describing visual air quality is a complex issue depending on: characteristics of observer optical...
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Describing Visual Air Quality Is AComplex Issue Depending On:
• characteristics of observer
• optical characteristics of target
• illumination of scene
• optical properties of atmosphere, ambient aerosols and gases
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Monte Carlo Radiation Transfer ModelTo Model Sky Radiance NS
• backwards photon trajectory• multiple scattering, spherical
geometry• Lambertian surface reflection• ANY complex inhomogeneous
distribution of wave-length dependent scattering and absorbing gases, aerosols or clouds
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Sky Radiance Measurements 5O Azimuth & 1O Elevation
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Measured vs. Modeled Sky RadiancesFebruary 19 - 21, 1995
Measured Sky Radiance (w/cm2/sr/nm)
5 6 7 8 9 10 11 12 13 14 15 16
Mo
de
led
Sky R
ad
ian
ce
5
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Horizon Sky: 5O Az & 1O El400 - 680 nm
modeled = 1.01 measured r2 = 0.86
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Measured/Modeled Horizon Sky RadianceFebruary 19 - 21, 1995 1100 - 1600 hrs
Radiance Ratio: Measured / Modeled (400 - 680 nm)0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
No
rm
ali
zed
Fre
qu
en
cy
0.0
0.1
0.2
0.3
0.4
0.5
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1.0
n = 754mean = 0.988sigma = 0.075
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Input 35 mm Color Slide
• Digitize into three color (RGB) files;
• calibrate with density vs. exposure data from manufacture;
• Mask to get distance (r) to each picture element (pixel);
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Estimating Radiance From Digitized Slide
• Exposure (E) = light intensity x time
• all picture elements (pixels) exposed for the same time
• thus, exposure can be used as a surrogate for radiance: E = f(density)
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Kodachrome 64(curve fit to published Kodak data)
Density
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2
Exp
osu
re (
lux-
seco
nd
s)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
Curve Fits:
Rexp = -0.07777 + 0.2992 * Density -1.013
Gexp = -0.04891 + 0.2231 * Density -1.089
Bexp = -0.03761 + 0.1909 * Density -1.209
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Input 35 mm Color Slide
• estimate sky radiance with Monte Carlo model and calibrated sky pixels from base slide;
• estimate extinction (bext) on day slide was taken;
• invert equation to calculate inherent radiance: NO = (Nr - NS(1-T)) / T
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Model New Image Extinction
• MIE model to get aerosol optical properties: phase function and single scattering albedo;
• generate model geometry assigning spatial distribution of aerosol;
• use Monte Carlo model to estimate new sky radiance;
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Aerosol Normalized Phase Functions
Scattering Angle O0 20 40 60 80 100 120 140 160 180
No
rma
lize
d I
nte
ns
ity
0.015
0.03
0.1
0.3
1
1.6
Regional Aerosol 450 nmRegional Aerosol 550 nmRegional Aerosol 650 nmUrban Aerosol 550 nm
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Model New Image Extinction
• assume inherent radiance is unchanged with changing extinction levels;
• use equilibrium radiance model to get new image radiances: Nr = NOT + NS(1-T)
• output new image.
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Seeing is a psychophysicalphenomenon not easily
modeled or reproduced by any process that is strictly physical
in origin.
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Requirements For Match
• Spectral Sensitivities
• Color Gamuts
• Dynamic Range
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Dynamic Range
• Human visual system: 10,000 : 1
• Projected slides: 500 : 1
• Best Computer Monitor: 200 : 1
• Reflection Prints: Highest Quality 50 : 1 Newspaper 15 : 1
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Projected 35 mm Slides Advantages
• Best Color Saturation and dynamic range.
• Proven to be valid representations of actual scenes for judgments of perceived scenic beauty, landscape preferences and visual air quality.
• Can easily design survey to include preview images, control scenes and multiple visual air quality scenarios
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Slides vs. Onsite Judgments
Slide Perceived Visual Air Quality
1 2 3 4 5 6 7 8 9 10
On
site
Per
ceiv
ed V
isu
al A
ir Q
ual
ity
1
2
3
4
5
6
7
8
9
10
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Projected 35 mm Slides Disadvantages
• Must be viewed in darkened room.
• Respondents must be brought to central facility.
• Slides degrade rapidly with projection time.
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Degradation of 35 mm slides
0
0.5
1
1.5
2
2.5
3
3.5
4 8 16 32
Cumulative Projection Time (mins)
delt
aE F
rom
Unp
roje
cted
Sli
de
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Reflection Prints Advantages
• Easily transportable so can be brought to respondents.
• Do not degrade as fast as 35 mm slides.
• Have been used in most previous CV studies of visibility benefits.
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Reflection Prints Disadvantages
• Limited dynamic range means small changes in visual air quality cannot be presented.
• Must use small focal length lens and large print size to mimic onsite field of view.
• Difficult to efficiently present multiple visual air quality levels with prints.
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Visibility Modeling Software
• Aerosol
• PLUVUE
• WinHaze
• VisualHaze
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Aerosol
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WinHaze
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WinHaze• Small image files: 900 x 600 pixels;
• Sky radiances calculated from fit to thousands of full Monte Carlo runs using a standard rural aerosol model;
• All base images have same Rayleigh sky
• Does not account for color shifts due to absorbing aerosols or gases;
• Optimized for viewing on CRT monitors only;
• IS FAST and simple to run.