Transcript Colour quality description of LED light sources for
Mesopic vision model and its application
János Schanda
Virtual Environments and Imaging Technologies Laboratory University of Pannonia
Overview
Mesopic vision fundamentals The five photosensitive cells in the human retina Luminance type and brightness type description The CIE TC 1-69 mesopic model Examples of application
Luminance levels
Mesopic vision
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 0.8
0.6
0.4
0.2
Cirk-Gall V'(λ) L(λ) M(λ) S(λ) 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
Perception: seeing details, perceiving brightness all 3 cone types & rods + ipRGC (?) slower Detection: only L & M cones + rods luminance like signal fast
Colour perception
Historic overview
The two stumbling blocks of mesopic photometry: Purkinje shift rod and cone interaction transition from cone to rod sp.resp. differs from Purkinje shift Helmhotz-Kohlrausch effect difference between luminance and brightness
Early investigations
Fovea: only cones Luminance like: rapid, contrast Brightness + colour: slower mechanism Peripheric vision: rods + cones In mesopic the influence of rods increases 5 4.5
4 3.5
3 2.5
2 1.5
400
Abramov-Gordon
0 -0.5
-1 -1.5
-2 -2.5
-3 -3.5
-4 400 500 600
wavelength, nm Stiles-Crawford (1935)
700 5', foveal 1.5°, foveal 1.5°, Exc.:45° 6.5°, Exc.:45° 700 foveal Exc.: 5° 500 600
wavelength, nm
Early investigations
Brightness description: Kokoschka 3 conew + rods Sagawa brightness model Contrast threshold investigations Non-linear!
Reaction time based models aV ( l )+(1 a ) V’ ( l )
Spectral responsivity at different luminance levels Walters & Wright : Proc. Roy. Soc. 1943.
At low mesopic levels first a shoulder is found at longer wavelength At mid-mesopic a transition from rod to cone takes place: broadening of curve At high mesopic narrowing to V(l) is found
Spectral responsivity at different luminance levels Kinney clearly identifies in 1955 that at high mesopic and photopic levels brightness sensitivity has a multi bump nature
CIE 1963 mesopic spectral sensitivities
1 0,8 0,6 0,4 0,2 0 400 450 500 550 600 wavelength, nm 650 700 10-5 cd/m2 10-3 cd/m2 10-1 cd/m2 10 cd/m2
Brightness perception
Observation Coloured lights brighter that white (or yellow) Influence of S cones Rods, even in daylight ipRGC, responsible also for the circadian rhythm
Quantitative descriptions of mesopic luminance
Equivalent luminance (CIE 1963): „The equivalent luminance of the field of an arbitrary spectral composition is the standard luminance of another field which has the colour temperature of 2042 K and which in particular photometric conditions seems to be equally bright to the first field.”
Mesopic 2 function brightness scale
Palmer (1966, 1967, 1968): Equivalent luminance ( L ) for large fields L ( S , P )=( MS + P 2 )/( M + P ) where: S: scotopic value P: photopic (10°) value M parameter, 6.28 . 10 -2 cd/m 2 for 15° field Other models used direct cone brightness functions (e.g. Ikeda & Shimozono, 1981; Nakano & Ikeda, 1986; Sagawa & Takeichi, 1983; modified Palmer formula)
Mesopic 4 function brightness
Helmhotz-Kohlrausch effect: brightness non additivity, influence of all three cone types Kokoschka model Trezona – Clarke model Problem of transition between luminance type (photopic) and brightness type (scotopic) experimental data: Viénot et al.
Kokoschka model
L
eq
X
10
Y
10
F x
F y
Z
10
Y
10
F z
S L
10
F S
L
10 where F x , F y , F z L 10 S X is scotopic luminance 10 functions depend on luminance level, is photopic 10°luminance , Y 10 , Z 10 are 10° tristimulus values
Mesopic models
Lighting Research Center of North America system: with 0,001 cd/m 2 < L mes < 0,6 cd/m 2 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 0,01 cd/m 2 < L mes < 10 cd/m 2
Kokoschka model functions and derived spectral luminous efficiency functions
Brightness/luminance discrepancy & rod/cone interaction + ipRGCs 1.
2.
3.
