Abstract

Digital image analysis of the cloudless sky's daytime and twilight chromaticities challenges some existing ideas about sky colors. First, although the observed colors of the clear daytime sky do lie near the blackbody locus, their meridional chromaticity curves may resemble it very little. Second, analyses of twilight colors show that their meridional chromaticity curves vary greatly, with some surprising consequences for their colorimetric gamuts.

© 1994 Optical Society of America

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References

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  1. C. F. Bohren, A. B. Fraser, “Colors of the sky,” Phys. Teach. 23, 267–272 (1985).
    [Crossref]
  2. E. R. Dixon, “Spectral distribution of Australian daylight,” J. Opt. Soc. Am. 68, 437–450 (1978).
    [Crossref]
  3. V. D. P. Sastri, S. R. Das, “Typical spectral distributions and color for tropical daylight,” J. Opt. Soc. Am. 58, 391–398 (1968).
    [Crossref]
  4. V. D. P. Sastri, S. R. Das, “Spectral distribution and color of north sky at Delhi,” J. Opt. Soc. Am. 56, 829–830 (1966).
    [Crossref]
  5. G. T. Winch, M. C. Boshoff, C. J. Kok, A. G. du Toit, “Spectroradiometric and colorimetric characteristics of daylight in the southern hemisphere: Pretoria, South Africa,” J. Opt. Soc. Am. 56, 456–464 (1966).
    [Crossref]
  6. D. B. Judd, D. L. MacAdam, G. Wyszecki, “Spectral distribution of typical daylight as a function of correlated color temperature,” J. Opt. Soc. Am. 54, 1031–1040 (1964).
    [Crossref]
  7. H. R. Condit, F. Grum, “Spectral energy distribution of daylight,” J. Opt. Soc. Am. 54, 937–944 (1964).
    [Crossref]
  8. Y. Nayatani, G. Wyszecki, “Color of daylight from north sky,” J. Opt. Soc. Am. 53, 626–629 (1963).
    [Crossref]
  9. Daylight terminology is confusing at best, although some authors have tried to codify it. See, for example, G. Wyszecki, W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae (Wiley, New York, 1982), p. 11.
  10. R. L. Lee, “Colorimetric calibration of a video digitizing system: algorithm and applications,” Col. Res. Appl. 13, 180–186 (1988).
    [Crossref]
  11. We used a Photo Research PR-704 spectroradiometer with a nominal 0.5° FOV.
  12. F. F. Hall, “Twilight sky colors: observations and the status of modeling,” J. Opt. Soc. Am. 69, 1179–1180, 1197 (1979).
    [Crossref]
  13. C. N. Adams, G. N. Plass, G. W. Kattawar, “The influence of ozone and aerosols on the brightness and color of the twilight sky,” J. Atmos. Sci. 31, 1662–1674 (1974).
    [Crossref]
  14. R. L. Lee, “What are ‘all the colors of the rainbow’?” Appl. Opt. 30, 3401–3407, 3545 (1991). The 1991 printing of Eq. (1) inadvertently omitted the overbars; the reported ĝ values are correct, however.
    [Crossref] [PubMed]
  15. R. L. Lee, “Horizon brightness revisited: measurements and a model of clear-sky radiances,” Appl. Opt. 33, 4620–4628 (1994).
    [Crossref] [PubMed]
  16. These achromatic stimuli are derived from the extraterrestrial solar irradiances reported by M. P. Thekaekara, “Solar energy outside the earth's atmosphere,” Sol. Energy 14, 109–127 (1973).
    [Crossref]
  17. “Relative azimuth” here means azimuth measured with respect to the Sun's azimuth; a relative azimuth of 0° points toward the Sun's azimuth. For the hemispheric daylight chromaticities shown in Figs. 1 and 3, 0° relative azimuth was defined by tilting the surface normal of the radiometer's cosine detector at an angle of 75° from the zenith and by pointing this surface normal toward the Sun's azimuth.
  18. G. J. Burton, I. R. Moorhead, “Color and spatial structure in natural scenes,” Appl. Opt. 26, 157–170 (1987).
    [Crossref] [PubMed]
  19. C. J. Bartleson, “Memory colors of familiar objects,” J. Opt. Soc. Am. 50, 73–77 (1960).
    [Crossref] [PubMed]
  20. Readers unfamiliar with three-dimensional color spaces and color solids may consult Secs. 3.3.9–3.7 of Ref. 9.
  21. We have not shown a stereogram for the University Park, Pa., chromaticities (Ref. 15, Plate 37) because their noisiness and broad luminance maximum make stereo interpretation difficult.
  22. A. Meinel, M. Meinel, Sunsets, Twilights, and Evening Skies, (Cambridge U. Press, Cambridge, 1983), pp. 51–61. Reference 23 offers a more comprehensive survey of twilight optics.
  23. G. V. Rozenberg, Twilight: A Study in Atmospheric Optics (Plenum, New York, 1966).
  24. M. G. J. Minnaert, Light and Color in the Outdoors, translated and revised by L. Seymour, (Springer-Verlag, New York, 1993), pp. 295–297.
  25. Strictly speaking, the twilight arch is defined only for solar depression angles of ∼7°–16°. See, for example, H. Neuberger, Introduction to Physical Meteorology (Penn State Press, University Park, Pa., 1957), p. 185. However, the yellow band in Plate 43 (solar depression ∼2°) is a local luminance maximum, and it remained so for solar depression angles >7°.
  26. T. Deshler, B. J. Johnson, W. R. Rozier, “Balloonborne measurements of Pinatubo aerosol during 1991 and 1992 at 41° N: vertical profiles, size distribution, and volatility,” Geophys. Res. Lett. 20, 1435–1438 (1993).
    [Crossref]
  27. Although the twilight seen in Hall's Plate 116 (dated 17 August 1978; see Ref. 12) may be the basis for his Fig. 1, Hall does not state this unambiguously, thus making the issue of stratospheric aerosol loading moot for his data. In any case, the pastel colors of Hall's Plate 116 are quite different from those of vivid posteruption twilights (e.g., our Plate 43).

