Abstract

In the clear sky there are three commonly known loci, the Arago, Babinet, and Brewster neutral points, where the skylight is unpolarized. These peculiar celestial points, bearing the names of their discoverers, have been the subject of many ground-based investigations, because their positions are sensitive indicators of the amount and type of atmospheric turbidity. According to theoretical considerations and computer simulations, there should exist an additional neutral point approximately opposite to the Babinet point, which can be observed only at higher altitudes in the air or space. Until now, this anonymous “fourth” neutral point has not been observed during air- or space-borne polarimetric experiments and has been forgotten, in spite of the fact that the neutral points were a basic tool in atmospheric research for a century. Here, we report on the first observation of this fourth neutral point from a hot air balloon. Using 180°-field-of-view imaging polarimetry, we could observe the fourth neutral point at 450, 550, and 650 nm from different altitudes between 900 and 3500 m during and after sunrise at approximately 22°–40° below the anti-solar point along the anti-solar meridian, depending on the wavelength and solar elevation. We show that the fourth neutral point exists at the expected location and has characteristics similar to those of the Arago, Babinet, and Brewster points. We discuss why the fourth neutral point has not been observed in previous air- or space-borne polarimetric experiments.

© 2002 Optical Society of America

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References

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  1. G. P. Können, Polarized Light in Nature (Cambridge U. Press, Cambridge, UK, 1985).
  2. K. L. Coulson, Polarization and Intensity of Light in the Atmosphere (A. Deepak, Hampton, Virginia, 1988).
  3. M. J. A. Barral, Oeuvres de Francois Arago I–V (Gide, Paris, 1858).
  4. J. Babinet, “Sur un nouveau point neutre dans l’atmosphère,” C. R. Acad. Sci. Paris 11, 618–620 (1840).
  5. D. Brewster, “On the existence of a new neutral point and two secondary neutral points,” Rep. Brit. Assoc. Adv. Sci. 2, 13–25 (1842).
  6. D. Brewster, “On the polarisation of the atmosphere,” Philos. Mag. J. Sci. 31, 444–454 (1847).
  7. M. J. L. Soret, “Influence des surfaces d’eau sur la polarisation atmosphérique et observation de deux points neutres à droite et à gauche de Soleil,” C. R. 107, 867–870 (1888).
  8. M. A. Cornu, “Observations relatives a la couronne visible actuellement autour du Soleil,” C. R. 99, 488–493 (1884).
  9. I. Pomozi, J. Gál, G. Horváth, R. Wehner, “Fine structure of the celestial polarization pattern and its temporal change during the total solar eclipse of 11 August 1999,” Rem. Sens. Environ. 76, 181–201 (2001).
    [CrossRef]
  10. G. Horváth, I. Pomozi, J. Gál, “Neutral points of skylight polarization observed during the total eclipse on 11 August 1999,” Appl. Opt. (to be published).
  11. H. Neuberger, “Arago’s neutral point: a neglected tool in meteorological research,” Bull. Am. Meteorol. Soc. 31, 119–125 (1950).
  12. G. V. Rozenberg, Twilight (Plenum, New York, 1966).
  13. F. M. Bréon, J. L. Deuzé, D. Tanré, M. Herman, “Validation of space-borne estimates of aerosol loading from sun photometer measurements with emphasis on polarization,” J. Geophys. Res. D 102, 17187–17195 (1997).
    [CrossRef]
