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Strictly Private and Confidential Paul Weindorf 3.6 Automotive Automatic Luminance Control

Automatic Luminance Control

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On October 23, 2014 at the Detroit SID (Society of Information Displays) Symposium, Paul Weindorf, Visteon technical fellow delivered a presentation on Automatic Luminance Controls. The objective of the paper/presentation is to formulate and describe methods that may be used to automatically adjust the display luminance in order to maintain a visible display under all lighting conditions.

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Page 1: Automatic Luminance Control

Strictly Private and Confidential

Paul Weindorf

3.6 Automotive Automatic Luminance Control

Page 2: Automatic Luminance Control

Outline

Introduction

System Requirements

Display Luminance Power Function

Constant Ratio Steps Concept

Luminance Table Structure Example

Logarithmic Light Sensor

Mathematical Schematic

Software Filtering

Conclusion

Page 2

Page 3: Automatic Luminance Control

Introduction

Number of displays used per vehicle is increasing year over year

Increasing level of focus on safety & readability, resulting in displays being placed higher

(closer to the eye line) and consequently increased vulnerability to reflections

Display luminance levels are increasing due to increased background reflections

High luminance required for high ambient conditions are distracting and cause undesirable

pupil contraction for lower ambient conditions

Backlight power levels are increasing!

Thermal management problems are leading

to active cooling resulting in more cost & noise

Problem: Historically Automatic Luminance Control

systems do not work properly

Solution: Properly Designed Automatic Luminance Control System

– Worked flawlessly on the Visteon Navigation Radio

– Only provides the higher luminance when needed

– Allows higher peak luminance levels with cooler thermal mass until de-rating occurs

Page 3

Kornacki break out

the dilithium crystals. We need

more power!!

Page 4: Automatic Luminance Control

System Requirements

Page 4

Photosensor

Automatic Luminance Control (Power Function)

Backlight

User Bias

Automatic Luminance Control adjusts the display luminance to maintain display

visibility as a function of:

– Light incident on the display (Must work over 5-8 decades of illuminance)

– User preference (Bias) with constant ratio luminance steps

– Power Function relating display emitted luminance as a function of reflected

background luminance based on user geometry

Page 5: Automatic Luminance Control

Display Luminance Power Function

Page 5

Figure 2.1-3: CR versus Background Luminance using Eq 2.1-1

Excerpt from “The ABC’s of Automatic Brightness Control” by Robin Merrifield

and Louis D. Silverstein –

These early “constant-contrast” mechanizations did not prove popular

or effective for several reasons. Display contrast requirements are not

constant over the range of cockpit ambient illumination.1 An observer’s

contrast sensitivity increases as ambient illumination increases.

Relatively high contrast is required at low ambient levels while relatively

low contrast is required at high cockpit illumination levels.

Page 6: Automatic Luminance Control

Display Luminance Power Function

Power Function Relationship: Required Display Luminance (ESL) as a function of

Background Luminance (DBL)

Page 6

CO DBLBESL

1

10

100

1000

10000

100000

1 100 10000

Emit

ted

Sym

bo

l Lu

min

ance

(E

SL)

Display Background Luminance (BGL)

Silverstein

ISO9241

13406

Visteon

ISO 9241 based on

Kokoschka mathematical

evaluation and not on

human factor studies

Page 7: Automatic Luminance Control

Constant Ratio Steps

Page 7

Tables constructed on the basis of luminance ratio (R) steps are easy for

software implementation and minimize processor throughput impacts

Luminance ratio (R) steps are commonly used by OEMs for night time luminance

control because the eye perceives luminance ratio steps as a linear increase in

luminance

Luminance step ratio (R) tables are convenient for user auto-luminance

preference offset control

– LSEL = Intermediate selected luminance ratio step

– LMax = Maximum luminance step luminance

– LMin = Minimum luminance step luminance

– T = Total number of luminance levels

– N = Step number selection out of the total steps, T

1

1

T

Min

Max

L

LR Eq 2.2-7

1T

NT

Min

Max

MaxSEL

L

L

LL Eq 2.2-15

Page 8: Automatic Luminance Control

Luminance Table Structure Example

Page 8

Example of user preference offset NB=2

Rule is to go up by 2 steps from the light sensor determined value

– R = Luminance step ratio

– VA/D = A/D converter reference rail voltage

– C =Silverstein power constant

– NA/D = A/D resolution

– VT = Logarithmic amplifier transistor thermal

voltage

– AV = Logarithmic amplifier gain (note that AV is

temperature compensated such that the AVVT

remains constant over temperature)