1994 (1)

1993 (1)

T. Deshler, B. J. Johnson, W. R. Rozier, “Balloonborne measurements of Pinatubo aerosol during 1991 and 1992 at 41° N: vertical profiles, size distribution, and volatility,” Geophys. Res. Lett. 20, 1435–1438 (1993).
[Crossref]

1991 (1)

R. L. Lee, “What are ‘all the colors of the rainbow’?” Appl. Opt. 30, 3401–3407, 3545 (1991). The 1991 printing of Eq. (1) inadvertently omitted the overbars; the reported ĝ values are correct, however.
[Crossref] [PubMed]

1988 (1)

R. L. Lee, “Colorimetric calibration of a video digitizing system: algorithm and applications,” Col. Res. Appl. 13, 180–186 (1988).
[Crossref]

1987 (1)

1985 (1)

C. F. Bohren, A. B. Fraser, “Colors of the sky,” Phys. Teach. 23, 267–272 (1985).
[Crossref]

1979 (1)

F. F. Hall, “Twilight sky colors: observations and the status of modeling,” J. Opt. Soc. Am. 69, 1179–1180, 1197 (1979).
[Crossref]

1978 (1)

1974 (1)

C. N. Adams, G. N. Plass, G. W. Kattawar, “The influence of ozone and aerosols on the brightness and color of the twilight sky,” J. Atmos. Sci. 31, 1662–1674 (1974).
[Crossref]

1973 (1)

These achromatic stimuli are derived from the extraterrestrial solar irradiances reported by M. P. Thekaekara, “Solar energy outside the earth's atmosphere,” Sol. Energy 14, 109–127 (1973).
[Crossref]

1968 (1)

1966 (2)

1964 (2)

1963 (1)

1960 (1)

Adams, C. N.

C. N. Adams, G. N. Plass, G. W. Kattawar, “The influence of ozone and aerosols on the brightness and color of the twilight sky,” J. Atmos. Sci. 31, 1662–1674 (1974).
[Crossref]

Bartleson, C. J.

Bohren, C. F.

C. F. Bohren, A. B. Fraser, “Colors of the sky,” Phys. Teach. 23, 267–272 (1985).
[Crossref]

Boshoff, M. C.