  14. T. Gehrels, ed. Planets, Stars, and Nebulae Studied with Photopolarimetry (University of Arizona Press, Tucson, Ariz., 1974).
  15. J. Gál, G. Horváth, A. Barta, R. Wehner, “Polarization of the moonlit clear night sky measured by full-sky imaging polarimetry at full moon: comparison of the polarization of moonlit and sunlit skies,” J. Geophys. Res. D 106(D19), 22647–22653 (2001).
    [CrossRef]
  16. J. Gál, G. Horváth, V. B. Meyer-Rochow, “Measurement of the reflection–polarization pattern of the flat water surface under a clear sky at sunset,” Rem. Sens. Environ. 76, 103–111 (2001).
    [CrossRef]
  17. J. Gál, G. Horváth, V. B. Meyer-Rochow, R. Wehner, “Polarization patterns of the summer sky and its neutral points measured by full-sky imaging polarimetry in Finnish Lapland north of the Arctic Circle,” Proc. R. Soc. London Ser. A 457, 1385–1399 (2001).
    [CrossRef]
  18. I. Pomozi, G. Horváth, R. Wehner, “How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation,” J. Exp. Biol. 204, 2933–2942 (2001).
    [PubMed]
  19. G. Horváth, A. Barta, J. Gál, B. Suhai, O. Haiman, “Ground-based full-sky imaging polarimetry of rapidly changing skies and its use for polarimetric cloud detection,” Appl. Opt. 41, 543–559 (2002).
    [CrossRef] [PubMed]
  20. G. Horváth, J. Gál, I. Pomozi, R. Wehner, “Polarization portrait of the Arago point: video-polarimetric imaging of the neutral points of skylight polarization,” Naturwissenschaften 85, 333–339 (1998).
    [CrossRef]
  21. G. Horváth, R. Wehner, “Skylight polarization as perceived by desert ants and measured by video polarimetry,” J. Comp. Physiol. A 184, 1–7 (1999).
    [CrossRef]
  22. G. P. Können, Royal Netherlands Meteorological Institute, P.O. Box 201, NL-3730 AE De Bilt, The Netherlands; e-mail: konnen@knmi.nl (personal communication, November2001).
  23. N. C. R. Rao, “Balloon measurements of the polarization of the light diffusely reflected by the earth’s atmosphere,” Planet. Space Sci. 17, 1307–1309 (1969).
    [CrossRef]
  24. K. L. Coulson, V. S. Whitehead, C. Campbell, “Polarized views of the earth from orbital altitude,” in Ocean Optics VIII, M. A. Blizard, ed., Proc. SPIE637, 35–41 (1986).
    [CrossRef]
  25. M. Herman, J. Y. Balois, L. Gonzalez, P. Lecomte, J. Lenoble, R. Santer, C. Verwaerde, “Stratospheric aerosol observations from a balloon-borne polarimetric experiment,” Appl. Opt. 25, 3573–3584 (1986).
    [CrossRef] [PubMed]
  26. J. L. Deuzé, C. Devaux, M. Herman, R. Santer, J. Y. Balois, L. Gonzalez, P. Lecomte, C. Verwaerde, “Photopolarimetric observations of aerosols and clouds from balloon,” Rem. Sens. Environ. 29, 93–109 (1989).
    [CrossRef]
  27. P. Y. Deschamps, F. M. Bréon, M. Leroy, A. Podaire, A. Bricaud, J. C. Buriez, G. Séze, “The POLDER mission: instrument characteristics and scientific objectives,” IEEE Trans. Geosci. Rem. Sens. 32, 598–615 (1994).
    [CrossRef]
  28. J. L. Deuzé, F. M. Bréon, P. Y. Deschamps, C. Devaux, M. Herman, “Analysis of the POLDER (POLarization and Directionality of Earth’s Reflectances) airborne instrument observations over land surfaces,” Rem. Sens. Environ. 45, 137–154 (1993).
    [CrossRef]
  29. D. K. Lynch (David.K.Lynch@aero.org) wrote to G. Horvath on 26November2001: “The 4th neutral point has been a favorite topic of mine for some years, yet I have never published the results. I have observed it twice, once in 1983 and again in 1994 from aircraft, but I have not published the results because my calibrations were not too good. The existence, however, is certain.”

