Required Grayscale Accuracy in Medical Monitors

Table of Contents
1. Preface ------------------------------------------------------------------------------------------------------2
2. Requirement for Perceptually Linear Display Characteristics ---------------------------------2
3. Composition of Imaging Display System ------------------------------------------------------------4
4. Contrast Resolution of Display System --------------------------------------------------------------5
5. Functions and Operations of LUT ---------------------------------------------------------------------5
6. Accuracy in Conformance to GSDF Curve ---------------------------------------------------------7
6-1. Deviation from GSDF Curve -------------------------------------------------------------------------7
6-2 Errors in Comparison with GSDF (Luminance Errors per Step of p-values) -------------8
6-3 "Number of JNDs" per Interval of p-values and Deviation (Grayscale Continuty) -------8
6-4 Contrast Response Error (Contrast per JND associated with GSDF) --------------------10
7. Comparison of GSDF Accuracies on Specific Systems ---------------------------------------11
7-1. 8-bit Input-Output System (LUT 8, 10, and 12bits) --------------------------------------------11(1) 8-8-8-bit System (Figures 17A-17D in Appendix-1) ---------------------------------------11(2) 8-10-8-bit System (Figures 18A-18D in Appendix-1) -------------------------------------12(3) 8-12-8-bit System (Figures 19A-19D in Appendix-1) -------------------------------------13(4) Summary of 8-bit Input-Output Systems ----------------------------------------------------13
7-2. 10-bit Input-Output System (LUT 10, 11, and 12bits) ----------------------------------------14(1) 10-10-10-bit System (Figures 20A-20D in Appendix-1) ---------------------------------14(2) 10-10-10-bit System (Figures 21A-21D in Appendix-1) ---------------------------------15(3) 10-12-10-bit System (Figures 22A-22D in Appendix-1) ---------------------------------16(4) Summary of 10-bit Input-Output Systems --------------------------------------------------16
7-3. 11-12-11-bit System (Figures 23A-23D in Appendix-1) --------------------------------------17
8. Realization of Multi-shade System ------------------------------------------------------------------18
8-1. Realization through Combination of Viewer, GC, and Monitor -----------------------------18
8-2. Realization through Combination of Viewer and Monitor ------------------------------------19
9. Summary --------------------------------------------------------------------------------------------------20
Appendix-1 ---------------------------------------------------------------------------------------------------24
Graphs Showing Accuracy Calculation Results ----------------------------------------------------24
Appendix-2 ---------------------------------------------------------------------------------------------------33
Psychology affect caused by human visual systems ----------------------------------------------33
Bibliography --------------------------------------------------------------------------------------------------34
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JVC White Paper

Grayscale Resolution

1. Preface
An image monitor used for filmless diagnosis is required to have high gray scale accuracy indisplaying monochrome images. Generally, the bit width of the look-up table (LUT) of a monitor isa specification item that represents the capability of the monitor in its catalog. This white paper
examines various errors in display systems to find out how the operations of LUTs and differencesof input-output bit widths are related to the accuracy in displaying images. It aims to, establish thesuperiority of JVC display systems and platform-independent approach to multi-shade* display
and consequently helps users understand gray-scale performances better.

2. Requirement for Perceptually Linear Display Characteristics
As shown in Figure 1, the minimum luminance difference that can be perceived with human eye
in two adjacent areas with different luminances (ΔL) varies depending on the mean luminance of
these areas (L). For example, ΔL =0.024cd/m2 at a low luminance of L=1cd/m2 and ΔL =3.3cd/m2
at a high luminance of L=500cd/m2, demonstrates a large difference. Figure 3 shows the limits ofperceivable luminance differences in comparison with mean luminances, which were calculated
based on the GSDF.
Figure 1. Minimum perceivable luminance change at mean luminances of 1cd/m2 and 500cd/m2

Luminance (cd/m2）

500

1

dL

L

1JND = 3.3cd/m2@ mean luminances = 500 cd/m2
dL/L =0.0067

L: Mean luminancedL:Detectable minimum luminance change
1JND = 0.024cd/m2@ mean luminances = 1cd/m2
L

dL

dL/L =0.024

Threshold contrast = dL/2L 1)

DICOM 3.14 defines the unit of minimum luminance difference that can be perceived with human
eye as one JND (Just Notice Difference) and the function of mean luminances and numbers of
JNDs as the Grayscale Standard Display Function (GSDF). GSDF, also called the DICOM curve(Figure 2), is used to convert a luminance value to a JND value and vice versa.
*multi-shade : more than 10 bits shades
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JVC White Paper
On the other hand, monitors have widely different grayscale characteristics (commonly called
gamma characteristics) depending on manufacturers, models, individual differences, or
luminance settings. An image displayed on monitors with different characteristics will naturally
look different, significantly interfering with imaging diagnosis. Medical image display systems,
therefore, are commonly calibrated according to the grayscale characteristics defined by GSDF
(Figure 5).
Figure 2. GSDF (Grayscale Standard Display Function)

Figure 3. Detectable minimum luminance modulation
0.1

1000

0.08

100

1

0.04
0.02

0.1
0.01

Calculated with GSDF

0.06

10

dL/L

Brightness （cd/m2）

10000

0

200

400

600

800

0
0

1000

200

JND index (1-1023)

400

600

800

1000

JND index (1-1023)

Figure 4 shows the comparison of different grayscale display images due to the difference inmonitor display characteristics. This figure shows that, on a monitor with GSDF characteristics,
differences between any two shades are perceived as the same degree of change in luminancein the grayscale displayed. On a monitor with gamma 2.2, however, there is a tendency that theshades are emphasized in a low luminance range and compressed in middle and high luminanceranges. In other words, it can be concluded that an image displayed on a monitor with gamma 2.2
will tend to look brighter on the whole than on the other monitor.
Figure 4. Gamma characteristics and displayed grayscale images
1000

GSDF
L Range 0.7-410cd/m

2

L (cd/m2）

100

10

GSDF
γ=2.2

1

0.1

0

32

64

96

128

160

192

224

256

Input Signal

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γ=2.2

Grayscale Resolution

Figure 5. Luminance of an 8-bit monitor calibrated to GSDF

350
L (cd/m2）

800

Luminance
JND index

400

700
600

300

500

250

400

200

300

150

200

100

100

50
0

JND index

450

0

32

64

96

128

160

192

224

255

0

p-value (0-255)
Increments in luminances in relation to increments in p-values are calibrated into perceptually linear.

