Sources, Surfaces and Scatter

Transcription

1 Sources, Surfaces and Scatter An investigation into the interaction of light sources, surfaces, eyes & the scattering of light by the atmosphere David M. Keith, FIES Jefferey F. Knox IESNA Roadway Lighting Committee, 2003

2 Lighting Basics: The Lumen Lumens = K * [ P(λ) * ν(λ) ] Spectral Power Distribution 150 HPS Photopic Sensitivity Function 1.40E Power (Watts) 1.20E E E E E E-01 Series1 Sensitivity Series1 0.00E Wavelength (nm) Wavelength Lumens source = K * [ S(λ) * ν(λ) ] Sum λ from 360 to 770 nanometers

3 Lighting Basics : The Lumen * Implications of the definition of the Lumen: the summation over the visible spectrum makes every lumen spectrally ignorant * A Lumen is a Lumen is a Lumen but are they the same? How could we tell? * for evaluating the magnitude of spectrally sensitive effects, such as atmospheric scatter, use radiation, NOT lumens!

4 Lighting Basics: Reflectance * Reflectance = Lumens off Lumens on this calculation is specific to a particular source * Average Reflectance What is average reflectance? Is it related to a specific source? For CRI we use the CIE s D65 and A (incandescent) * Because reflectance is lumen-based, it too is spectrally ignorant

5 Lighting Basics: Reflectivity * reflectance may be spectrally sensitive, so: do not convert to lumens until at the retina! replace average reflectance with Reflectivity = [ P(λ) on * ρ(λ)] [ P(λ) on ] ρ(λ) from NASA s ASTER Spectral Library

6 Lighting Basics: Reflectivity 100% 90% 80% 70% Reflectivity 60% 50% 40% Concrete Asphalt 30% 20% 10% 0% wavelength (nm)

7 Lighting Basics: Skyglow * What is the problem? light gets in the way of seeing the night sky * How does the light get in the way? the light is redirected by the atmosphere * Where does the light come from? uplight can be direct from sources uplight generally occurs from reflectance

8 Why uplight matters.. * uplight is the unavoidable result of exterior lighting (and windows without curtains!) * since we want to see objects, we make them reflective and luminous themselves * the same light that we can use to see can also escape up into the sky * when there is more uplight there is more skyglow too!

9 An observer looks at the sky..

10 .. where he sees skyglow..

11 .. which is caused by uplight..

12 .. scattered by the atmosphere. * atmospheric scattering redirects light like surfaces do - but without a surface! * Rayleigh scatter is what makes the sky blue also causes the colors at sunrise and sunset scatter is extremely dependent on wavelength redirection can be extreme, normal or minimal * Mie scatter causes halo around the sun or moon, small change in direction, small effect * Inelastic scatter is minor contribution

13 Rayleigh Scatter Index * Rayleigh scatter is proportional to the: quantity of light, separated into bands (S( )) wavelength of the light (, in nanometers) to the inverse 4th power! * so the equation for RSI is: RSI = k (S( ) / 4 ) for from 360 to 770 nm, with constant k = 5.0E11 for S( ) at 1 W over spectrum

14 What does RSI indicate? * RSI is a comparative measure of the likelihood that light going up will be redirected by Rayleigh scatter, the dominant contributor to skyglow when atmospheric conditions are most appropriate for viewing the night sky - clear sky, few clouds, little pollution or particulates, corresponding to blue sky conditions of daytime

15 Sunlight: Blackbody at 6600K Radiant Energy (W) Source wavelength (nm)

16 Scattered Sunlight Radiant Energy (W) Source Scatter wavelength (nm)

17 Color Shift for Scattered Sunlight

18 Compare Blackbody Sources Radiant Energy (W) K Source 2856K Source wavelength (nm)

19 Compare Sources & Scatter Radiant Energy (W) K Source 6600K Scatter 2856K Source 2856K Scatter wavelength (nm)

20 Scattered Radiation Profiles * the RSI calculation creates a spectral radiation profile that represents the relative distribution of skyglow for each source same procedure applied to different data compare magnitudes & distributions between sources overall radiant energy photopic lumens scotopic lumens specific bands: e.g. between 455 & 465 nm identified as significant for melatonin & diurnal cycle

21 Equal Energy, CIE D65 & CIE A Power (Watts per nm) EqEn CIE D65 CIE A wavelength (nm)

22 RSI Results: 100 Photopic Lms EqEn D65 CIE A Radiant Watts Sources Asphalt Concrete

23 High Pressure Sodium W Power (Watts per nm) C100 C150 C250 C wavelength (nm)

24 RSI for HPS: 100 Photopic Lms C100 C150 C250 C400 Radiant W Sources Asphalt Concrete

25 Metal Halide 250W & 400W Power (Watts per nm) M25H M25U M40H M40U wavelength (nm)

26 RSI for MH: 100 Photopic Lms M25H M25U M40H M40U Radiant W Sources Asphalt Concrete

27 Average MH & HPS Sources Radiant Energy (W) MH Source HPS Source wavelength (nm)

28 Average MH & HPS Scatter Radiant Energy (W) MH Scatter HPS Scatter wavelength (nm)

29 Radiation from the sky per 100 lms AvgMH AvgHPS MH-to-HPS Initial W % Sources % Asphalt % Concrete %

30 Relative Skyglow from 100 Lumens AvgMH AvgHPS MH-to-HPS RSI (radiant W) % Photopic Lumens % Scotopic Lumens % nm (W) % nm (W) %

31 Average MH & HPS Scatter Radiant Energy (W) MH Scatter HPS Scatter wavelength (nm)

32 Skyglow & Spectral Sources * the phenomenon of skyglow is caused by atmospheric scattering (when the sky is clear!) * the dominant form of atmospheric scattering is Rayleigh scattering (inverse 4th power of ) * Rayleigh Scattering Index (RSI) provides relative information about different SPD s * HPS produces less skyglow compared to MH from the same amount of lumens or radiation

33 Sources, Surfaces and Scatter Get a copy of this presentation at: IESNA Roadway Lighting Committee, 2003 David M. Keith, FIES Jefferey F. Knox