Burton, G. J.

Condit, H. R.

Das, S. R.

Deshler, T.

T. Deshler, B. J. Johnson, W. R. Rozier, “Balloonborne measurements of Pinatubo aerosol during 1991 and 1992 at 41° N: vertical profiles, size distribution, and volatility,” Geophys. Res. Lett. 20, 1435–1438 (1993).
[Crossref]

Dixon, E. R.

du Toit, A. G.

Fraser, A. B.

C. F. Bohren, A. B. Fraser, “Colors of the sky,” Phys. Teach. 23, 267–272 (1985).
[Crossref]

Grum, F.

Hall, F. F.

F. F. Hall, “Twilight sky colors: observations and the status of modeling,” J. Opt. Soc. Am. 69, 1179–1180, 1197 (1979).
[Crossref]

Johnson, B. J.

T. Deshler, B. J. Johnson, W. R. Rozier, “Balloonborne measurements of Pinatubo aerosol during 1991 and 1992 at 41° N: vertical profiles, size distribution, and volatility,” Geophys. Res. Lett. 20, 1435–1438 (1993).
[Crossref]

Judd, D. B.

Kattawar, G. W.

C. N. Adams, G. N. Plass, G. W. Kattawar, “The influence of ozone and aerosols on the brightness and color of the twilight sky,” J. Atmos. Sci. 31, 1662–1674 (1974).
[Crossref]

Kok, C. J.

Lee, R. L.

R. L. Lee, “Horizon brightness revisited: measurements and a model of clear-sky radiances,” Appl. Opt. 33, 4620–4628 (1994).
[Crossref] [PubMed]

R. L. Lee, “What are ‘all the colors of the rainbow’?” Appl. Opt. 30, 3401–3407, 3545 (1991). The 1991 printing of Eq. (1) inadvertently omitted the overbars; the reported ĝ values are correct, however.
[Crossref] [PubMed]

R. L. Lee, “Colorimetric calibration of a video digitizing system: algorithm and applications,” Col. Res. Appl. 13, 180–186 (1988).
[Crossref]

MacAdam, D. L.

Meinel, A.

A. Meinel, M. Meinel, Sunsets, Twilights, and Evening Skies, (Cambridge U. Press, Cambridge, 1983), pp. 51–61. Reference 23 offers a more comprehensive survey of twilight optics.

Meinel, M.

A. Meinel, M. Meinel, Sunsets, Twilights, and Evening Skies, (Cambridge U. Press, Cambridge, 1983), pp. 51–61. Reference 23 offers a more comprehensive survey of twilight optics.

Minnaert, M. G. J.

M. G. J. Minnaert, Light and Color in the Outdoors, translated and revised by L. Seymour, (Springer-Verlag, New York, 1993), pp. 295–297.

Moorhead, I. R.

Nayatani, Y.

Neuberger, H.

Strictly speaking, the twilight arch is defined only for solar depression angles of ∼7°–16°. See, for example, H. Neuberger, Introduction to Physical Meteorology (Penn State Press, University Park, Pa., 1957), p. 185. However, the yellow band in Plate 43 (solar depression ∼2°) is a local luminance maximum, and it remained so for solar depression angles >7°.

Plass, G. N.

C. N. Adams, G. N. Plass, G. W. Kattawar, “The influence of ozone and aerosols on the brightness and color of the twilight sky,” J. Atmos. Sci. 31, 1662–1674 (1974).
[Crossref]

Rozenberg, G. V.

G. V. Rozenberg, Twilight: A Study in Atmospheric Optics (Plenum, New York, 1966).

Rozier, W. R.

T. Deshler, B. J. Johnson, W. R. Rozier, “Balloonborne measurements of Pinatubo aerosol during 1991 and 1992 at 41° N: vertical profiles, size distribution, and volatility,” Geophys. Res. Lett. 20, 1435–1438 (1993).
[Crossref]

Sastri, V. D. P.

Stiles, W. S.

Daylight terminology is confusing at best, although some authors have tried to codify it. See, for example, G. Wyszecki, W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae (Wiley, New York, 1982), p. 11.

