Visteon Confidential

Luminance and Contrast Decreasedue to White Point Calibration

Hans-Ulrich Lauer, Osama Amin

Nuernberg, 15.03.2017

How large is the luminance decrease ldecr for given target White xW , yW ?

• White point calibration of TFT-LCDs is required in modern automotive display applications

• White point calibration is achieved by adjusting the intensities of Red, Green and Blue by their digital grey

levels nR,nG,nB

White luminance decreases from initially to :

Contrast decreases proportionally, as Black luminance remains the same:

• Display manufacturers specify normally color coordinates , , , , , , , of full Red, Green,

Blue and initial White

Introduction

01i

W

calW

decrY

Yl

Gx GyRx Ry Bx By iWx i

Wy

iWY cal

WY

2

t t

decr

Blacki

W

Blackcal

Wdecr l

YY

YYCR 1 (2)

(1)

• White point calibration is achieved by adjusting the intensities of Red, Green and Blue by their digital grey

levels nR,nG,nB

• X,Y,Z of any color is obtained from the color addition model

X,Y,Z Tristimulus values of resulting color

XR,YR, .. ,ZB Tristimulus values of full Red, Green, Blue

sR(nR), sG(nG), sB(nB) Scaling factors for tristimulus values sR(nR) sG(nG) sB(nB) (n/255) 1

nR,nG,nB Digital grey levels 0 nR,nG,nB 255

kR,kG,kB Correction factors kR,kG,kB 1

• Note on solvability: “Correction factors” are needed to avoid over-determination of equation system (3),

if given values for X,Y,Z are entered

3

Color addition model

X RRRR Xnsk )( GGGG Xnsk )( BBBB Xnsk )(

Y RRRR Ynsk )( GGGG Ynsk )( BBBB Ynsk )(

Z RRRR Znsk )( GGGG Znsk )( BBBB Znsk )(

(3)

• Calculate normalized tristimulus values X,Y,Z for Red, Green, Blue, initial and target White out of given x,y :

Note on solvability: Transformation from x,y to X,Y,Z is under-determined Choose Z = 1

• Normalized tristimulus values for initial White:

• Normalized, but yet unbalanced tristimulus values for Red, Green, Blue and target White:

• Balanced tristimulus values for Red, Green, Blue are obtained in next step, using correction factors kR,kG,kB

4

Calculation of luminance decrease (1)

iW

iW

iWi

Wyx

xX

1 iW

iW

iWi

Wyx

yY

11i

WZ

RR

RuR

yx

xX

1 RR

RuR

yx

yY

1 GG

GuG

yx

xX

1 GG

GuG

yx

yY

1

BB

BuB

yx

xX

1 BB

BuB

yx

yY

1

ZYX

Xx

ZYX

Yy

ZYX

Zz

yx

xZX

1 yx

yZY

1, , ,

, ,

(4)

(5a)

(5b)

, , , , ,1uRZ 1u

GZ

, , , , ,1uBZ

tW

tW

tWut

Wyx

xX

1

,

tW

tW

tWut

Wyx

yY

1

, 1, utWZ

• Enter normalized tristimulus values of full Red, Green, Blue and initial White into eq. (3), and set sR=sG=sB=1:

• Calculate correction factors kR,kG,kB :

with determinants

• Calculate normalized and balanced tristimulus values XR,YR, .. ,ZB acc. eq. (6), e.g. ,

5

Calculation of luminance decrease (2)

kD

RkDGkD

BkD uR

uG

uB

uR

uG

uB

uR

uB

uB

uG

uG

uR YXYXYXYXYXYX

iW

uG

uB

iW

uG

uB

iW

uB

uB

uG

uG

iW YXYXYXYXYXYX u

RiW

uB

uR

iW

uB

uR

uB

uB

iW

iW

uR YXYXYXYXYXYX

uR

uG

iW

uR

uG

iW

uR

iW

iW

uG

uG

uR YXYXYXYXYXYX

kkR DDkR

(6)

kkG DDkG

kkB DDkB

uRRR XkX BB kZ

iWX

uRR Xk 1

uGG Xk 1 u

BB Xk 1 RX GX BX

iWY

uRR Yk 1 u

GG Yk 1uBB Yk 1 RY GY BY

1 11 Rk 11 Gk 11 Bk RZ GZ BZ

, ,

(7b)