2002 (1)

2001 (5)

J. Gál, G. Horváth, A. Barta, R. Wehner, “Polarization of the moonlit clear night sky measured by full-sky imaging polarimetry at full moon: comparison of the polarization of moonlit and sunlit skies,” J. Geophys. Res. D 106(D19), 22647–22653 (2001).
[CrossRef]

J. Gál, G. Horváth, V. B. Meyer-Rochow, “Measurement of the reflection–polarization pattern of the flat water surface under a clear sky at sunset,” Rem. Sens. Environ. 76, 103–111 (2001).
[CrossRef]

J. Gál, G. Horváth, V. B. Meyer-Rochow, R. Wehner, “Polarization patterns of the summer sky and its neutral points measured by full-sky imaging polarimetry in Finnish Lapland north of the Arctic Circle,” Proc. R. Soc. London Ser. A 457, 1385–1399 (2001).
[CrossRef]

I. Pomozi, G. Horváth, R. Wehner, “How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation,” J. Exp. Biol. 204, 2933–2942 (2001).
[PubMed]

I. Pomozi, J. Gál, G. Horváth, R. Wehner, “Fine structure of the celestial polarization pattern and its temporal change during the total solar eclipse of 11 August 1999,” Rem. Sens. Environ. 76, 181–201 (2001).
[CrossRef]

1999 (1)

G. Horváth, R. Wehner, “Skylight polarization as perceived by desert ants and measured by video polarimetry,” J. Comp. Physiol. A 184, 1–7 (1999).
[CrossRef]

1998 (1)

G. Horváth, J. Gál, I. Pomozi, R. Wehner, “Polarization portrait of the Arago point: video-polarimetric imaging of the neutral points of skylight polarization,” Naturwissenschaften 85, 333–339 (1998).
[CrossRef]

1997 (1)

F. M. Bréon, J. L. Deuzé, D. Tanré, M. Herman, “Validation of space-borne estimates of aerosol loading from sun photometer measurements with emphasis on polarization,” J. Geophys. Res. D 102, 17187–17195 (1997).
[CrossRef]

1994 (1)

P. Y. Deschamps, F. M. Bréon, M. Leroy, A. Podaire, A. Bricaud, J. C. Buriez, G. Séze, “The POLDER mission: instrument characteristics and scientific objectives,” IEEE Trans. Geosci. Rem. Sens. 32, 598–615 (1994).
[CrossRef]

1993 (1)

J. L. Deuzé, F. M. Bréon, P. Y. Deschamps, C. Devaux, M. Herman, “Analysis of the POLDER (POLarization and Directionality of Earth’s Reflectances) airborne instrument observations over land surfaces,” Rem. Sens. Environ. 45, 137–154 (1993).
[CrossRef]

1989 (1)

J. L. Deuzé, C. Devaux, M. Herman, R. Santer, J. Y. Balois, L. Gonzalez, P. Lecomte, C. Verwaerde, “Photopolarimetric observations of aerosols and clouds from balloon,” Rem. Sens. Environ. 29, 93–109 (1989).
[CrossRef]

1986 (1)

1969 (1)

N. C. R. Rao, “Balloon measurements of the polarization of the light diffusely reflected by the earth’s atmosphere,” Planet. Space Sci. 17, 1307–1309 (1969).
[CrossRef]

1950 (1)

H. Neuberger, “Arago’s neutral point: a neglected tool in meteorological research,” Bull. Am. Meteorol. Soc. 31, 119–125 (1950).

1888 (1)

M. J. L. Soret, “Influence des surfaces d’eau sur la polarisation atmosphérique et observation de deux points neutres à droite et à gauche de Soleil,” C. R. 107, 867–870 (1888).

1884 (1)

M. A. Cornu, “Observations relatives a la couronne visible actuellement autour du Soleil,” C. R. 99, 488–493 (1884).

1847 (1)

D. Brewster, “On the polarisation of the atmosphere,” Philos. Mag. J. Sci. 31, 444–454 (1847).

1842 (1)

D. Brewster, “On the existence of a new neutral point and two secondary neutral points,” Rep. Brit. Assoc. Adv. Sci. 2, 13–25 (1842).