Thekaekara, M. P.

These achromatic stimuli are derived from the extraterrestrial solar irradiances reported by M. P. Thekaekara, “Solar energy outside the earth's atmosphere,” Sol. Energy 14, 109–127 (1973).
[Crossref]

Winch, G. T.

Wyszecki, G.

D. B. Judd, D. L. MacAdam, G. Wyszecki, “Spectral distribution of typical daylight as a function of correlated color temperature,” J. Opt. Soc. Am. 54, 1031–1040 (1964).
[Crossref]

Y. Nayatani, G. Wyszecki, “Color of daylight from north sky,” J. Opt. Soc. Am. 53, 626–629 (1963).
[Crossref]

Daylight terminology is confusing at best, although some authors have tried to codify it. See, for example, G. Wyszecki, W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae (Wiley, New York, 1982), p. 11.

Appl. Opt. (3)

R. L. Lee, “What are ‘all the colors of the rainbow’?” Appl. Opt. 30, 3401–3407, 3545 (1991). The 1991 printing of Eq. (1) inadvertently omitted the overbars; the reported ĝ values are correct, however.
[Crossref] [PubMed]

R. L. Lee, “Horizon brightness revisited: measurements and a model of clear-sky radiances,” Appl. Opt. 33, 4620–4628 (1994).
[Crossref] [PubMed]

G. J. Burton, I. R. Moorhead, “Color and spatial structure in natural scenes,” Appl. Opt. 26, 157–170 (1987).
[Crossref] [PubMed]

Col. Res. Appl. (1)

R. L. Lee, “Colorimetric calibration of a video digitizing system: algorithm and applications,” Col. Res. Appl. 13, 180–186 (1988).
[Crossref]

Geophys. Res. Lett. (1)

T. Deshler, B. J. Johnson, W. R. Rozier, “Balloonborne measurements of Pinatubo aerosol during 1991 and 1992 at 41° N: vertical profiles, size distribution, and volatility,” Geophys. Res. Lett. 20, 1435–1438 (1993).
[Crossref]

J. Atmos. Sci. (1)

C. N. Adams, G. N. Plass, G. W. Kattawar, “The influence of ozone and aerosols on the brightness and color of the twilight sky,” J. Atmos. Sci. 31, 1662–1674 (1974).
[Crossref]

J. Opt. Soc. Am. (9)

Phys. Teach. (1)

C. F. Bohren, A. B. Fraser, “Colors of the sky,” Phys. Teach. 23, 267–272 (1985).
[Crossref]

Sol. Energy (1)

These achromatic stimuli are derived from the extraterrestrial solar irradiances reported by M. P. Thekaekara, “Solar energy outside the earth's atmosphere,” Sol. Energy 14, 109–127 (1973).
[Crossref]

Other (10)

“Relative azimuth” here means azimuth measured with respect to the Sun's azimuth; a relative azimuth of 0° points toward the Sun's azimuth. For the hemispheric daylight chromaticities shown in Figs. 1 and 3, 0° relative azimuth was defined by tilting the surface normal of the radiometer's cosine detector at an angle of 75° from the zenith and by pointing this surface normal toward the Sun's azimuth.

We used a Photo Research PR-704 spectroradiometer with a nominal 0.5° FOV.

Daylight terminology is confusing at best, although some authors have tried to codify it. See, for example, G. Wyszecki, W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae (Wiley, New York, 1982), p. 11.

Readers unfamiliar with three-dimensional color spaces and color solids may consult Secs. 3.3.9–3.7 of Ref. 9.

We have not shown a stereogram for the University Park, Pa., chromaticities (Ref. 15, Plate 37) because their noisiness and broad luminance maximum make stereo interpretation difficult.

A. Meinel, M. Meinel, Sunsets, Twilights, and Evening Skies, (Cambridge U. Press, Cambridge, 1983), pp. 51–61. Reference 23 offers a more comprehensive survey of twilight optics.

G. V. Rozenberg, Twilight: A Study in Atmospheric Optics (Plenum, New York, 1966).

M. G. J. Minnaert, Light and Color in the Outdoors, translated and revised by L. Seymour, (Springer-Verlag, New York, 1993), pp. 295–297.