Figure 5 shows the relationship of the luminance and JND values to input signal levels (p-values)
on a GSDF-calibrated monitor. The luminances are shown as a red curve against the left-side
vertical axis and the JND values as a blue line against the right-side vertical axis.
This chart shows that, on a GSDF-calibrated monitor, the input signal levels and JND values arein a linear proportionality relation and that increments in the input signal levels and consequent
increments in the luminances are displayed as a perceptually linear relationship.

3. Composition of Imaging Display System
Figures 6a and 6b show a composition example of a standard imaging display system.
A look-up table (LUT) used to convert the display-specific shading characteristics to the GSDF isprovided in the computer's graphics card on some systems (Figure 6a) and in the monitor onother systems (Figure 6b), the latter of which are increasing due to a widespread use of medical
imaging monitors in recent years.
Images transferred from modality system are normally 12 bits wide but are output to the monitor
with a bit width downscaled by the computer's operating system, viewer software, or graphics
card. The bit width of this output signal represents the maximum number of simultaneously available shades.
On common display systems, the maximum number of simultaneously available shades is limited
to 256 because 8-bit viewer software and graphics cards are used. Recently, however, a special
approach using color channels was developed to realize a system capable of handling output witha 10-bit width or better on a commercial basis. This system, although not yet standardized like the
8-bit input-output systems, can display 1024 shades or more simultaneously.
How to realize a multi-shade display system is explained in details in Chapter 8.

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JVC White Paper

Figure 6a

Image memory

Figure 6. Image observation display systems

Workstation
ViewerGraphics
Card

SingnalProcessor

MonitorLCD Panel

LUT

12
bit

Figure 6b

Image memory

Workstation
ViewerGraphics
Card

SingnalProcessor

MonitorLCD Panel

LUT

12
bit

4. Contrast Resolution of Display System
The contrast resolution, if defined as a display system's "ability to display images with finely
discriminated luminance changes," corresponds to the bit width of imaging signals that are input
to the monitor. More accurately, however, the contrast resolution of a display system should be
defined as the "maximum number of shades that can be displayed on the monitor due to input
signals" because the number of shades sometimes decreases when grayscale characteristics
are converted via the viewer and LUT. The degree of decrease in the number of shades isdependent on the input-output bit width of the LUT, gamma characteristics of the display, andsettings of maximum and minimum luminances (Lmax and Lmin) used for calibration.

5. Functions and Operations of LUT
LUT is the memory inserted between the monitor and the imaging signals transmitted as p-values
and included in the graphics processing circuit. The LUT has a function of specifying the monitor's
digital driving level (DDL) in order to output the shading data of pixels recorded in the video
memory as luminance values. Figure 7 shows an example of a monitor with a built-in LUT and aconceptual illustration of operations for 10-bit LUT with 8-bit input-output (8-bit LUT with a 10-bit
width).
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Grayscale Resolution

Figure 7. Conceptual illustration of LUT operations on a display system (monitor)
with a built-in 10-bit LUT with 8-bit input-output
Video signal with an 8-bit p-value
p-values

Luminanc
Display

255 410.0410
0.7
254 403.4 0.7

127 41.85
126 41.00

1
0
p-

410
403.0 410

41.83

0.7
0.7

410
410

41.08

0.749
0.700

0.750
0.7

DDL

0.7
0

255
254

410.0
403.0

1023
1016

127
126

41.83
41.08

361
358

1
0

0.750
0.700

18
0

410
410

Luminance values for 1024 shades 0 - 410cd/m2
1
10bit

410
1023

1022

DDL

LUT
Target luminance required for
GSDF (cm/d2)

256 luminance values closest
to the GSDF luminances are
selected out of the monitor’s
shades 0-1023 and displayed.

Display-specificshading characteristics

For the sake of discussion in this paper, a "monitor" means a section between the port of thecomputer's graphics card that outputs p-values and the unit that receives the signals and displays
an image and an "LCD panel" means an internal section of a monitor that converts processed
image signals (DDLs) into luminances and displays them.
Figure 8. Example of LUT conversion characteristics

data of 8 bits (256 shades). For the sake of1024

more specific explanation, ideal luminance896

values (target luminances) for the digital values

768

to be displayed in conformity with the GSDF arelisted. The horizontal axis represents the LUT
output values, which drive a display systemcapable of simultaneous display of 10-bit data.
For the image data that has been input, the LUT
is used to select a display luminance closest to

LUT output (DDL 0-1023)

The vertical axis of the LUT represents pixel

LUT used to convert Gamma 2.2 to GSDF

640
512
384
256
128
0

the target luminance. The larger output bit
width of the LUT, the more finely discriminated

0

32

64

96

128

160

192

224

256

LUT input (p-value 0-255)

the luminances and, consequently, the more the choices and the closer to the target luminances
are the displayed images. Figure 8 shows an example of characteristics of a LUT used to convert
the display with Gamma 2.2 to the GSDF characteristics.
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JVC White Paper
6. Accuracy in Conformance to GSDF Curve
This chapter examines some items concerning the accuracy in conversion of the monitor-specificdisplay characteristics to the GSDF characteristics.
The display luminance accuracy is dependent on the bit width of the LUT, the gamma
characteristics, and the luminance range specific to the display system. This section assumes
that the display-specific gamma is 2.2 and that the luminance range is 0.7-410 cd/m2.

6-1. Deviation from GSDF Curve
The GSDF is an analog, continuous curve but, on an actual system driven digitally, the luminanceresponses of the monitor are in a step-like pattern and, as shown in Figure 9, have errors incomparison with the GSDF curve. The current study is about converting luminance errors into
JND values and deriving errors in them in comparison with the ideal JND values. To minimize thecalculation errors in sampling, the JND values in the horizontal axis are calculated at 14-bit
intervals.
Figure 9. Deviation of luminance responses from the GSDF on a digitally driven monitor
25

20

L (cd/m2)

GSDF curve

15

Actual luminance

10

5

0

50

Lum error (%)

0

100
1

2

3

150
4

JND index

200

250

300

200

250

300

200

250

300

p-value

+
↑

0

0
↓

-

50

100

150

50

100

150

JND index

Error in JND

+

↑
20
0
0
↓

-

JND index

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Grayscale Resolution
6-2. Errors in Comparison with GSDF (Luminance Errors per Step of
p-values)
The figure below shows how accurately the luminance values per step of input signals (e.g.,
256-point luminance values for an 8-bit width signal) are displayed in comparison with the luminance values per step defined in the GSDF. The error, expressed as the "number of JNDs," isused as an indicator of the system accuracy performance.

