Mesopic vision models and their application

1. Mesopic vision models and their application János Schanda1 and Agnes Vidovszky-Nemeth2 1. Virtual Environments and Imaging Technologies Laboratory, University of Pannonia, Hungary 2. National Transport Authority, Hungary

2. Overview • 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

3. Luminance levels

4. 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

5. Spectral responsivity of light sensitive cells in the human retina 3 types of cones, rodsand ipRGCs (Cirk.-Gall) 5

6. Perception and detection • Perception: • seeing details, perceiving brightness • all 3 cone types & rods • + ipRGC (?) • slower • Detection: • only L & M cones + rodsluminance like signal • fast

7. 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

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

9. 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)

10. 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

11. CIE supplementary system of photometry, CIE 200:2011 for equivalent luminance System for brightness description in the photopic, mesopic, scotopic region

12. 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.

13. Forerunner 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

14. 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

15. CIE Recommended System for Mesopic Photometry based on Visual Performance CIE Publ 191, prepared by TC 1-58, 1 • Compromise solution between the two experimental systems, main input data: • achromatic contrast • reaction time (see ball in windshield of virtual reality simulation)

16. CIE 191, Part 2 • The system is not for visual performance : • 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!

17. 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

18. Calculating mesopic luminance, 2 • m is calculated using iteration • Start with m0=0.5 • Calculate Lmes,n from Lmes, n-1: where

19. Vmes at different m values

20. An often encountred mistake 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

21. Spectral luminous efficacy One could define the candela at an other wavelength, e.g 528 nm

22. 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

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

24. Some examples • q= 0.0016 and • q= 0.032 • Typical light source S/P values:

28. Numeric evaluation

29. Problems with the application of the new mesopic photometry What is adaptation luminance? Elderly observer Visual acuity – contrast -eccentricity Effect of radiation with short wavelength radiation Foveal vision photopic

30. There should be enough mesopic contrast. But to what do we adapt in this situation? Bodrogi: CIE mesopic Workshop, 2012.

31. Visual field – adaptation field?

32. What will be the adaptation luminance?

33. Different sources in the visual field, different S/P ratios Fixed Illumi- nation Car head- lamp Blattner: CIE Mesopic Workshop 2012

34. Elderly observer Alferdinck: CIE Mesopic Workshop 2012 Change of ocular transmission with age, normalized to the 30 years old observer

35. Visual acuity and lamp spectrum • 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

36. 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

37. Visual acuity - eccentricity • Change of visual acuity with adaptation luminance For a given visual acuity the needed contrast is colour dependent and increases with excentricity Völker: CIE Mesopic Workshop 2012

38. Further problems 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

39. 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

40. Thanks for your kind attention! This publication has been supported by the TÁMOP-4.2.2/B-10/1-2010-0025 project.