In photopic regime one assumes: luminance to be a combination of the L- and M cone signals Brightness to be a combination of all three cone signals after a transformation into magno-, parvo- and conio-cellular signals In mesopic regime rod contribution has to be added.
Recently found ipRGCs have influence on pupillary reflex, influencing light reaching the retina
Photopic regime: brightness/luminance Magnocellular pathway: luminance like Brightness: all 3 channels B=(L 2 +d 2 +t 2 ) 1/2 (Guth model) Influence of ipRGCs?
Different brightness of metameric samples
Mesopic: rod contribution Two pathways for rod cone interaction Classical: via rod bipolar (RB) and amacrine (RA) cells to cone bipolars (DCB & HCB) Direct pathway via gap junctions From Buck SL: Rod-cone interaction in human vision, The visual neuroscience
2 new CIE reports CIE supplementary system of photometry, CIE 200:2011 Recommended system for mesopic photometry based on visual performance, based on MOVE and X models
Scotopic system V'(
λ
)input x(
λ
)input y(
λ
)input z(
λ
)input Photopic system
Scotopic luminance
L'
Purkinje effect Photopic luminance
L
C y/b C r/g
H elm holtz-K ohlrausch effect
a
=
L L +
a
1-a a
(L') · (L) ·10
c
a c c
= =
a a c
c
L
· [
f(x,y) kL 1/2 1/2 +
b - 0.078] (adaptation coefficient; achromatic) (adaptation coefficient; chromatic)
Equivalent luminance, L
eq
Parameters: a
= 0.05 cd/m 2 ,
b
= 2.24 cd/m 2 ,
k
= 1.3, f(x,y)=Nakano (1999)
Detection
Traffic situation Detecting the presence of an obstacle Rapid action necessary 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.
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 TC 1-58 system, 1
Compromise solution between the two experimental systems, main input data: achromatic contrast reaction time (see ball in windshield of virtual reality simulation)
CIE TC 1-58 system, 2
The system is not for visual performance : if chromatic channel signals are important: S/P ratio very higy or low if target has narrow band spectral power distributions if brightness evaluation is required Mesopic limits: 0,005 cd/m 2 < L mes < 5 cd/m 2 The TC 1-58 system is for adaptation luminance, i.e. background luminance, not for calculating mesopic luminance of target Foveal vision is photopic!
Calculating mesopic luminance, 1
L
v 683 780nm 380nm
L
e
V L
v 1700 780nm 380nm
L
e
V
Photopic luminance Scotopic luminance Mesopic luminance: where and V mes ( l 0 )= V mes (555nm) m =1 if L mes >5.0 cd/m 2 m =0 if L mes <0.005 cd/m 2 M ( m ) is a normalizing constant: V mes,max =1
Calculating mesopic luminance, 2
m is calculated using iteration Start with m 0 =0.5
Calculate L mes,n from L mes, n-1 : where
V mes
at different
m
values
Calculation from pavement illuminance
Input data: Photopic luminance: L p Luminance coefficient of road surface ( q = L / E ) S/P ratio of light source, where
S
1700 780nm 380nm
S P
683 780nm 380nm
S
l l and S ( l ) is the rel.sp. power distribution (SPD) of the lamp to be used l l l l
Calculation from pavement illuminance
Calculate L p = qE Calculate with Calculate with L p S / P and determine L mes,1 m 0 =0.5
L s from And do the iteration, usually 5 to 10 iterations are needed to get final L mes If Vmes is required, used
Some examples
q = 0.0016 and q = 0.032
Typical light source S/P values:
LPS HPS S/P 0,25 0,75 LED-2700K 1,12 LED-4000K 1,91
Numeric evaluation
Visual acuity and lamp spectrum
Transmission of eye media changes with age 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/m 2 and 1 cd/m 2
Visual acuity and lamp spectrum, results
Young observers have less errors at 0,1 cd/m 2 At 1 cd/m 2 the difference is not significant under CW-LED
Summary
The mesopic photometry model is valid for background adaptation luminance It refers to reaction time type of tasks, not brightness For foveal vision V ( photometry) is valid!
l ) based metric (photopic It is an experimental model for trial, has to be validated with real street lighting tests and accident simulations In preparing new recommendations spectral vision differences between young and old observers should be considered
Peter Kaisers vision of the future photometer in 1981
Modern image taking photometers are almost there + info: background ipRGCs