– ∆ADC = A/D count difference between

successive luminance ratio steps

N LN DBLNLn

10 bit A/D

Linear

10 bit A/D

0 38.71 0.68 23 0.68

1 50.00 1.41 123 1.42

2 64.58 2.94 223 2.95

3 83.41 6.10 323 6.13

4 107.72 12.66 423 12.74

5 139.13 26.30 523 26.46

6 179.69 54.64 623 54.96

7 232.08 113.49 723 114.15

8 299.74 235.73 823 237.11

9 387.13 489.63 923 492.51

10 500.00 1017.03 1023 1023.00

NB=2Log Light Sensor ADC Value

cV

RVAADC

DA

N

TVDA

/

ln12 /

Ea A2-25B

If constant luminance ratio table is used, each luminance

step will correspond to an equal A/D count increase to

provide the power function when a log sensor is utilized

1

1

T

Min

Max

L

LR Eq 2.2-7

Page 9: Automatic Luminance Control

Logarithmic Light Sensor

Page 9

Minimize

Dark

Current

with 0V

across

light

sensor

Single Power

Supply =

ADC Voltage

Single Power Supply

Rail-to-Rail Output

Op Amps with Low

Input Bias Currents

2

1lnI

IVV T

I2 Reference

Current

Generator

VT Temperature

Compensation

Logarithmic Light Sensor is a Key Element since 5-8 Decades of Dynamic

Range is Required.

Th

p

TVO VI

iVAV

2

ln

Page 10: Automatic Luminance Control

Mathematical Schematic

Page 10

Log Light Sensor(5.1) A/D Converter(5.2) Calculate Step N(5.3)

ip VoTh

p

TV VI

iVA

2

ln

AV

Gain

ADC

N

V

DA 12 /

NA/D

A/DResolution

VADC

A/DRail

Voltage

VT=KT/q

ThermalVoltage

I2

ReferenceCurrent

VTh

TheveninOffset

Voltage

ADC

ADC

ADCADC O

ADCO

ADCCount

N=0

∆ADC

# of A/DCounts/

Step

N

User Bias(5.4)

N+NB

Luminance Table(5.5)

NB

UserBias

Math Simplification(5.6)

BNN

T

Max RR

L

T=Total # of

LuminanceLevels

1

1

T

Min

Max

L

L

LMax

LMin

R

LSEL

∆ADC

R

CV

RVA

ADC

N

TVDA

ln12 /

AV=Gain

VT=Thermal Voltage

VADC=A/D Rail Volts C=Silverstein Power Constant

LSEL

ADC

ADCV

VO

ADC

ThN DA

12 /

NOS

DBL Conversion(5.7)

R

xxR

ln

lnlog

NA/D

∆ADC VTh ADCO

ip

C

C

P

N

T

MaxNDBL

IKR

R

LR OSB

2

LSEL CO

N

SEL DBLBRL B

BO

BO Substitution(5.8)

C

P

N

T

Max

IKR

R

LOS

2

PK

DBL=Display Background Luminance

β=Reflectance Factor

KP=Lux/ipµA

C

pN

T

MaxN

I

iR

R

LR OSB

2

The log base conversion law

is the what allows equal

ADC counts to relate to

luminance ratio steps and

provide the power function

Page 11: Automatic Luminance Control

Software Filtering

Page 11

Eliminate display luminance “breathing” changes due to picket fence effects

Works like a peak detector for worst case sunload (brighter is better)

Large sudden ambient illumination changes Abnormal Shadowing Event

Small ambient illumination changes Permanent “True” changes

Operational principal:

– Determine number of luminance steps ∆N from current level (step)

– Change by 1 step after waiting ∆N x (Time Constant)

Can be described as an “Inverse Filter” since it is opposite to an exponential type

response (slow in the beginning and changes faster when it approaches the final

value)

Page 12: Automatic Luminance Control

Software Filtering – Intermittent Shadowing

Page 12

0

20

40

60

80

100

120

0

50

100

150

200

250

300

350

400

450

500

0 2 4 6 8 10

Ste

p #

Lum

inan

ce (

cd/m

2)

Time (seconds)

Variable Time Constant Filter Response (Tcup=0.01sec, Tcdown=0.1sec)

Algorithm Luminance

BS# Output

BS#N Input

• 40 step table

• Step #1 Luminance = 79.43 cd/m2

• Step #40 Luminance = 430.36 cd/m2

• Tcdown Time Constant = 0.1 seconds

• Tcup Time Constant = 0.01 seconds

Page 13: Automatic Luminance Control

Summary/Conclusion

The power function is the correct relationship for automatic luminance control

The use of a logarithmic (or other suitable compression function) light sensor is the

key to working successfully over 5-8 decades of illuminance

The use of a logarithmic light sensor allows equal ADC deltas to relate to luminance

ratio steps and provide the desired power function.

The use of the inverse filter software is one method to successfully deal with

“unnatural” large changes in illuminance

Page 13

Page 14: Automatic Luminance Control

Page 14

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