1840 (1)

J. Babinet, “Sur un nouveau point neutre dans l’atmosphère,” C. R. Acad. Sci. Paris 11, 618–620 (1840).

Babinet, J.

J. Babinet, “Sur un nouveau point neutre dans l’atmosphère,” C. R. Acad. Sci. Paris 11, 618–620 (1840).

Balois, J. Y.

J. L. Deuzé, C. Devaux, M. Herman, R. Santer, J. Y. Balois, L. Gonzalez, P. Lecomte, C. Verwaerde, “Photopolarimetric observations of aerosols and clouds from balloon,” Rem. Sens. Environ. 29, 93–109 (1989).
[CrossRef]

M. Herman, J. Y. Balois, L. Gonzalez, P. Lecomte, J. Lenoble, R. Santer, C. Verwaerde, “Stratospheric aerosol observations from a balloon-borne polarimetric experiment,” Appl. Opt. 25, 3573–3584 (1986).
[CrossRef] [PubMed]

Barral, M. J. A.

M. J. A. Barral, Oeuvres de Francois Arago I–V (Gide, Paris, 1858).

Barta, A.

G. Horváth, A. Barta, J. Gál, B. Suhai, O. Haiman, “Ground-based full-sky imaging polarimetry of rapidly changing skies and its use for polarimetric cloud detection,” Appl. Opt. 41, 543–559 (2002).
[CrossRef] [PubMed]

J. Gál, G. Horváth, A. Barta, R. Wehner, “Polarization of the moonlit clear night sky measured by full-sky imaging polarimetry at full moon: comparison of the polarization of moonlit and sunlit skies,” J. Geophys. Res. D 106(D19), 22647–22653 (2001).
[CrossRef]

Bréon, F. M.

F. M. Bréon, J. L. Deuzé, D. Tanré, M. Herman, “Validation of space-borne estimates of aerosol loading from sun photometer measurements with emphasis on polarization,” J. Geophys. Res. D 102, 17187–17195 (1997).
[CrossRef]

P. Y. Deschamps, F. M. Bréon, M. Leroy, A. Podaire, A. Bricaud, J. C. Buriez, G. Séze, “The POLDER mission: instrument characteristics and scientific objectives,” IEEE Trans. Geosci. Rem. Sens. 32, 598–615 (1994).
[CrossRef]

J. L. Deuzé, F. M. Bréon, P. Y. Deschamps, C. Devaux, M. Herman, “Analysis of the POLDER (POLarization and Directionality of Earth’s Reflectances) airborne instrument observations over land surfaces,” Rem. Sens. Environ. 45, 137–154 (1993).
[CrossRef]

Brewster, D.

D. Brewster, “On the polarisation of the atmosphere,” Philos. Mag. J. Sci. 31, 444–454 (1847).

D. Brewster, “On the existence of a new neutral point and two secondary neutral points,” Rep. Brit. Assoc. Adv. Sci. 2, 13–25 (1842).

Bricaud, A.

P. Y. Deschamps, F. M. Bréon, M. Leroy, A. Podaire, A. Bricaud, J. C. Buriez, G. Séze, “The POLDER mission: instrument characteristics and scientific objectives,” IEEE Trans. Geosci. Rem. Sens. 32, 598–615 (1994).
[CrossRef]

Buriez, J. C.

P. Y. Deschamps, F. M. Bréon, M. Leroy, A. Podaire, A. Bricaud, J. C. Buriez, G. Séze, “The POLDER mission: instrument characteristics and scientific objectives,” IEEE Trans. Geosci. Rem. Sens. 32, 598–615 (1994).
[CrossRef]

Campbell, C.

K. L. Coulson, V. S. Whitehead, C. Campbell, “Polarized views of the earth from orbital altitude,” in Ocean Optics VIII, M. A. Blizard, ed., Proc. SPIE637, 35–41 (1986).
[CrossRef]

Cornu, M. A.

M. A. Cornu, “Observations relatives a la couronne visible actuellement autour du Soleil,” C. R. 99, 488–493 (1884).

Coulson, K. L.

