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 3 types of cones, rods and ipRGCs (Cirk.-Gall) 5

Perception and detection 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 8

Early investigations Fovea: only cones Peripheric vision: rods + cones Luminance like: rapid, contrast Brightness + colour: slower mechanism Peripheric vision: rods + cones In mesopic the influence of rods increases

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

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/m2 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 where Fx, Fy, Fz functions depend on luminance level, L10 is photopic 10°luminance S is scotopic luminance X10, Y10, Z10 are 10° tristimulus values

Mesopic models 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 0,01 cd/m2 < Lmes < 10 cd/m2

Kokoschka model functions and derived spectral luminous efficiency functions

Brightness/luminance discrepancy & rod/cone interaction + ipRGCs 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=(L2+d2+t2)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

Detection Traffic situation Can be approximated by and additive system 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/m2 < Lmes < 5 cd/m2 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 Photopic luminance Scotopic luminance Mesopic 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 m is calculated using iteration Start with m0=0.5 Calculate Lmes,n from Lmes, n-1: where

Vmes at different m values

Calculation from pavement illuminance Input data: Photopic luminance: Lp Luminance coefficient of road surface (q=L/E) S/P ratio of light source, where and S(l) is the rel.sp. power distribution (SPD) of the lamp to be used

Calculation from pavement illuminance Calculate Lp=qE Calculate S/P and with Lp determine Ls Calculate Lmes,1 from with m0=0.5 And do the iteration, usually 5 to 10 iterations are needed to get final Lmes If Vmes is required, used

Some examples q= 0.0016 and q= 0.032 Typical light source S/P values: LPS 0,25 HPS 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/m2 and 1 cd/m2

Visual acuity and lamp spectrum, results Young observers have less errors at 0,1 cd/m2 under CW-LED At 1 cd/m2 the difference is not significant

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(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 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