matches what is being displayed on the radiologist softcopy workstation at the hospital?

???

?

Calibrating Medical MonitorsCalibrating Medical Monitors DICOM - OverviewDICOM - Overview

The image is also sent to another radiologist for a second opinion. This radiologist has a diagnostic viewing station from another vendor - a different monitor and video display card. As a matter of fact, he has a new flat panel display instead of a conventional CRT.

How do we make sure that the image again looks identical?

???

? ?


• Everyone misses something among the myriads of pixels that are tape archived for colleagues and lawyers to examine with 20/20 hindsight vision.

• Perhaps the missed findings were apparent in the image data, but a once-new PACS monitor had lost its ability to display those subtle differences.

• How can you know? What will you say when they ask what steps you took to assure that the display equipment you used was up to par?


A digital signal from an image can be measured, characterized, transmitted, and reproduced objectively and accurately. However, the visual interpretation of that signal is dependent on the varied characteristics of the systems displaying that image. [Without a standard] images produced by the same signal may have completely different visual appearance, information, and characteristics on different display devices.

In medical imaging it is important that there be a visual consistency in how a given digital image appears, whether viewed, for example, on the display monitor of a workstation or

as a film on a light-box.

excerpts taken from the Forward toDigital Imaging and Communications in Medicine (DICOM)

Part 14: Grayscale Standard Display Function


• These issues are addressed the Digital Imaging and Communications in Medicine (DICOM) standard (PS 3.14-2003)

• Part 14 of this standard deals with grayscale consistency.

• Jointly developed by American College of Radiology (ACR) & National Electrical Manufacturers Association (NEMA)

• Standard method for transferring images and associated information

• Allows images to be displayed consistently on different softcopy and hardcopy devices

– devices manufactured by various vendors

– devices having different technologies (CRT, LCD)

– devices with different characteristics (max. Brightness and/or Density)

– different ambient light environments.

Calibrating Medical MonitorsCalibrating Medical Monitors DICOM - Display GammaDICOM - Display Gamma

CRT and LCD monitors have a fixed power-law relationship between output luminance and input drive signal (gamma) of about 2.5:

output luminance = input drive2.5

Signals applied to CRTs &LCD require an inverse gamma offset to counteract the display’s gamma

This correction produces a linear relationship between the driving signal and resulting light output


• Our eye-brain detection system is not linear– much less sensitive in the blacks than in the whites– why map all black values into luminance changes to display on the monitor

when we can’t notice the differences between small black value changes?

• DDL* values of darker areas mapped into large luminance increments• DDL* values of whiter areas mapped to smaller luminance increments• Resulting curve is standardized as the DICOM Grayscale Standard

Display Function (GSDF)

DICOM - Display GammaDICOM - Display Gamma

*Digital Driving Level (DDL): digital value applied to a Display System to produce a luminance output

Calibrating Medical MonitorsCalibrating Medical Monitors DICOM - Perceptual LinearizationDICOM - Perceptual Linearization

• GSDF produces a display function that matches the perceptual characteristics of our eyes

• Just-Noticeable Difference (JND) - smallest luminance change that an average person can just perceive (approximately 256 shades of gray)

• “Perceptually linearized” display - equal changes in driving levels produce perceptually equivalent changes in luminance across the entire luminance range.

• Most digital radiologic images display 256 shades of gray - the approximate maximum number of Just Noticeable Differences that we can perceive


.01

.1

1

10

100

1000

200 400 600 800 1000

Grayscale Standard Display Function

JND Index

Same number of Just Noticeable Difference == same perceived contrast

Different changein absolute luminance

Equal steps in perceived brightness represent equal steps in the acquired image data

DICOM - Perceptual LinearizationDICOM - Perceptual Linearization

Calibrating Medical MonitorsCalibrating Medical Monitors DICOM - Perceptual LinearizationDICOM - Perceptual Linearization


• GSDF provides a way to standardize image displayed on monitors having different inherent brightness and response curves

• Used to recalibrate monitors whose luminance and response curves have changed over time

• Simply calibrate the monitor & printers according to this curve

• Calibration requires luminance meter that can be placed on the monitor sothe light output (luminance) can be measured at multiple digital driving levels (DDLs) / luminance levels

• Luminance meter output is compared with the actual Grayscale Standard Display Function (GSDF) and differences can be calculated and saved

• A corrective transfer function is then loaded directly to the video display controller

DICOM - Perceptual LinearizationDICOM - Perceptual Linearization

Calibrating Medical MonitorsCalibrating Medical Monitors DICOM - Color Lookup TableDICOM - Color Lookup Table

Video Card Look Up Table

• Digitized images are made up of a matrix of pixels, each possessing at least three dimensions: two (or more) spatial and one intensity value. The quantized dimensions are stored on the computer as a file of binary numbers.

• In order to see the image on the computer monitor, the image pixel values must be mapped, one-to-one, to screen pixel values, via a Look Up Table (LUT). The LUT transfer function determines what screen values correspond to image pixel intensity values at all coordinates in the image.


• If each image intensity value is mapped to its corresponding screen value the LUT is a linear function and can be graphed as a 45° straight line.

• By modifying the slope of the transfer function and screen mapping, LUT image intensity values may be selectively increased or decreased to perceptually equalize luminance steps in the image

DICOM - Color Lookup TableDICOM - Color Lookup Table

Calibrating Medical MonitorsCalibrating Medical Monitors DICOM - Calibrating DisplaysDICOM - Calibrating Displays

Procedure for Calibrating Gray scale (B&W) Monitors

• Calibrate each monitor regularly

• Calibrate under normal viewing & lighting conditions

• Adjust monitor contrast and brightness before/during calibration and never again

• Calibrate with always the same background intensity (about 20%, or a value comparable to the average intensity emitted from the monitor)

• Calibrate to match the standard display function (DICOM)


Your Sencore contact:

Scott Reardon

1-800-SENCOREwww.sencore.com

Questions?Questions?

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To Delivering High Quality Images in Medical Monitors.

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