(7a)

• Enter balanced tristimulus values XR,YR, ..,ZB of full Red, Green, Blue and of target White into eq. (3):

• Calculate preliminary scaling factors , , :

with determinants

6

Calculation of luminance decrease (3)

sD

RsD

GsD

BsD

BRGGBRRGBGRBRBGBGR ZYXZYXZYXZYXZYXZYX

But

WGGBut

WGBGut

WBBGBGut

W ZYXZYXYXZYXYXZYX ,,,, 11

BRut

WBRRut

WBRBRBut

WBut

WR ZYXYXZYXYXZYXZYX ,,,, 11

11 ,,,, RGGut

WRRGut

WGRut

WRut

WGGR YXZYXZYXZYXZYXYX

ssR DDsR

*

(8)

ssG DDsG

*ssB DDs

B*

utWX ,

RR Xs* GG Xs* BB Xs*

utWY ,

RR Ys* GG Ys* BB Ys*

1 RR Zs* GG Zs* BB Zs*

, ,

(9b)

(9a)

***BGR sss

Luminance decrease ldecr calculated out of x,y with normalized X,Y,Z values

Calculation of luminance decrease (4)

7

• White point calibration is achieved by adjusting the intensities of Red, Green and Blue

Final scaling factors sR, sG, sB 1 :

• Normalized White luminance after calibration:

• Luminance decrease with acc. eq. (5a):

• Note: Calculation of luminance decrease requires determination of scaling factors sR(nR), sG(nG), sB(nB),

but not of related grey levels nR,nG,nB

*** ,,max BGRs sssk sGG kss *sRR kss * sBB kss *, , (10)

suBBB

uGGG

uRRRBBGGRR

calW kYksYksYksYsYsYsY /)( *** (11)

01i

W

calW

decrY

Yl ((1))

iWY

,

• Calculation template

in Excel

• Input:

x, y of Red, Green,

Blue, initial and

target White

• Output:

Luminance decrease

8

Luminance decrease: Examples (1)

• Examples show consistency between calculation and measurement:

• Minor differences between calculation and measurement are due to

- limited measurement repeatability or / and

- limited color depth (8bit per color) of the applied TFT-LCDs

9

Luminance decrease: Examples (2)

Example #1 #2 #3 #4

Redx = 0,65919y = 0,30279

x = 0,64090y = 0,33205

x = 0.64133y = 0.33466

x = 0.63958y = 0.33774

Greenx = 0,28950y = 0,66838

x = 0,30722y = 0,61313

x = 0.29359y = 0.60889

x = 0.32741y = 0.61875

Bluex = 0,13851y = 0,07285

x = 0,15342y = 0,05741

x = 0.14650y = 0.06989

x = 0.15945y = 0.08918

Initial Whitex = 0,30090y = 0,32023

x = 0,32007y = 0,34049

x = 0.30682y = 0.34969

x = 0.33079y = 0.37521

Target Whitex = 0.307y = 0.321

x = 0.315y = 0.318

x = 0.315y = 0.318

x = 0.318y = 0.318

Calculated luminance decrease -5.4% -15.3% -24.2% -34.1%

Measured luminance decrease -5.6% -15.5% -24.2% -34.0%

Strongest luminance decrease when initial White is in direction of Cyan

• Example a)

Possible luminance decrease for

non-centered, quadratic tolerance

field of initial White point

• Example b)

Possible luminance decrease for

centered, hexagonal tolerance

field of initial White point

• Decrease trend changes

- in corners of tolerance fields

- at intersections of tolerance

border with rays from target

White to full Red, Green and

Blue

Luminance decrease: Examples (3)

