Describing Visual Air Quality Is A
Complex 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 Model
To 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 Radiances
February 19 - 21, 1995
16
Horizon Sky: 5O Az & 1O El
15
400 - 680 nm
Modeled Sky Radiance
14
13
12
11
10
9
8
7
modeled = 1.01 measured
r2 = 0.86
6
5
5
6
7
8
9
10
11
12
13
Measured Sky Radiance
2
(w/cm /sr/nm)
14
15
16
Measured/Modeled Horizon Sky Radiance
February 19 - 21, 1995 1100 - 1600 hrs
1.0
0.9
n = 754
mean = 0.988
sigma = 0.075
Normalized Frequency
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
Radiance Ratio: Measured / Modeled (400 - 680 nm)
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)
2.0
1.9
Kodachrome 64
1.8
(curve fit to published Kodak data)
Exposure (lux-seconds)
1.7
1.6
Curve Fits:
1.5
Rexp = -0.07777 + 0.2992 * Density -1.013
1.4
Gexp = -0.04891 + 0.2231 * Density -1.089
1.3
Bexp = -0.03761 + 0.1909 * Density -1.209
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Density
2.0
2.2
2.4
2.6
2.8
3.0
3.2
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
1.6
Normalized Intensity
1
Regional Aerosol 450 nm
Regional Aerosol 550 nm
Regional Aerosol 650 nm
Urban Aerosol 550 nm
0.3
0.1
0.03
0.015
0
20
40
60
80
100
120
Scattering Angle
O
140
160
180
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 psychophysical
phenomenon 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
Onsite Perceived Visual Air Quality
10
9
8
7
6
5
4
3
2
1
1
2
3
4
5
6
7
8
Slide Perceived Visual Air Quality
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.
deltaE From Unprojected Slide
Degradation of 35 mm slides
3.5
3
2.5
2
1.5
1
0.5
0
4
8
16
Cumulative Projection Time (mins)
32
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.