6-3. "Number of JNDs" per Interval of p-values and Deviation(Grayscale Continuity)
On an ideal system, an increment of the display luminance should be one JND per increment of
one step of input signal.
However, this condition cannot be easily met on an actual digital system mainly because of alimitation due to the bit width of input signals and quantization errors that occur during grayscaleconversion.
Figure 10. "Number of JNDs" per interval of p-values and deviation from GSDF

If the "number of JNDs" islarge per interval between
steps of input signals, a
discontinuity of luminance

Lmax
GSDF curveis discernable in part ofwithluminance

gradualchanges.

To

eliminate this discontinuity,
you need to decrease the"number

of

JNDs"

per

p-value interval to less than

Luminance (cd/m2)

imagesActual luminance response

p-value interval as well as asmall deviation from themean "number of JNDs" at

2.0
1.5

one JND. In other words, asmall "number of JNDs" per

2.5

1.0
Lmin

0.5
0.0

J1
50

smooth grayscale.

70

1p

all the p-value intervals are
the conditions for realizing a

60

J2

p0

J3
80

90

1p

p1

100
JND index
1p

p2

Jn
110
1p

p3
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p n-1

pn

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JVC White Paper
On monitors with a display luminance range from 0.7cd/m2 to 410cd/m2 as shown in Figure 11, forexample, the "number of JNDs" existing in this luminance range is 676.4-57.8=618.6. If the input
signal is 8 bits wide, the number of steps is 256. The mean value for "numbers of JNDs" perp-value interval (Jmean) is 618.6/255=2.43. Thus, a discontinuity of luminance may bediscernable in part of images with gradual luminance changes. If the input signal is 10 bits wide,
Jmean is 618.6/1024=0.60, less than one JND. Perceptually, therefore, it can be represented asa continuous luminance change.
The "number of JNDs" per p-value interval has a deviation due to a quantization error that occurs
when the gamma specific to the display is converted to the GSDF. This deviation is smaller for alarger output bit width of the LUT.

Figure 11. Calculating the mean value (Jmean) for the "number of JNDs" per interval of p-values
1000
Lmax
(410)

GSDF

Luminance (cd/m2)

100

Dynamic range

10

1
Lmin
(0.7)

JNDs = JND(p.n) - JND(p.0)
Jmean = 618.6/n

0.1

0.01

JND(p.0)(57.8)
0

100

p.0

9

JND(p.n)(676.4)
200

300
400
JND index
p-value(0-n)

500

600

700

p.n

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800

Grayscale Resolution
6-4. Contrast Response Error (Contrast per JND associated with GSDF)
The evaluation of a contrast response error is a method of quantitative and detailed inspection onwhether increments in luminances in relation to increments in p-values demonstrate aperceptually linear relationship. This method is defined as the Advanced Evaluation in
AAPM-TG18 (American Association of Medical Physicists of Medicine – Task Group 18). As
shown in Figure 12, the luminance change rate per p-value interval (dL/L: contrast) is normalized
to a value per JND and contrasted with that of the GSDF.
Figures 17D to 23D in Appendix-1 are graphs showing the comparison of the contrast response
characteristics of the systems with those in the GSDF.
How much the extent of an error influences the perception needs to be examined in anotherstudy. AAPM TG18 recommends that the contrast response evaluated using 18 steps of p-values
should be less than ±10% of the GSDF.
In this paper, all the systems are compared under the same condition, a p-value of 256. There isa tendency that, the fewer the p-values to be evaluated, the smaller error value. This is becausea larger p-value interval averages out a localized discontinuity.
Figure 12. Calculating the contrast response

Lmax
Monitor Luminance

GSDF curve

L (cd/m2)

3.0

2.5

Ldi

2.0

L’i

Ldi-1

1.5

L’i-1

1.0
Lmin

J mean0.5
J mean(J i-1 +J i )/2

J mean

0.0
50

p0

60

70

80

J i-1

90

p i-1

100
JND index

p-value

110

J i

pi

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pn

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JVC White Paper
In Figure 12, the contrast response at the point with a JND index of ( Ji-1 +Ji )/2 can be defined asfollows7):diContrast response of GSDF δ =2(Ldi - L di-1)(Ldi + Ldi-1 )(Ji -J i-1 )Contrast response of monitor δ i =2(L'i - L 'i-1)(L'i + L'i-1 )(Ji -J i-1 )The following section is about the comparison of the GSDF conformance accuracies on specificsystems.

7. Comparison of GSDF Accuracies on Specific Systems
7-1. 8-bit Input-Output System (LUT 8, 10, and 12bits)
On this system, 12-bit image data stored on a computer is compressed by an 8-bit viewer and agraphics card into image signals with 8-bit, 256-step shades that drive the monitor.
The monitor has a built-in LUT 8 bits wide or better. In generally 8-bit input-output systems havea LUT with an output bit width of 8, 10, or 12 bits. In this paper, we compare the characteristics ofthese three systems.
Systems with a built-in LUT in the graphics card are handled in the same way as above.

(1) 8-8-8-bit System (Figures 17A-17D in Appendix-1)
ComputerMonitorImage data
Viewer

12
bit

8

bit

Graphics
card

8

8

Monitordisplay

8

bit

bit

bit

8

8

8

bit

11

LUT
of monitor

bit

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bitGrayscale Resolution
The term "8-8-8-bit system" is used because each of the graphics card output, LUT, anddisplayed image has a bit depth of 8 bits (this naming convention is used for all the systems
described hereafter).
A typical example is a system that includes an ordinary PC monitor. On an 8-8-8-bit system witha few numbers of selectable shades, the error in comparison with the GSDF reaches 2.16 JNDs
at the maximum.
Furthermore, the same luminance value is selected for some steps so that there are actually
fewer number of shades in the monitor. Under the conditions shown in Table 1, for example, about
44 p-values result in the same luminance values so that the actual number of shades in thedisplay is 256-44=212, making it impossible to display accurate grayscale information.

