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

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

Sky Radiance Measurements 5O Azimuth & 1O Elevation

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

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6

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9

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14

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16

Horizon Sky: 5O Az & 1O El400 - 680 nm

modeled = 1.01 measured r2 = 0.86

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

0.6

0.7

0.8

0.9

1.0

n = 754mean = 0.988sigma = 0.075

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);

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)

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

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1.0

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1.2

1.3

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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

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

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;

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

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.

Seeing is a psychophysicalphenomenon not easily

modeled or reproduced by any process that is strictly physical

in origin.

Requirements For Match

• Spectral Sensitivities

• Color Gamuts

• Dynamic Range

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

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

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

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5

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9

10

Projected 35 mm Slides Disadvantages

• Must be viewed in darkened room.

• Respondents must be brought to central facility.

• Slides degrade rapidly with projection time.

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

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.

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.

Visibility Modeling Software

• Aerosol

• PLUVUE

• WinHaze

• VisualHaze

Aerosol

WinHaze

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.