10

0,29

0,30

0,31

0,32

0,33

0,34

0,35

0,36

0,37

0,38

0,26 0,27 0,28 0,29 0,30 0,31 0,32 0,33 0,34 0,35

A B

CD

Initial

White

Target

White

x

y

a)

-80

-70

-60

-50

-40

-30

-20

-10

0

A B C D (A)

Lu

min

ance d

ecre

ase l

dec

r[%

]

Initial White point location around tolerance border

0,27

0,28

0,29

0,30

0,31

0,32

0,33

0,34

0,35

0,36

0,27 0,28 0,29 0,30 0,31 0,32 0,33 0,34 0,35 0,36

A B

C

DE

F

y

x

b)

-50

-40

-30

-20

-10

0

A B C D (A)E F

Lu

min

ance d

ecre

ase l

dec

r[%

]

Initial White point location around tolerance border

Initial =Target White

• Initial White point can be tuned by designing individual aperture of Red, Green and Blue dots:

Initial dot aperture, e.g. 60..70%

, , Scaling factors acc. eq. (10), sR, sG, sB 1

, , Modified dot aperture

Luminance decreases similarly to electronic White point calibration

Color coordinates of Red, Green, Blue mostly kept; little influence from changed Black luminance of dots

Contrast does not decrease, see following derivation

• Introduction of “dot luminance” , , :

- Luminance inside single dots, excluding dot aperture

- Can be calculated out of “spot luminance”

• Introduction of “spot luminance” , :

- Average of luminance of many dots, including dot aperture

- User relevant, measurable value

11

Tuning of initial White point by individual dot aperture

iG

mG asa i

RmR asa i

BmB asa , , (12)

ia

mB

mG

mR aaa

BGR sss

„Dot luminance“ „Spot luminance“

ia

dB

dG

dR YYY

sBlack

sW YY

Tuning of dot aperture decreases luminance, but preserves contrast

Luminance and contrast for individual dot aperture

12

• White and Black spot luminance , and spot contrast for initial dot aperture:

• White and Black spot luminance , and spot contrast for modified dot aperture acc. eq. (12):

No contrast decrease, but similar luminance decrease as for electronic White point calibration:

(13a)iW

idB

dG

dR

isW YaYYYY

)1(.eq

31, )( Blackd

idB

dG

dR

isBlack Y

CR

aYYYY

)2(.eq

31, )(

Blacki

Wdis

Blackis

Wis YYCRYYCR ,,, (13b)

,

isWY , is

BlackY , isCR ,

msWY , ms

BlackY , msCR ,

isW

idBB

dGG

dRR

msW YaYsYsYsY ,

31, )( is

Blackd

idBB

dGG

dRR

msBlack Y

CR

aYsYsYsY ,

31, )( (14a)

(14b)isdmsBlack

msW

ms CRCRYYCR ,b).(13eq

,,,

,

decr

BGR

BBGGRRdB

dG

dR

dBB

dGG

dRR

isW

msWm

decr lYYY

YsYsYs

YYY

YsYsYs

Y

Yl

)11(),6(),1.(eq

,

,

111

(15)

• Initial White - compared to target - in direction of Cyan

Luminance decrease of -24% typ.

• Tuning of dot aperture

Initial mean White point moved to target White point

Color coordinates of Red, Green, Blue kept

Luminance decrease similar to initial design

Contrast preserved

13

Tuning of dot aperture: Example

TFT-LCD color filter with

same dot aperture for

Red, Green and Blue

TFT-LCD color filter with

reduced dot aperture for

Green and Blue

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8

x

y

Initial color

coordinates

Color coord.

after tuning of

dot aperture

after dot

aperture

tuning

initial

• White point calibration can decrease White luminance and contrast significantly

Product capability for luminance and contrast might be seriously affected

• Luminance and contrast decrease calculated out of given color coordinates x, y

Consistency with measurements demonstrated

Strongest luminance decrease when initial White point is in direction of Cyan

• Tuning of dot aperture

Sets initial mean White point to a defined target

Keeps mostly the color coordinates of Red, Green and Blue

Decreases White luminance similarly to the electronic White point calibration

Preserves contrast

14

Summary