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to Delivering High Quality Images in Medical Monitors. Welcome!. Your Instructor & Sencore contact. Scott Reardon. 1-800-SENCORE www.sencore.com. About Sencore. Founded 1951 Located in Sioux Falls, SD

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to

Delivering High Quality Images

in Medical Monitors

Welcome!


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Your Instructor & Sencore contact

Scott Reardon

1-800-SENCORE

www.sencore.com


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About Sencore

  • 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


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Introduction

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


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Introduction

Why Perform Video Calibration?

  • 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


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Video Review - Producing an Image

Neg. bias

6.3v

400-800V

Approx. 20-40% of HV

25KV

CRT Operation

  • 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


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Video Review - Producing an Image

How do video displays make B&W?

  • 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

No light output

Medium light output

Maximum light output

High bias

Medium bias

Low bias


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Video 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


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Video Review - Producing an Image

  • 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


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Video Review - Producing Color

How do video displays make color?

  • Color CRTs use three electron guns

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

  • Red + Green + Blue = white


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Video Review - Producing Color

No light output

  • 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 light output

Maximum light output

Medium bias

Low bias



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Video Review - Signal Types

Composite Video vs.

S-Video (Y/C)

  • 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.


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Video Review - Signal Types

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

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

  • Fewer artifacts with modern video sources like DSS and DVD


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Video Review - Signal Types

Advantage of Component vs. “S-video”:

  • Increased color resolution


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Video Review - Signal Types

Advantage of RGB over component:

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

  • Provides best possible picture quality for analog CRT displays

HDTV uses component rather than RGB:

  • Requires less bandwidth

  • More content can be stored on a DVD or broadcast.


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Video 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



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Video Review - Resolution

  • 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


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Light & Color Theory

  • 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


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Light & 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.


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Light & Color Theory

  • 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.


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Human Vision

  • 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


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Human Vision

Three characteristics define how our eye/brain sees light:


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Human Vision

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


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Human Vision

  • Saturation: Degree of purity from light of other wavelengths

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


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Human Vision

  • Brightness: Perceived light energy level


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Light & Color Measurement

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 candelaper square meter

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


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Light & Color Measurement

Light Measurement Units:

Color (hue and saturation):

  • CIE chromaticity coordinates (x,y): From 1931 CIE Chromaticity Diagram (Kelly Chart)

What color?


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Color Measurement

CIE Chromaticity Diagram

  • 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.


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Color Measurement

How do video displays make white?

  • By combining the proper mix of RGB

  • All three guns are turned on to make white


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Color Measurement

  • 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)


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Color Measurement

How do video displays make black?

  • All three guns are turned off to make black.

  • The same point on the chart represents white and black


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Color Measurement

How do video displays make color?

  • 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.


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Color Measurement

Color Temperature

  • 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).


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Color 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)


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Color Measurement

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

  • Accurate calibration is done using CIE coordinates


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Color Measurement

  • 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.


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Calibration Overview

1. Calibrate Display Geometry

  • Size

  • Centering

  • Pincushion

  • Trapezoid

  • Rotation

  • Linearity


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Calibration Overview

2. Calibrate Display Convergence

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

  • Eliminate all color fringing effects.


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Calibration Overview

3. Calibrate Display White Balance

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


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Calibration Overview

4. Calibrate Display User Controls

  • Brightness (Black Level)

  • Contrast (White Level)

  • Color (Saturation)

  • Tint (Hue)

  • Sharpness


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Video Generator

Introducing the VP400 & 401 ‘VideoPro’

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.


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Video Generator

DVI

(VP401)

NTSC RF

(VP401)

Battery operated

(8 hours)

Component/RGB VESA/HDTV

Composite

S-Video


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Video Generator

SMPTE Bar

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


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Video Generator

Color Bar

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


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Video Generator

Focus

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


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Video Generator

Crosshatch (4:3)

Check and adjust convergence and linearity.


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Video Generator

Anamorphic

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


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Video Generator

Overscan

5% overscan

(green line)

10% overscan

(red line)

electrical center

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


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Video Generator

Staircase

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


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Video Generator

Horizontal Staircase

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


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Video Generator

Pluge(Picture Line-Up Generator Equipment)

Black

10 IRE

Alternating

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


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Video Generator

Needle

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


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Video Generator

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.


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Video Generator

Window

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


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Video Generator

Regulate

Black / White

alternating

1 pixel wide

Check high voltage and scan current regulation.


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Video Generator

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.


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Video Generator

Checker

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


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Video Generator

Multiburst

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


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Video Generator

Sharpness

Clear, same

brightness

Straight, same width

No blooming

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.


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Color Calibration

3 chip DLP Projector

CRT Projector

LCD Projector

Direct View CRT

  • 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


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Color 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


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Color Calibration

CP5000 Color Analyzer

CP5001 ‘ColorPro’

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

CP5000 ‘ColorPro’

Color Analyzer


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Color Calibration

ColorPro Analyzer overview

  • 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.


