Post on 09-Jul-2015
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Strictly Private and Confidential
Paul Weindorf
3.6 Automotive 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
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!!
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
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.
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
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
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
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
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
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)
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
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
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