Transcript Colour quality description of LED light sources for

Mesopic vision models and
their application
János Schanda1 and Agnes VidovszkyNemeth2
1. Virtual Environments and Imaging Technologies
Laboratory, University of Pannonia, Hungary
2. National Transport Authority, Hungary
Overview
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Mesopic vision fundamentals
The five photosensitive cells in the human retina
Luminance type and brightness type description
CIE Supplementary System of Photometry, Publ. 200
CIE Recommended System for Mesopic Photometry
based on Visual Performance, Publ. 191
Examples of application and open questions
Luminance levels
Mesopic vision
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Classical interpretation
 Daylight: photopic –
cones
 Dark adaptation:
scotopic – rods
 Twilight vision: mesopic
– cones + rods
Present day knowledge
 Foveal vision: photopic
 Pupil diameter:
intrinsically
photosensitive Retinal
Ganglion Cells
(ipRGC)?
 Difference between
perception and
detection
Spectral responsivity of light sensitive cells
in the human retina
1.2
1
rel. units
0.8
Cirk-Gall
0.6
V'(λ)
L(λ)
0.4
M(λ)
S(λ)
0.2
0
350 400 450 500 550 600 650 700 750 800
wavelength, nm
3 types of cones, rods and ipRGCs (Cirk.-Gall)
5
Perception and detection
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Perception:
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seeing details,
perceiving
brightness
all 3 cone types &
rods
+ ipRGC (?)
slower
Detection:
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only L & M cones +
rods
luminance like
signal
fast
Mesopic: rod contribution
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Two pathways for rodcone interaction
Classical: via rod bipolar
(RB) and amacrine (RA)
cells to cone bipolars
(DCB & HCB)
 Direct pathway via gap
junctions
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From Buck SL: Rod-cone interaction in human
vision, The visual neuroscience
Early investigations
Fovea: only cones
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Luminance like:
rapid, contrast
Brightness + colour:
slower mechanism
0
-0.5
log. sensitivity
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Abramov-Gordon
In mesopic the
influence of rods
increases
5', foveal
-1.5
1.5°, foveal
-2
1.5°, Exc.:45°
-2.5
6.5°, Exc.:45°
-3
-4
400
Peripheric vision:
rods + cones
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-1
-3.5
500
600
700
wavelength, nm
Stiles-Crawford (1935)
5
4.5
log. rel. sensitivity
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4
3.5
foveal
Exc.: 5°
3
2.5
2
1.5
400
500
600
wavelength, nm
700
Early investigations
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Brightness description:
Kokoschka 3 conew +
rods
 Sagawa brightness model
 Contrast threshold
investigations
Non-linear!
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Reaction time based
models
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aV(l)+(1-a)V’(l)
Brightness
perception
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Observation
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Coloured lights
brighter that
white (or yellow)
Influence of
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S cones
Rods, even in
daylight
ipRGC,
responsible also
for the circadian
rhythm
CIE supplementary system of photometry,
CIE 200:2011 for equivalent luminance
Photopic system
Scotopic system
x(λ)input y(λ )input z(λ)input
V'(λ )input
Scotopic
luminance
L'
Photopic
luminance
L
Cy/b Cr/g
H elm holtz-K ohlrausch
effect
Purkinje effect
a=
1-a
L
L+ a
a
(L') · (L) ·10
(adaptation coefficient; achromatic)
c
cc =a
= acc·[ f(x,y)
f(x,y) - 0.078]
1/2
ac = kL
1/2
L +b
(adaptation coefficient; chromatic)
Equivalent luminance, Leq
Parameters:
a = 0.05 cd/m2, b = 2.24 cd/m2, k = 1.3, f(x,y)=Nakano (1999)
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System for brightness description in the
photopic, mesopic, scotopic region
Detection
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Traffic situation
Detecting the presence of an obstacle
 Rapid action necessary
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Can be approximated by and additive
system
Abney’s law holds
photometry possible
 Should have smooth transition to photopic
and scotopic at the two ends.