(2) 8-10-8-bit System (Figures 18A-18D in Appendix-1)
ComputerMonitorImage data
Viewer

12
bit

8

bit

Graphics
card

LUT
of monitor

Monitordisplay

8

10
bit

bit

8

10

8

bit

bit

bit

8

bit

On an 8-10-8-bit system, the monitor has a number of shades as finely discriminated as 1024 andthe 256 p-values can be displayed with an error of 0.52 JNDs or less in comparison with the target
luminance values of the GSDF. Furthermore, the number of shades in the monitor is maintained
so that, actually, 256 shades can be rendered.

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JVC White Paper(3) 8-12-8-bit System (Figures 19A-19D in Appendix-1)
ComputerMonitorLUTof monitor

Image data
Graphics
card

Viewer

12

8

bit

bit

Monitordisplay

8

12
bit

bit

8

12

8

bit

bit

8

bit

bit

On an 8-12-8-bit system, the monitor has a larger number of shades of 4096, more finely
discriminated, and the 256 p-values can be displayed with an extremely high accuracy, i.e., amaximum error of 0.31 JNDs in comparison with the target luminance values of the GSDF.

(4) Summary of 8-bit Input-Output Systems
Table 1 shows the result obtained from the evaluation of these 8-bit systems concerning theerrors in comparison with the GSDF in terms of four perspectives.
Table 1. Comparison of accuracies of 8-bit input-output systems(Trial calculation assuming Lmin=0.7cd/m2, Lmax=410cd/m2, and the display-specific gamma =2.2)
Deviation from
GSDF curve (JNDs)

Error in comparison
with GSDF (JNDs)

JNDs per step

8-8-8-bit system

Max.

4.6

Max.
p-p

2.16

Ave.

2.43

4.11

Ave.

4.76

8-10-8-bit system

Max.

3.0

Max.
p-p

0.52

Ave.

2.43

0.99

Ave.

1.52

8-12-8-bit system

Max.

2.6

Max.
p-p

0.16

Ave.

2.43

0.31

Ave.

0.47

Contrast
response error (%)

Actual number ofshades in display

Max.

100

212

Max.

30.4

256

Max.

9.8

256

On an 8-8-8-bit system, the increments in display luminances in relation to increments in p-values
present large fluctuation from 0 to 4.76 JNDs. Such a system is not suitable for imaging diagnosis
because it is incapable of presenting continuous grayscale representations and have a fewer
number of shades that can actually be displayed, causing significant deterioration from an original
image.
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Grayscale Resolution
However, using a LUT with a larger number of output bits can put the 256 luminance points closer
to the GSDF target luminances, consequently decreasing the variations of JNDs per step anddramatically improving the display quality.
When seen from a different point of view, this fact proves that a 12-bit LUT has a significantly
reduced contrast response error and is capable of smooth grayscale display.
In consideration of the above result, an 8-bit input-output system to be used for imaging diagnosis
requires at least a 10-bit LUT and can realize ideal imaging display if a 12-bit LUT is used.

7-2. 10-bit Input-Output System (LUT 10, 11, and 12bits)
On a 10-bit input-output system, 12-bit original image data is compressed into 10-bit image
signals that drive the monitor. Concerning this group of systems, this section compares systems
with LUTs with output bit widths of 10, 11, and 12 bits.
These systems are ready for input image signals with four times as many shades as an 8-bit
input-output system, which result in about one-fourths of deviations from the GSDF curve andcome closer to more analogous representations. Since the mean value of JNDs per step is also
improved to less than one JND, an even smoother grayscale representation can be realized.

(1) 10-10-10-bit System (Figures 20A-20D in Appendix-1)
ComputerImage data

12
bit

MonitorViewerGraphics
card

LUT
of monitor

Monitordisplay

10

10

10

10

10

10

10

bit

bit

bit

bit

bit

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bit

bit

14

JVC White Paper
On a 10-10-10-bit system, 1024 p-values are used to select values closest to the GSDF target
values out of the 1024 luminance values determined according to the monitor's own display characteristics. As described before, the LUTs have the same number of input-output bits so that the
tonal expression is lost during conversion of display characteristics and consequently there are a
fewer number of shades that can actually be displayed. On the other hand, this system build inwider bit LUT, so that an error in comparison with the GSDF in each shade of gray can be limited
to 0.54 JNDs at the maximum.
Since the variation of JNDs per step reaches 1.19 JNDs at the maximum, the discontinuity of
tonal representations is discernable in part of images with gradual luminance changes.

(2) 10-11-10-bit System (Figures 21A-21D in Appendix-1)
ComputerImage data

12
bit

MonitorLUTof monitor

ViewerGraphics
card

10

10

11

10

10

11

10

bit

bit

bit

bit

bit

Monitordisplaybitbit

On a 10-11-10-bit system, there are 2048 choices of luminance values that are available on themonitor for 1024 p-values so that the steps of input signals can be brought closer to the GSDF
target values.
When the errors in comparison with the GSDF are considered, the deviation from the GSDF
curve is 0.87 JNDs, the error of each shade in comparison with the GSDF is 0.28 JNDs, and thevariation of JNDs per step is 0.76 JNDs. Since all of these errors are less than one JND, assumed
to be the limit of average human perception, more or less satisfactory display quality can beobtained.

15

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Grayscale Resolution(3) 10-12-10-bit System (Figures 22A-22D in Appendix-1)
ComputerImage data

12

MonitorViewerGraphics
card

10

10

12

10

10

12

10

bit

bit

LUT
of monitor

Monitordisplay

bit

bit

bit

bit

bit

bit

On a 10-12-10-bit system, there are 4096 choices of luminances that are available on the monitor
so that the each input signals match the GSDF target values very accurately.
When the errors in comparison with the GSDF are considered, the deviation from the GSDF
curve is 0.76 JNDs, the error of each shade in comparison with the GSDF is 0.17 JNDs, and thevariation of JNDs per step is 0.45 JND. This system has an ideal performance as a 10-bit
input-output system.

