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ILLUMINA project

Heterogeneous modeling of artificial sky radiance

Martin Aubé, Ph.D.

Light pollution induced sky radiance arise when light of artificial origin is throw toward the sky. A part of this light is returned back towards the Earth by the atmospheric constituents (molecules and aerosols). This phenomenon is highly correlated with an inapropriate and excessive use of the artificial light. In addition to limiting the access to the starry skies, light pollution acts negatively on fauna and the flora, on human health and it also contributes to the growth of energy needs and thus to the production of greenhouse gases.

GRAPHYCS is one of the research groups to have developed a hyper-spectral detector dedicated to the measurement of light pollution. Moreover, GRAPHYCS developed a sophisticated light pollution numerical model making it possible to simulate with precision human induced sky radiance under a variety of conditions. The novelty of this model lies in the fact that it makes it possible to take account of the heterogeneity of the environment in addition to being able to simulate the spectral behavior of the phenomenon. The present page is devoted to the description of this model named ILLUMINA.

The final objective of the project ILLUMINA is to develop a new methodology to allow the detection of aerosols during the night. The method is based on the fact that aerosols are partly responsible (with molecules of the atmosphere) for artificial sky radiance. However, in the shorter term, an important part of our research resources are used to study light pollution.

In this page, we dont want to treat in detail the problems surrounding light pollution, the reader is invited to consult the site for further information.

We chose to follow a research track based on the development and the use of numerical modeling. We seek to develop a model allowing to predict the level of artificial sky radiance according to a knowledge of the geographical distribution and physical properties of the light sources, a knowledge of the reflectance of the ground, of the topography, and of the atmospheric contents in aerosols and molecules. The model allow the simulation of the radiative properties of the atmosphere and thus we can compute the signal which would be detected by an instrument.

This model can be used independently to simulate the fraction of light pollution due to the molecules and to atmospheric aerosols. By adjusting the aerosol content in an iterative way we seek to minimize the difference between modeling prediction and a measurement. The method rely on a certain number of simplifying assumptions concerning the population of aerosols and on the assumption of an horizontal isotropy of the aerosols concentrations on a scale comparable with the size of the modeling domain (typically about 100 km). In other words we suppose that at the time of an observation, the composition and the vertical profile of aerosols are horizontally uniform over the modeling domain.

A funny aspect of this new approach lies in the fact that we are taking advantage of the presence of a pollution type (light) to detect some other (aerosols).

Characteristics of the 3D radiative model ILLUMINA

  • Physics and mathematics of ILLUMINA In progress and partly in french
  • 3D Calculation of single and double scattering with or without reflection on the ground
    To download animation
  • Ajustable horizontal model resolution
    Actually the only constraint is that a horizontal cell contains several sources since we consider a horizontal isotropy of angular light pattern of lighting fixtures
  • Taking into account for the heterogeneity of ground reflectance
    Up to now only lambertian surfaces are considered but simulated reflectance can vary from one cell to another
  • Taking into account for luminaires total luminosity for each horizontal element
  • Calculation of shadows
  • Correction for subgrid obstacles (trees, buildings)
  • Integration of the angular light pattern of the sources
    Up to now, we are only considering vertical anisotropy. It is thus necessary to average the function over all the azimuth directions. A script named ies2fctem.bash makes it possible to convert a photometric file in IES format into a horizontally averaged file such as required by ILLUMINA. The user can apply the tilt angle of the luminaire before performing the average
  • Maximum of 9 different kind of sources which can differ by their angular light pattern and post height.
  • Integration of the topography
  • Matrix size limit: 50 X 1024 X 1024
    The 50 th vertical level reach an altitude of ~ 30km so that we can account for about 99% of the atmosphere
  • Logarithmic based vertical scale.
    Thickness of the first level=0.5 m, thickness of the last level=5 km
  • Typical horizontal resolution of about 100 to 1000 m
  • Integrated spectrometer simulator
  • Statistical computing improvement:

Model inputs

  • Horizontal resolution
  • Geographical distribution of lighting devices spectral luminosity
  • Up to 99 distinct angular light pattern can be simulated at the same time
  • Geographical distribution of the luminaire height
  • Gridded ground reflectance
  • Digital elevation model
  • Angular light patterns (photometric file) for each kind of luminaire.
  • Aerosol optical properties (cross sections, phase function, extinction optical thickness)
  • Subgrid obstacles height and mean free path between obstacles.
  • Spectrometer optical properties (optional)
  • Maximum range of the second order scattering
  • Minimal altitude of the modeling domain and atmospheric pressure at this altitude

GlossyBlue theme adapté par David Gilbert et Martin Aubé
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