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
Lighting Basics: The Lumen Lumens = K * [ P(λ) * ν(λ) ] Spectral Power Distribution 150 HPS Photopic Sensitivity Function 1.40E+00 1.2 Power (Watts) 1.20E+00 1.00E+00 8.00E-01 6.00E-01 4.00E-01 2.00E-01 Series1 Sensitivity 1 0.8 0.6 0.4 0.2 Series1 0.00E+00 0 380 411 442 473 504 535 566 597 628 659 360 394 428 462 496 530 564 598 632 666 700 734 768 802 690 721 752 Wavelength (nm) Wavelength Lumens source = K * [ S(λ) * ν(λ) ] Sum λ from 360 to 770 nanometers
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!
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
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
Lighting Basics: Reflectivity 100% 90% 80% 70% Reflectivity 60% 50% 40% Concrete Asphalt 30% 20% 10% 0% 360 390 420 450 480 510 540 570 600 630 660 690 720 750 wavelength (nm)
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
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!
An observer looks at the sky..
.. where he sees skyglow..
.. which is caused by uplight..
.. 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
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
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
Sunlight: Blackbody at 6600K 0.016 0.014 0.012 Radiant Energy (W) 0.01 0.008 0.006 Source 0.004 0.002 0 360 390 420 450 480 510 540 570 600 630 660 690 720 750 wavelength (nm)
Scattered Sunlight 0.016 0.014 0.012 Radiant Energy (W) 0.01 0.008 0.006 Source Scatter 0.004 0.002 0 360 390 420 450 480 510 540 570 600 630 660 690 720 750 wavelength (nm)
Color Shift for Scattered Sunlight
Compare Blackbody Sources 0.016 0.014 0.012 Radiant Energy (W) 0.01 0.008 0.006 6600K Source 2856K Source 0.004 0.002 0 360 390 420 450 480 510 540 570 600 630 660 690 720 750 wavelength (nm)
Compare Sources & Scatter 0.016 0.014 0.012 Radiant Energy (W) 0.01 0.008 0.006 6600K Source 6600K Scatter 2856K Source 2856K Scatter 0.004 0.002 0 360 390 420 450 480 510 540 570 600 630 660 690 720 750 wavelength (nm)
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
Equal Energy, CIE D65 & CIE A 0.004 0.003 Power (Watts per nm) 0.002 0.001 EqEn CIE D65 CIE A 0 360 390 420 450 480 510 540 570 600 630 660 690 720 750 wavelength (nm)
RSI Results: 100 Photopic Lms EqEn D65 CIE A Radiant Watts 0.56 0.49 0.61 Sources 4.4 3.8 2.3 Asphalt 4.1 3.6 2.3 Concrete 3.5 3.1 2.4
High Pressure Sodium 100-400W 0.014 0.012 Power (Watts per nm) 0.01 0.008 0.006 0.004 C100 C150 C250 C400 0.002 0 360 390 420 450 480 510 540 570 600 630 660 690 720 750 wavelength (nm)
RSI for HPS: 100 Photopic Lms C100 C150 C250 C400 Radiant W 0.25 0.25 0.26 0.25 Sources 1.2 1.2 1.2 1.2 Asphalt 1.2 1.2 1.2 1.2 Concrete 1.2 1.2 1.3 1.2
Metal Halide 250W & 400W 0.014 0.012 Power (Watts per nm) 0.01 0.008 0.006 0.004 M25H M25U M40H M40U 0.002 0 360 390 420 450 480 510 540 570 600 630 660 690 720 750 wavelength (nm)
RSI for MH: 100 Photopic Lms M25H M25U M40H M40U Radiant W 0.31 0.30 0.32 0.30 Sources 2.3 2.4 2.5 2.3 Asphalt 2.2 2.3 2.4 2.2 Concrete 2.0 2.0 2.1 1.9
Average MH & HPS Sources 0.01 0.009 0.008 Radiant Energy (W) 0.007 0.006 0.005 0.004 0.003 MH Source HPS Source 0.002 0.001 0 360 390 420 450 480 510 540 570 600 630 660 690 720 750 wavelength (nm)
Average MH & HPS Scatter 0.01 0.009 0.008 Radiant Energy (W) 0.007 0.006 0.005 0.004 0.003 MH Scatter HPS Scatter 0.002 0.001 0 360 390 420 450 480 510 540 570 600 630 660 690 720 750 wavelength (nm)
Radiation from the sky per 100 lms AvgMH AvgHPS MH-to-HPS Initial W 0.31 0.26 121% Sources 2.4 1.2 200% Asphalt 2.3 1.2 189% Concrete 2.0 1.2 162%
Relative Skyglow from 100 Lumens AvgMH AvgHPS MH-to-HPS RSI (radiant W) 2.4 1.2 200% Photopic Lumens 5.1 4.4 115% Scotopic Lumens 12.3 4.5 278% 455-465 nm (W) 0.42 0.21 205% 360-460 nm (W) 1.10 0.15 728%
Average MH & HPS Scatter 0.01 0.009 0.008 Radiant Energy (W) 0.007 0.006 0.005 0.004 0.003 MH Scatter HPS Scatter 0.002 0.001 0 360 390 420 450 480 510 540 570 600 630 660 690 720 750 wavelength (nm)
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
Sources, Surfaces and Scatter Get a copy of this presentation at: http://resodance.com/mdi/scatter.html IESNA Roadway Lighting Committee, 2003 David M. Keith, FIES Jefferey F. Knox