(4) Summary of 10-bit Input-Output Systems
Table 2 shows the summary of evaluations of 10-bit input-output systems.
Table 2. Comparison of accuracies of 10-bit input-output systems(Trial calculation assuming Lmin=0.7cd/m2, Lmax=410cd/m2, and the display-specific gamma =2.2)
Deviation from
GSDF curve (JNDs)

Error in comparison
with GSDF (JNDs)

10-10-10-bit system

Max.

1.10

Max.
p-p

10-11-10-bit system

Max.

0.87

10-12-10-bit system

Max.

0.76

JNDs per step

0.54

Ave.

0.60

1.05

Ave.

1.19

Max.
p-p

0.28

Ave.

0.60

0.55

Ave.

0.76

Max.
p-p

0.17

Ave.

0.60

0.32

Ave.

0.45

Contrast
response error (%)

Actual number ofshades in display

Max.

31.8

849

Max.

19.7

1024

Max.

9.3

1024

A 10-bit input-output system is ready for handling four times as much shading information as an
8-bit input-output system. With the deviation from the GSDF curve about one-third of that of an
8-bit input-output system, a 10-bit input-output system can display images more similar to ananalog system.
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16

JVC White Paper
Generally speaking, a 10-bit input-output system is far superior to an 8-bit input-output system.
However, a 10-10-10-bit input-output system, with its actual number of shades in the display
significantly decreased, cannot prove its worth as a 10-bit input-output system. A 10-bit input-output system needs to have at least an 11-bit or better LUT and, if provided with a 12-bit LUT, candeliver a more sophisticated display performance.

7-3. 11-12-11-bit System (Figures 23A-23D in Appendix-1)
ComputerImage data

12

MonitorViewerGraphics
card

LUT
of monitor

Monitordisplay

11

11

12

11

11

12

11

bit

bit

bit

bit

bit

bit

bit

bit

On an 11-bit input-output system, 12-bit original image data is compressed into 11-bit image
signals that drive the monitor.
It can simultaneously display 2048 shades, twice as many as a 10-bit input-output system.
Table 3 shows the superiority of an 11-12-11-bit system to a 10-12-10-bit system.
Table 3. Comparison of accuracies of 11-12-11-bit and 10-12-10-bit input-output systems(Trial calculation assuming Lmin=0.7cd/m2, Lmax=410cd/m2, and the display-specific gamma =2.2)
Deviation from
GSDF curve (JNDs)

Error in comparison
with GSDF (JNDs)

10-12-10-bit system

Max.

0.76

Max.
p-p

11-12-11-bit system

Max.

0.45

Max.
p-p

JNDs per step

0.17

Ave.

0.60

0.32

Ave.

0.45

0.17

Ave.

0.30

0.32

Ave.

0.42

Contrast
response error (%)

Actual number ofshades in display

Max.

9.3

1024

Max.

9.7

2048

As is apparent from this table, an 11-12-11 system is advantageous even to a 10-12-10-bit
system whose high fidelity to the GSDF curve was confirmed in the previous section in terms of
performance indicated by the deviation from the GSDF curve and JNDs per step. Although thelimit of average human perception is allegedly one JND, there are variations among individuals
and a radiologist with great sensitivity may be able to perceive a contrast of less than one JND.
17

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Grayscale Resolution
Therefore, this system that can achieve 50% or less of one JND ranks as a top-of-the-line medical imaging display monitor.

8. Realization of Multi-shade System
Then, how is it possible to realize a multi-shade system with a high fidelity to the GSDF curve,
which was concluded to be the best above?

8-1. Realization through Combination of Viewer, GC, and Monitor (Figure 13)
This section explains an approach to realization with a focus on the transmission of digital image
signals for an LCD monitor.
The commonly used operating systems (OS) such as Windows, MacOS, SUN Solaris, and Linuxare specified to transmit image signals through three channels R, G, and B, covering 8 bits eachand 24 bits in total.
Although a monochromatic system needs only single-color signals and thus uses only one channel, it cannot perform over-8-bit data transfer due to the above reason, i.e., a limitation of the OS.
A medical display system, as shown in the figure 13, realizes over-8-bit data transfer by sending
monochromatic image data via all of the R, G, and B channels. For example, 8-bit data and 2-bit
data are transferred via the R-channel and B-channel, respectively, to half of the screen and thus
10-bit data is used to drive the monitor. For the remaining half of the screen, 8-bit data and 2-bit
data are transferred via the G-channel and B-channel, respectively, to half of the screen and thus
10-bit data is used to drive the monitor. Therefore, 10-bit display is realized for the entire screen.
Since this signal transfer system is not a specified for the hardware or operating system, thedriver software for the graphics card must be provided with special software (an application
program interface or API). In other words, this software allows 10-bit signals in an application to
be converted on the graphics card to special output.
Figure 13. Example of a conventional multi-shade display system

PC system

8bit

APIs
SpecialGC

ViewerNon-OS-standard
signals using the APIsof a special GC

2bit
2bit

8bit
Non-OS-standard
video signals

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JVC White Paper
This system has the following difficulties:(1) Since the multi-shade transmission system is not a standardized for specified the hardware
or operating system, an API must be prepared for each graphics card (GC) or operating system(OS).
(2) Since there is no standard way for deciding which channel should be the main one for themulti-shade transmission system or which bit of which channel should be the multi-shade bit,the input-output interface of an LCD monitor should conform to the interface of a GC.
(3) The system will not be compatible at all with an ordinary 8-bit system or common 32-bit
mode.
An unexpected problem may occur if you try to run a general application such as Web browser in
addition to the viewer. In sum, this system is not versatile due to its many restrictions.