Strictly speaking, the twilight arch is defined only for solar depression angles of ∼7°–16°. See, for example, H. Neuberger, Introduction to Physical Meteorology (Penn State Press, University Park, Pa., 1957), p. 185. However, the yellow band in Plate 43 (solar depression ∼2°) is a local luminance maximum, and it remained so for solar depression angles >7°.

Although the twilight seen in Hall's Plate 116 (dated 17 August 1978; see Ref. 12) may be the basis for his Fig. 1, Hall does not state this unambiguously, thus making the issue of stratospheric aerosol loading moot for his data. In any case, the pastel colors of Hall's Plate 116 are quite different from those of vivid posteruption twilights (e.g., our Plate 43).

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Figures (13)

Fig. 1
Fig. 1

Comparison of CIE 1976 UCS chromaticity curves derived from photographic and spectroradiometer data for the same 0.5°-FOV meridional clear-sky scan made at ∼1605 GMT on 6 October 1992 at University Park, Pa. See Ref. 15, Plate 37 for the original photograph.

Fig. 2
Fig. 2

Chromaticity curves of daytime clear skies for Plates 37–40 (Ref. 15) and Plate 41 are compared with a portion of the blackbody locus. See Fig. 3 for a detailed view of these curves. The color of sunlight outside the atmosphere is marked by an ×.

Fig. 3
Fig. 3

Detailed view of Fig. 2. The daylight chromaticities (marked with ×'s) are estimated from hemispheric spectral irradiances measured at 0° relative azimuth and at the solar elevations listed in Table 1.

Fig. 4
Fig. 4

Relative luminance versus view elevation angle for Plates 37–40 (Ref. 15) and Plate 41. Compare these relative luminances with their stereo representations in Figs. 58.

Fig. 5
Fig. 5

Stereogram pair of the Bald Eagle Mountain sky's meridional luminance and chromaticity scan (see Ref. 15, Plate 38 for the original photograph). In this figure and in Figs. 68 and 12, (a) shows the left-hand side of the stereogram pair and (b) shows the right-hand side of the stereogram pair.

Fig. 6
Fig. 6

Stereogram pair of the Antarctic sky's meridional luminance and chromaticity scan (see Ref. 15, Plate 39 for the original photograph).

Fig. 7
Fig. 7

Stereogram pair of the Bermuda sky's meridional luminance and chromaticity scan (see Ref. 15, Plate 40 for the original photograph).

Fig. 8
Fig. 8

Stereogram pair of the Chesapeake sky's meridional luminance and chromaticity scan (see Plate 41 for the original photograph).

Fig. 9
Fig. 9

Chromaticity curves of twilight clear skies from Plate 42 (Manchester, N.H.) and Plate 43 (Philadelphia, Pa.) are compared with the near-twilight sky of Plate 41 (Chesapeake). AM and PM denote morning and evening twilights, respectively. See Fig. 10 for a detailed view of these curves.

Fig. 10
Fig. 10

Detailed view of Fig. 9's twilight chromaticity curves, labeled with view elevation angles corresponding to the horizon and the upper edges of Plates 41–43.

Fig. 11
Fig. 11

Relative luminance versus view elevation angle for Plates 41–43. Compare the Philadelphia twilight's relative luminances with their stereo representation in Fig. 12.

Fig. 12
Fig. 12

Stereogram pair of the Philadelphia twilight sky's meridional luminance and chromaticity scan (see Plate 43 for the original photograph).

Fig. 13
Fig. 13

Comparison of the naked-eye twilight chromaticities reported by Hall (Ref. 12, Fig. 1) and those plotted in Fig. 10. View elevation angles are labeled for most curves. The labels solar and antisolar indicate the relative azimuth of each meridional scan.

Tables (1)

Tables Icon

Table 1 Summary of Viewing Geometry and Chromaticity Information for Plates 37–43a

Equations (2)

Equations on this page are rendered with MathJax. Learn more.

g = { [ i = 1 N ( u i u ¯ ) 2 + ( υ i υ ¯ ) 2 ] / N } 1 / 2 .
ĝ = g / g s .

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