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Applied Optics

Applied Optics


  • Editor: James C. Wyant
  • Vol. 47, Iss. 34 — Dec. 1, 2008
  • pp: H106–H115

Measuring overcast colors with all-sky imaging

Raymond L. Lee, Jr.  »View Author Affiliations

Applied Optics, Vol. 47, Issue 34, pp. H106-H115 (2008)

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Digital images of overcast skies as seen from the earth’s surface open new windows onto the angular details of overcast colors and visible-wavelength spectra. After calibration with a spectroradiometer, a commercial CCD camera equipped with a fisheye lens can produce colorimetrically accurate all-sky maps of overcast spectra. Histograms and azimuthally averaged curves of the resulting chromaticities show consistent, but unexpected, patterns in time-averaged overcast colors. Although widely used models such as LOWTRAN7 and MODTRAN4 cannot explain these characteristic patterns, a simple semiempirical model based on the radiative transfer equation does, and it provides insights into the visible consequences of absorption and scattering both within and beneath overcasts.

© 2008 Optical Society of America

OCIS Codes
(010.1290) Atmospheric and oceanic optics : Atmospheric optics
(290.1090) Scattering : Aerosol and cloud effects
(330.1730) Vision, color, and visual optics : Colorimetry
(010.1615) Atmospheric and oceanic optics : Clouds
(010.7295) Atmospheric and oceanic optics : Visibility and imaging
(010.1690) Atmospheric and oceanic optics : Color

Original Manuscript: April 18, 2008
Manuscript Accepted: June 5, 2008
Published: September 12, 2008

Raymond L. Lee, Jr., "Measuring overcast colors with all-sky imaging," Appl. Opt. 47, H106-H115 (2008)

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  1. M. Minnaert, Light and Color in the Outdoors, translated and revised by L. Seymour (Springer-Verlag, 1993), p. 325. This edition's foreword indicates that the book was first published in 1937, so Minnaert's earliest research predates the mid-1930s.
  2. W. E. K. Middleton, Vision through the Atmosphere (U. Toronto Press, 1952), pp. 155-172.
  3. A pioneering work on irradiance-based overcast color is W. E. K. Middleton's “The color of the overcast sky,” J. Opt. Soc. Am. 44, 793-798 (1954). [CrossRef]
  4. S. Nann and C. Riordan, “Solar spectral irradiance under clear and cloudy skies: measurements and a semiempirical model,” J. Appl. Meteorol. 30, 447-462 (1991). [CrossRef]
  5. J. Hernández-Andrés, R. L. Lee, Jr., J. Romero, and J. L. Nieves, “Color and spectral analysis of daylight in southern Europe,” J. Opt. Soc. Am. A 18, 1325-1335 (2001). [CrossRef]
  6. R. L. Lee, Jr. and J. Hernández-Andrés, “Colors of the daytime overcast sky,” Appl. Opt. 44, 5712-5722 (2005). [CrossRef] [PubMed]
  7. One early example is L. T. Maloney and B. A. Wandell, “Color constancy: a method for recovering surface spectral reflectance,” J. Opt. Soc. Am. A 3, 29-33 (1986). [CrossRef] [PubMed]
  8. R. L. Lee, Jr., “Colorimetric calibration of a video digitizing system: algorithm and applications,” Color Res. Appl. 13, 180-186 (1988). [CrossRef]
  9. D. Connah, S. Westland, and M. G. A. Thomson, “Recovering spectral information using digital camera systems,” Coloration Technol. 117, 309-312 (2001). [CrossRef]
  10. J. L. Nieves, E. M. Valero, S. M. C. Nascimento, J. Hernández-Andrés, and J. Romero, “Multispectral synthesis of daylight using a commercial digital CCD camera,” Appl. Opt. 44, 5696-5703 (2005). [CrossRef] [PubMed]
  11. PR-650 spectroradiometer from Photo Research, Inc., 9731 Topanga Canyon Place, Chatsworth, Calif. 91311. According to Photo Research, at specified radiance levels a properly calibrated PR-650 measures luminance and radiance accurate to within ±4%, has a spectral accuracy of ±2 nm, and its CIE 1931 colorimetric errors are x<0.001, y<0.001 for a 2856 K blackbody (CIE standard illuminant A).
  12. J. Romero, A. García-Beltrán, and J. Hernández-Andrés, “Linear bases for representation of natural and artificial illuminants,” J. Opt. Soc. Am. A 14, 1007-1014 (1997). [CrossRef]
  13. J. Hernández-Andrés, J. L. Nieves, E. M. Valero, and J. Romero, “Spectral-daylight recovery by use of only a few sensors,” J. Opt. Soc. Am. A 21, 13-23 (2004). [CrossRef]
  14. For any pair of chromaticities separated by Δu′ and Δv′, calculate this error as the Euclidean distance Δu′v′=[(Δu′)2+(Δv′)2]½.
  15. G. Wyszecki and W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed. (Wiley, 1982), pp. 306-310.
  16. All photographs are corrected for the effective integrated transmissivity of this fisheye lens as a function of angle from its optical axis, but tests show that no additional spectral corrections are needed to the transform matrix F.
  17. R. L. Lee, Jr. and J. Hernández-Andrés, “Short-term variability of overcast brightness,” Appl. Opt. 44, 5704-5711 (2005). [CrossRef] [PubMed]
  18. For example, see Spatial Distribution of Daylight--CIE Standard Overcast Sky and Clear Sky, CIE Standard S 003/E-1996 (Commission Internationale de l'Eclairage, 1996), p. 3.
  19. R. L. Lee, Jr. and D. E. Devan, “Observed brightness distributions in overcast skies,” Appl. Opt. 47, H116-H127 (2008). [CrossRef] [PubMed]
  20. For a definition of g^, see R. L. Lee, Jr., “Twilight and daytime colors of the clear sky,” Appl. Opt. 33, 4629-4638, 4959 (1994). [CrossRef] [PubMed]
  21. R. L. Lee, Jr., “Horizon brightness revisited: measurements and a model of clear-sky radiances,” Appl. Opt. 33, 4620-4628, 4959 (1994). [CrossRef] [PubMed]
  22. D. B. Judd, D. L. MacAdam, and G. Wyszecki, “Spectral distribution of typical daylight as a function of correlated color temperature,” J. Opt. Soc. Am. 54, 1031-1040 (1964). [CrossRef]
  23. J. Hernández-Andrés, R. L. Lee, Jr., and J. Romero, “Calculating correlated color temperatures across the entire gamut of daylight and skylight chromaticities,” Appl. Opt. 38, 5703-5709 (1999). [CrossRef]
  24. , pp. 224-225.
  25. C. F. Bohren and E. E. Clothiaux, Fundamentals of Atmospheric Radiation (Wiley-VCH, 2006), p. 294.
  26. C. F. Bohren, Clouds in a Glass of Beer: Simple Experiments in Atmospheric Physics (Wiley, 1987), p. 149.

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