8-2. Realization through Combination of Viewer and Monitor (Figure 14)
This system, based on a general 32-bit color mode (24-bit signal transmission mode), encodes
monochromatic multi-shade signals into color signals when transmitting them and decodes thesignals in the monitor to realize multi-shade display.
Figure 14. Example of a conventional multi-shade display system

PC system

Conversion
table

ViewerOS-standard signals
using the conversion
table

Conversion
table

GeneralpurposeGC
OS-standard
video signals

Specifically, this system transmits multi-shade signals using a table that converts monochromaticshades into color signals, R, G, and B. Theoretically, representation of about 16 million shades ispossible because there are 24-bit signal lines including R, G, and B with 8 bits each. However,
only about 4,000 shades (12 bits) are actually necessary at the most and therefore about half ofthese signal lines are required to transmit a necessary and sufficient number of shades of gray.
This system has the following advantages:(1) The use of a standard image signal transmission system (24-bit full-color transmission) of
Windows or other OS (such as MacOS and Linux) eliminates the need of special APIs
dependent on the graphics card.
(2) The installation of a monochrome-to-color conversion table into an application easily
realizes this system.
(3) The adoption of an OS standard video output enables the use of a general-purpose
19

graphics card and eliminates dependence on graphics card manufacturers.
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Grayscale Resolution(4) A possible increase in the number of shades from 10-bit, 11-bit, 12-bit, and so on can be
dealt with only by modifying the conversion table, a table used to assign combinations of color
signals to shades of monochromatic signals, eliminating the need of rewriting other software or
upgrading of hardware, etc. in principle.
In comparison of the above two systems, the latter JVC system is dependent only on the viewer
and monitor and thus apparently advantageous to the other.

9. Summary
This paper compared the relationships between the LUT input-output bit width and the GSDF
accuracy that can be theoretically achieved on each of 8-bit, 10-bit, and 11-bit systems assuming
the monitor's displayable range as 0.7-410cd/m2 and the display native gamma as 2.2. The
following summarizes what is described in previous chapters and examines evaluation items andresults for grayscale accuracy required for medical imaging monitor.
"JNDs per step" means:"Increments in luminances in relation to increments per step of input signals." Ideally, it is desirable that both the mean value and variation of "JNDs per step" should be no more than one JND,
which is the limit of human perception. The mean value, dependent on the input signal bit width,is 2.4 JNDs for an 8-bit input-output system, 0.6 JNDs for a 10-bit input-output system, and 0.3
JNDs for an 11-bit input-output system. The variation decreases if the output bit width of the LUT
increases.
"Deviation from the GSDF Curve" means:"An error resulting in loss of shades due to sampling." In other words, this term means how mucherror an original image is finally displayed with on the monitor if the original image is assumed to
be analog. It is desirable that this error should be no more than one JND to reproduce an image
with sufficient fidelity to the original image.
"Error in comparison with the GSDF" means:"An error that represents a difference between the luminance values of all the steps of input
signals and the luminance values defined in the GSDF (target luminances)." In this paper, anerror is expressed not as a percent error of a luminance value but converted to a "number of
JNDs" because the expression as a perceptually linear scale enables comparison actually by
numbers and facilitates understanding. Note that there is a significant difference in terms ofperception between 1% in a low-luminance area and 1% in a high-luminance area, as describedin Chapter 2.

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JVC White Paper"Contrast response error" means:"How much error the increment in luminance per increment in input signals (contrast) has incomparison with the value defined in the GSDF." In other words, this means how much error acontrast between all the neighboring steps of input signals (the luminance difference divided by
the mean luminance) has in comparison with the value in the GSDF. This parameter can be used
to strictly evaluate the conformity to the GSDF.
The performances of the systems described in this paper can be compared in terms of the above
four perspectives as follows:
8-bit Input-Output System
Has discernable boundaries of shades in part of images with gradual luminance changes
because the "mean value of JNDs per step" is 2.4 JNDs. An 8-bit LUT, in particular, makes thevariation so wide that it causes significant jumps and drops in shades of gray, which may
adversely influence diagnosis. The use of a 10-bit LUT considerably reduces the variation andmakes few jumps and drops so that the system is in a range of practical use. The use of a 12-bit
LUT makes a necessary and sufficient monitor for an 8-bit input-output system.
10-bit input-output system
Is a desirable system because the "mean value of JNDs per step" is 0.6 JNDs and the incrementof luminance per step is less than the limit of perception. However, a 10-bit LUT, which will cause
jumps and drops in shades of gray, is insufficient for a 10-bit input-output system. It needs to beequipped with at least an 11-bit LUT. The use of a 12-bit LUT allows you to make the 10-bit
input-output system to deliver its best performance.
11-bit input-output system
Is a desirable system because the "mean value of JNDs per step" is 0.3 JNDs and the incrementof luminance per step is less than the limit of perception. It is an ideal imaging display systembecause the deviation from the GSDF is not more than half of one JND, which is considered alimit of perception, and thus it provides image representation with extremely high fidelity to an
original image.
The grayscale accuracy required of an imaging display system depends on the type of diagnosis
image, image reading environment, luminance range of the monitor, or depth of sensitivity of theradiologist. Generally, however, it is desirable that a 1- to 2-megapixel monitor is at least provided
with an "8-10-8 bit system" and that a 3- to 5-megapixel monitor is provided with at least an"8-12-8-bit system" or ideally an "11-12-11-bit system."21

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Grayscale Resolution
JVC, based on the above result, realized an 8-10-8-bit system for 1-megapixel resolutions and an
11-12-11-bit systemAGforcoating
2- to 6-megapixel
resolutions,
thusAR
providingsolution for each of
AG + ARfilterSpecialcoating an ideal
Glarethese resolutions from the perspective of accurate medical imaging monitor.
Furthermore, a research paper was published to report that, as a result of carrying out psychological and physical evaluation by human observers on monitor systems supporting 8-bit-plus
multi-shade imaging, they were able to make out on an 11-bit system some images that they
could not on an 8-bit system.
With these facts considered, it can be concluded that the ideal theoretical accuracy of a display
system should be targeted at no less than one-half JND.
Lastly, let us examine the superiority of an 11-bit system. For example, a 0.5-450cd/m2 monitor,
when the performance is converted to a "number of JNDs," can display about 644 JNDs. Somepeople claim that, on a 10-bit input-output system with a p-value of 1024 and thus more than 644
JNDs, no system with a higher number of bits is necessary. Let us, then, examine whether a
10-bit-plus system is really unnecessary. A digital display system has an error close to the "meannumber of JNDs" per step at the maximum due to a quantization error. Figure 16 shows aclose-up view of part of the luminance response characteristics of 10-bit and 11-bit input-output
systems. The black line represents the grayscale of an original image and the two stepped lines
represent the luminance responses of 10-bit and 11-bit input-output systems, respectively. This
chart shows, in specific values, how a low-contrast luminance difference existing in the original
image is represented on the above two systems when an original image with sufficient grayscale
information close to an analog counterpart is displayed. The luminance difference between thetwo red dots shown in this figure is 1.10 JNDs. The luminance difference between the two areas,
therefore, is perceivable on a display system that can reproduce the image with fidelity. If thisoriginal image is displayed on 10-bit and 11-bit input-output systems, the luminance difference
between the two areas will be 0.63 JNDs and 0.94 JNDs, respectively.