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Forerunner mesopic models
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Lighting Research Center of North America system:
with 0,001 cd/m2 < Lmes < 0,6 cd/m2
 MOVE model, based on
 Ability to detect target
 Speed of detection
 Ability to identify details of target
with soft transition to scotopic and photopic at
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0,01 cd/m2 < Lmes < 10 cd/m2
Comparing the two systems
Two lamps with S/P ratio: 0.65 and 1.65: difference
of mesopic lum. to photopic lum. in the two systems
CIE Recommended System for Mesopic
Photometry based on Visual Performance
CIE Publ 191, prepared by TC 1-58, 1
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Compromise solution
between the two
experimental
systems, main input
data:
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achromatic contrast
reaction time (see ball
in windshield of virtual
reality simulation)
CIE 191, Part 2
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The system is not for visual performance :
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if chromatic channel signals are important:
if target has narrow band spectral power distributions
if brightness evaluation is required
Mesopic limits: 0,005 cd/m2 < Lmes < 5 cd/m2
The CIE 191 system is for adaptation luminance, i.e.
background luminance, not for calculating mesopic
luminance of target
Foveal vision is photopic!
Calculating mesopic luminance, 1
780nm
Lv  683
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Le  l  V (l )dl
780nm
Lv  1700
380nm
Photopic luminance
Mesopic luminance:
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Le  l  V '(l )dl
380nm
Scotopic luminance
where
and Vmes(l0)=Vmes(555nm)
m =1 if Lmes>5.0 cd/m2
m =0 if Lmes<0.005 cd/m2
M(m) is a normalizing constant: Vmes,max=1
Calculating mesopic luminance, 2
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m is calculated using iteration
Start with m0=0.5
Calculate Lmes,n from Lmes, n-1:
where
Vmes at different m values
An often encountred mistake
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One often encountered picture with title „spectral sensitivity”, it is a
photometry artefact: at 555 nm K(l) and K’(l) have to be equal: 683 lm/W
Spectral luminous efficacy
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One could define the candela at an other wavelength, e.g 528 nm
Calculation from pavement
illuminance
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Input data:
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Photopic luminance: Lp
Luminance coefficient of
road surface (q=L /E )
S/P ratio of light source,
780nm
where
S  1700
S (l )V '(l )dl

380nm
780nm
P  683
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S (l )V (l )dl
380nm
and S(l) is the rel.sp. power distribution
(SPD) of the lamp to be used
Calculation from pavement
illuminance
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Calculate Lp=qE
Calculate S /P and
with Lp determine Ls:
S / P = Ls / Lp
Calculate Lmes,1 from
with m0=0.5
And do the iteration,
usually 5 to 10
iterations are
needed to get final
Lmes
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If Vmes is required
Some examples
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q= 0.0016 and
q= 0.032
Typical light source
S/P values:
LPS
S/P
0,25
HPS
0,75
LED-2700K
1,12
LED-4000K
1,91
Numeric evaluation
Problems with the application of
the new mesopic photometry
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What is adaptation luminance?
Elderly observer
Visual acuity – contrast -eccentricity
Effect of radiation with short wavelength
radiation
Foveal vision photopic
There should be enough
mesopic contrast.
But to what do we adapt in
this situation?
Bodrogi: CIE mesopic Workshop, 2012.
Visual field – adaptation field?
What will be the adaptation
luminance?
Different sources in the visual field, different S/P
ratios
Fixed
Illumination
Car
headlamp
Blattner: CIE Mesopic Workshop 2012
Elderly observer
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Change of ocular transmission with age, normalized to the 30 years old
observer
Alferdinck: CIE Mesopic Workshop 2012
Visual acuity and lamp spectrum
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Test with cool-white and warm-white LEDs
Young observers: < 30 years of age
Old observers: > 65 years of age
Reading Snellen table at 0.1 cd/m2 and 1 cd/m2
Visual acuity and lamp spectrum,
results
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Young observers have less errors at 0,1 cd/m2 under CW-LED
At 1 cd/m2 the difference is not significant
Visual acuity - eccentricity
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For a given visual acuity the
needed contrast is colour
dependent and increases with
excentricity
Völker: CIE Mesopic Workshop 2012
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Change of visual acuity with
adaptation luminance
Further problems
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Re-adaptation from bright surrounding to
dark is long, increases with age
In foggy wheather light scatttering at
shorter wavelength increases.
Insects sensitivity to short wavelength is
higher
Astrological observations are more
sensitive to short wavelength stray light
Summary
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The mesopic photometry model is valid for background
adaptation luminance
It refers to reaction time type of tasks, not brightness
For foveal vision V(l) based metric (photopic
photometry) is valid!
It is an experimental model for trial, has to be validated
with real street lighting tests and accident simulations
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In preparing new recommendations spectral vision
differences between young and old observers should be
considered
Thanks for your kind attention!
This publication has been supported by the
TÁMOP-4.2.2/B-10/1-2010-0025 project.