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Describing Visual Air Quality Is A Complex Issue Depending On:

This article discusses the complexity of describing visual air quality, including the characteristics of the observer, target, scene illumination, and optical properties of the atmosphere. It also explores the use of Monte Carlo Radiation Transfer Model and sky radiance measurements to estimate radiance and extinction levels. The advantages and disadvantages of using projected slides and reflection prints for visual air quality assessments are compared. Finally, visibility modeling software options are mentioned.

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Describing Visual Air Quality Is A Complex Issue Depending On:

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  1. 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

  2. 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

  3. Sky Radiance Measurements 5O Azimuth & 1O Elevation

  4. 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);

  5. 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)

  6. 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

  7. 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;

  8. 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.

  9. Seeing is a psychophysicalphenomenon not easily modeled or reproduced by any process that is strictly physical in origin.

  10. Requirements For Match • Spectral Sensitivities • Color Gamuts • Dynamic Range

  11. 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

  12. 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

  13. Slides vs. Onsite Judgments

  14. Projected 35 mm Slides Disadvantages • Must be viewed in darkened room. • Respondents must be brought to central facility. • Slides degrade rapidly with projection time.

  15. Degradation of 35 mm slides

  16. 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.

  17. 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.

  18. Visibility Modeling Software • Aerosol • PLUVUE • WinHaze • VisualHaze

  19. Aerosol

  20. WinHaze

  21. 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.

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