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22

JVC White Paper
From this figure, it can be concluded that 11bit system is a superior to 10 bit input-output system.
Figure 15. Comparison of 10-bit and 11-bit systems
10-bit system

11-bit system

Comparison of luminance responses (magnified view of part)
Trial calculation assuming Lmin=0.5cd/m2, Lmax=450cd/m2

110.6

Original
image(GSDF)

110.4

JND vaule

cd/m2
2.247

110.2

2.239

110.0

2.231

109.8

2.223
Luminance value
between the twopoints of theoriginal image=1.10JND

109.6
109.4

Luminance difference
available on 10-bit
system =0.63JND

109.2
109.0

2.215

Luminance
difference
available on
11-bit system=0.94JND

2.207
2.199
2.191

108.8
108.6

2.183

11bit

198

10bit

99

199

200

100

201

p-value

202

203

101

204
102

Company names and product names are the trademarks or registered trademarksof the respective companies.

23

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LGrayscale Resolution
Appendix-1
Accuracy Calculation Results
8-8-8-bit system
Figure 17A Shows the deviation from the GSDF curve on an 8-8-8-bit system by the"numbers of JNDs."
8-8 A

Deviation from GSDF (JNDs)

2.0
1.0
0.0
-1.0
-2.0
-3.0
-4.0
-5.0

0

100

200

300

400

500

600

700

800

JND index (14bit)

Eror in comparison with GSDF (JNDs)

Figure 17B Shows the luminance error in comparison with the GSDF on an 8-8-8-bit
system by the "numbers of JNDs."
8-8 B

3
2
1
0
-1
-2
-3

0

32

64

96

128
160
p-value(0-255)

192

224

256

Figure 17C Shows the "numbers of JNDs" per p-value interval on an 8-8-8-bit system.
8-8 C

5

JNDs/step

4
3
2
1
0

0

32

64

96

128

160

192

224

256

p-value (0-255)
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JVC White Paper

Figure 17D Shows the contrast per JND on an 8-8-8-bit system.
The values evaluated using 256 steps of p-values are shown.
8-8 D

0.035

GSDF
8-8bitdL/L per 1JND

0.030
0.025
0.020
0.015
0.010
0.005
0.000

0

32

64

96

128
160
p-value (256 step)

192

224

256

8-10-8-bit system
Figure 18A Shows the deviation from the GSDF curve on an
8-10-8-bit system by the numbers of JNDs.
8-10 A

Deviation from GSDF (JNDs)

2.0
1.0
0.0
-1.0
-2.0
-3.0
-4.0
-5.0

0

100

200

300

400

500

600

700

800

JND index (14bit)

Error in comparison with GSDF (JNDs)

Figure 18B Shows the luminance error in comparison with the GSDF on an
8-10-8-bit system by the "numbers of JNDs."
8-10 B

3
2
1
0
-1
-2
-3
0

32

64

96

128

160

192

p-value (0-255)

25

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224

256

Grayscale Resolution

Figure 18C Shows the "numbers of JNDs" per p-value interval on an 8-10-8-bit system.
8-10 C

5

JNDs/step

4
3
2
1
0

0

32

64

96

128

160

192

224

256

p-value (0-255)

Figure 18D Shows the contrast per JND on an 8-10-8-bit system.
The values evaluated using 256 steps of p-values are shown.
8-10 D

0.035

GSDF
8-10bitdL/L per 1JND

0.030
0.025
0.020
0.015
0.010
0.005
0.000

0

32

64

96

128

160

192

224

256

p-value (256 step)

8-12-8-bit system
Figure 19A Shows the deviation from the GSDF curve on an
8-12-8-bit system by the "numbers of JNDs."
8-12 A

Deviation from GSDF (JNDs)

2.0
1.0
0.0
-1.0
-2.0
-3.0
-4.0
-5.0

0

100

200

300

400

500

600

700

800

JND index (14bit)

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JVC White Paper

Error in comparison with GSDF (JNDs)

Figure 19B Shows the luminance error in comparison with the GSDF on an
8-12-8-bit system by the "numbers of JNDs."
8-12 B

3
2
1
0
-1
-2
-3
0

32

64

96

128

160

192

224

256

p-value (0-255)

Figure 19C Shows the "numbers of JNDs" per p-value interval on an 8-12-8-bit system.
8-12 C

5

JNDs/step

4
3
2
1
0
0

32

64

96

128

160

192

224

256

p-value (0-255)

Figure 19D Shows the contrast per JND on an 8-12-8-bit system.
The values evaluated using 256 steps of p-values are shown.
8-12 D

0.035

GSDF
8-12bit0.030
dL/L per 1JND

0.025
0.020
0.015
0.010
0.005
0.000
0

32

64

96

128

160

192

p-value (256 step)27

Copyright © 2020 JVCKENWOOD Corporation. All rights reserved.

224

256

Grayscale Resolution
10-10-10-bit system
Figure 20A Shows the deviation from the GSDF curve on a
10-10-10-bit system by the "numbers of JNDs."