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Color Calibration

ColorPro Analyzer overview

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

  • Selectable color reference

  • Change resolution for fine touchup adjustments


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Color Calibration

ColorPro Analyzer overview

  • 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


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Color Calibration

ColorPro Analyzer overview

  • 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)


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Color Calibration

ColorPro Analyzer overview

Identifies when next calibration is due

Documentation that display is optimized

Shows before and after color temperatures


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Color Calibration

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


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White Balance Calibration

White Balance/Gray-Scale Tracking Overview

  • 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).


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White Balance Calibration

White Balance/Gray-Scale Tracking Overview - continued

  • 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 Luminance

Poor tracking

Lower Luminance

Good tracking


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

White Balance Calibration

White Balance - preliminary setup


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Pluge Pattern

Black

10 IRE

alternating 0/7.5 IRE


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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 Calibration

White Balance - preliminary setup


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White Balance Calibration 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 - CRT displays

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.


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White Balance Calibration 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 - 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 %


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White Balance Calibration 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

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


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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.

White Balance Calibration


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White Balance Calibration coordinates to which the display is to be adjusted.

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.


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White Balance Calibration coordinates to which the display is to be adjusted.

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.


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White Balance Calibration coordinates to which the display is to be adjusted.

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.


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White Balance Calibration coordinates to which the display is to be adjusted.

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.


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Calibrating User Controls coordinates to which the display is to be adjusted.

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


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Brightness Control coordinates to which the display is to be adjusted.

  • 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


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Contrast Control coordinates to which the display is to be adjusted.

  • 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)


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Enhancement Controls - coordinates to which the display is to be adjusted.

(Sharpness, Picture, Aperture, Scan Velocity Modulation)

  • Artificially enhance picture transitions between black & white

  • Recommend turning SVM off


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Calibrating User Controls coordinates to which the display is to be adjusted.

HDTV

0 /- 4 IRE

0 IRE

10 IRE

0 IRE

NTSC

0 /7.5 IRE

7.5 IRE

10 IRE

7.5 IRE

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.


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Calibrating User Controls coordinates to which the display is to be adjusted.

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

than

black)

97.5 %

100 %

(white)

95 %


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Calibrating User Controls coordinates to which the display is to be adjusted.

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.

2.5 %

0 %

(black)

5 %

- 4IRE

(blacker

than

black)

97.5 %

100 %

(white)

95 %


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Calibrating User Controls coordinates to which the display is to be adjusted.

Sharpness/Enhancement Controls - Fixed Pixel & CRT displays

  • 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

Straight, same width,

no ghosting


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Calibrating User Controls coordinates to which the display is to be adjusted.

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


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Calibrating User Controls coordinates to which the display is to be adjusted.

Contrast Control - CRT Displays

  • 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.

same width

(no blooming)

Straight

(no raster distortion)


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Calibrating User Controls coordinates to which the display is to be adjusted.

Top Outside Bar

Large Outside Bar

Small Center Bar

Small Center Bar

Color 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.


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Calibrating User Controls coordinates to which the display is to be adjusted.

Cyan

Magenta

Magenta

Cyan

Tint 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.


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Color, Hue and Decoder/matrix coordinates to which the display is to be adjusted.

  • 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.

Decoder Adjust


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75% white reference coordinates to which the display is to be adjusted.

Primary color sections

Secondary

color bars

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.


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Benefits of a Properly Calibrated Video Display: coordinates to which the display is to be adjusted.

  • 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


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DICOM Overview coordinates to which the display is to be adjusted.

Imagine the following:

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.


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DICOM Overview coordinates to which the display is to be adjusted.

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?

?

?

?

?


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DICOM - Overview coordinates to which the display is to be adjusted.

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?

?

?

?

?

?


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DICOM - Overview coordinates to which the display is to be adjusted.

  • 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?


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DICOM - Overview coordinates to which the display is to be adjusted.

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 to

Digital Imaging and Communications in Medicine (DICOM)

Part 14: Grayscale Standard Display Function


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DICOM - Overview coordinates to which the display is to be adjusted.

  • 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.


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DICOM - Display Gamma coordinates to which the display is to be adjusted.

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


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DICOM - Display Gamma coordinates to which the display is to be adjusted.

  • 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)

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


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DICOM - Perceptual Linearization coordinates to which the display is to be adjusted.

  • 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


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DICOM - Perceptual Linearization coordinates to which the display is to be adjusted.

Grayscale Standard Display Function

1000

100

10

1

200

400

600

800

1000

.1

.01

JND Index

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

Different change

in absolute luminance

Same number of Just Noticeable Difference == same perceived contrast


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DICOM - Perceptual Linearization coordinates to which the display is to be adjusted.


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DICOM - Perceptual Linearization coordinates to which the display is to be adjusted.

  • 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


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DICOM - Color Lookup Table coordinates to which the display is to be adjusted.

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.


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DICOM - Color Lookup Table coordinates to which the display is to be adjusted.

  • 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


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DICOM - Calibrating Displays coordinates to which the display is to be adjusted.

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)


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Questions? coordinates to which the display is to be adjusted.

Remember:

Your Sencore contact:

Scott Reardon

1-800-SENCORE

www.sencore.com


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