Deviation from GSDF (JNDs)

10-10 A

2.0
1.0
0.0
-1.0
-2.0
-3.0
-4.0
-5.0

0

100

200

300

400

500

600

700

800

JND index (14bit)

Error in comparison with GSDF (JNDs)

Figure 20B Shows the luminance error in comparison with the GSDF on a
10-10-10-bit system by the "numbers of JNDs."
10-10 B

3
2
1
0
-1
-2
-3
0

128

256

384

512

640

768

896

1024

p-value (0-1023)

Figure 20C Shows the "numbers of JNDs" per p-value interval on a 10-10-10-bit system.
10-10 C

5

JNDs/step

4
3
2
1
0
0

128

256

384

512

640

768

896

1024

p-value (0-1023)

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JVC White Paper
Figure 20D Shows the contrast per JND on a 10-10-10-bit system.
The values evaluated using 256 steps of p-values are shown.
10-10 D

0.035

GSDF
10-10bitdL/L per 1JND

0.030
0.025
0.020
0.015
0.010
0.005
0.000

0

128

256

384

512

640

768

896

1024

p-value (256 step)

10-11-10-bit system
Figure 21A Shows the deviation from the GSDF curve on a
10-11-10-bit system by the "numbers of JNDs."
10-11 A

Deviation from GSDF (JNDs)

2.0
1.0
0.0
-1.0
-2.0
-3.0
-4.0
-5.0
0

100

200

300

400

500

600

700

800

JND index (14bit)

Error in comparison with GSDF (JNDs)

Figure 21B Shows the luminance error in comparison with the GSDF on a
10-11-10-bit system by the "numbers of JNDs."
10-11 B

3
2
1
0
-1
-2
-3
0

128

256

384

512

640

768

896

p-value (0-1023)

29

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1024

Grayscale Resolution
Figure 21C Shows the "numbers of JNDs" per p-value interval on a 10-11-10-bit system.
10-11 C

5

JNDs/step

4
3
2
1
0
0

128

256

384

512

640

768

896

1024

p-value (0-1023)

Figure 21D Shows the contrast per JND on a 10-11-10-bit system.
The values evaluated using 256 steps of p-values are shown.
10-11 D

0.035

GSDF
10-11bitdL/L per 1JND

0.030
0.025
0.020
0.015
0.010
0.005
0.000
0

128

256

384

512

640

768

896

1024

p-value (256 step)

10-12-10-bit system
Figure 22A Shows the deviation from the GSDF curve on a
10-12-10-bit system by the "numbers of JNDs."
10-12 A

Deviation from GSDF (JNDs)

2.0
1.0
0.0
-1.0
-2.0
-3.0
-4.0
-5.0

0

100

200

300

400

500

600

700

800

JND index (14bit)

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JVC White Paper

Error in comparison with GSDF (JNDs)

Figure 22B Shows the luminance error in comparison with the GSDF on a
10-12-10-bit system by the "numbers of JNDs."
10-12 B

3
2
1
0
-1
-2
-3
0

128

256

384

512

640

768

896

1024

p-value (0-1023)

Figure 22C Shows the "numbers of JNDs" per p-value interval on a 10-12-10-bit system.
10-12 C

5

JNDs/step

4
3
2
1
0
0

128

256

384

512

640

768

896

1024

p-value (0-1023)

Figure 22D Shows the contrast per JND on a 10-12-10-bit system.
The values evaluated using 256 steps of p-values are shown.
10-12D

0.035

GSDF
10-12bit

0.030
dL/L per 1JND

0.025
0.020
0.015
0.010
0.005
0.000
0

128

256

384

512

640

768

p-value (256 step)

31

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896

1024

Grayscale Resolution
11-12-11-bit system
Figure 23A Shows the deviation from the GSDF curve on an
11-12-11-bit system by the "numbers of JNDs."
11-12 A

Deviation from GSDF (JNDs)

2.0
1.0
0.0
-1.0
-2.0
-3.0
-4.0
-5.0

0

100

200

300

400

500

600

700

800

JND index (14bit)

Error in comparison with GSDF (JNDs)

Figure 23B Shows the luminance error in comparison with the GSDF on an
11-12 11-bit system by the "numbers of JNDs."
11-12 B

3
2
1
0
-1
-2
-3
0

256

512

768

1024

1280

1536

1792

2048

p-value (0-2047)

Figure 23C Shows the "numbers of JNDs" per p-value interval on an 11-12-11-bit system.
11-12 C

5

JNDs/step

4
3
2
1
0
0

256

512

768

1024

1280

1536

1792

2048

p-value (0-2047)

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JVC White Paper
Figure 23D Shows the contrast per JND on an 11-12-11-bit system.
The values are evaluated using 256 steps of p-values.
11-12 D

0.030

GSDF
11-12bit

0.025
dL/L per 1JND

0.020
0.015
0.010
0.005
0.000
0

256

512

768

1024

1280

1536

1792

2048

p-value (256 step)

Appendix-2
Psychology affect caused by human visual systems
People do not always perceive figures exactly as their physical existence. This appendix showssome examples.

1. Mach Effect4)

Mach effectAround a spatial boundary between step
lighter and the dark side looks darker.

Luminance cd/m2

patterns of shading, the light side looks

Luminance
Perception

100
80
60
40
20
0

-5

-3

-1

1

3

5

Edge Width (mm)2. Craik-O'Brien Effect4)
Craik-O'Brien effect4）

In the figure on the right, for example, boththe sides of a vertical line in the center,
if they have a difference in shading, asshown by dotted lines, if there is a gradientof shading between them.

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Luminance

having exactly the same luminance, look as

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7

9

Grayscale Resolution

3. Spatial Frequency Characteristics of Human Eyes5)
When a pattern with luminance modulation is seen, the luminance differences are perceived withdifferent sensitivities depending on the spacing of light and dark stripes. In the entire visualsystem, the peak threshold value is around four cycles per visual angle (the spacing of one shading cycle is about 1.3 mm if the pattern is observed from a distance of 30 cm). If the stripes are
narrower or broader than this value, the sensitivity to perception of light and dark stripes declines.

The visual system’s sensitivity to contrast depends on
the spatial frequency (cycle per degree)

4. Adaptation6)
The visibility is different between photopic and scotopic visions. Between images with high andlow mean luminances, the differences of luminances in the images are perceived with different
sensitivities. Refer to other papers for more information on visual characteristics, which are not
the aim of this paper.

Bibliography
1) AAPM TG18 v.9.0 pp83
2) DICOM 3.14-1999 pp13
3) AAPM TG18 v.9.0 pp84
4) Tadahiko Fukuda, Biological Information System, pp78-79, Sangyo Tosho
5) Tadahiko Fukuda, Biological Information System, pp62, Sangyo Tosho
6) AAPM TG18 v.10 pp85, Tadahiko Fukuda, Biological Information System, pp56, Sangyo Tosho
7) AAPM TG18 v.10 p87

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