K. L. Coulson, Polarization and Intensity of Light in the Atmosphere (A. Deepak, Hampton, Virginia, 1988).

K. L. Coulson, V. S. Whitehead, C. Campbell, “Polarized views of the earth from orbital altitude,” in Ocean Optics VIII, M. A. Blizard, ed., Proc. SPIE637, 35–41 (1986).
[CrossRef]

Deschamps, P. Y.

P. Y. Deschamps, F. M. Bréon, M. Leroy, A. Podaire, A. Bricaud, J. C. Buriez, G. Séze, “The POLDER mission: instrument characteristics and scientific objectives,” IEEE Trans. Geosci. Rem. Sens. 32, 598–615 (1994).
[CrossRef]

J. L. Deuzé, F. M. Bréon, P. Y. Deschamps, C. Devaux, M. Herman, “Analysis of the POLDER (POLarization and Directionality of Earth’s Reflectances) airborne instrument observations over land surfaces,” Rem. Sens. Environ. 45, 137–154 (1993).
[CrossRef]

Deuzé, J. L.

F. M. Bréon, J. L. Deuzé, D. Tanré, M. Herman, “Validation of space-borne estimates of aerosol loading from sun photometer measurements with emphasis on polarization,” J. Geophys. Res. D 102, 17187–17195 (1997).
[CrossRef]

J. L. Deuzé, F. M. Bréon, P. Y. Deschamps, C. Devaux, M. Herman, “Analysis of the POLDER (POLarization and Directionality of Earth’s Reflectances) airborne instrument observations over land surfaces,” Rem. Sens. Environ. 45, 137–154 (1993).
[CrossRef]

J. L. Deuzé, C. Devaux, M. Herman, R. Santer, J. Y. Balois, L. Gonzalez, P. Lecomte, C. Verwaerde, “Photopolarimetric observations of aerosols and clouds from balloon,” Rem. Sens. Environ. 29, 93–109 (1989).
[CrossRef]

Devaux, C.

J. L. Deuzé, F. M. Bréon, P. Y. Deschamps, C. Devaux, M. Herman, “Analysis of the POLDER (POLarization and Directionality of Earth’s Reflectances) airborne instrument observations over land surfaces,” Rem. Sens. Environ. 45, 137–154 (1993).
[CrossRef]

J. L. Deuzé, C. Devaux, M. Herman, R. Santer, J. Y. Balois, L. Gonzalez, P. Lecomte, C. Verwaerde, “Photopolarimetric observations of aerosols and clouds from balloon,” Rem. Sens. Environ. 29, 93–109 (1989).
[CrossRef]

Gál, J.

G. Horváth, A. Barta, J. Gál, B. Suhai, O. Haiman, “Ground-based full-sky imaging polarimetry of rapidly changing skies and its use for polarimetric cloud detection,” Appl. Opt. 41, 543–559 (2002).
[CrossRef] [PubMed]

I. Pomozi, J. Gál, G. Horváth, R. Wehner, “Fine structure of the celestial polarization pattern and its temporal change during the total solar eclipse of 11 August 1999,” Rem. Sens. Environ. 76, 181–201 (2001).
[CrossRef]

J. Gál, G. Horváth, V. B. Meyer-Rochow, “Measurement of the reflection–polarization pattern of the flat water surface under a clear sky at sunset,” Rem. Sens. Environ. 76, 103–111 (2001).
[CrossRef]

J. Gál, G. Horváth, A. Barta, R. Wehner, “Polarization of the moonlit clear night sky measured by full-sky imaging polarimetry at full moon: comparison of the polarization of moonlit and sunlit skies,” J. Geophys. Res. D 106(D19), 22647–22653 (2001).
[CrossRef]

J. Gál, G. Horváth, V. B. Meyer-Rochow, R. Wehner, “Polarization patterns of the summer sky and its neutral points measured by full-sky imaging polarimetry in Finnish Lapland north of the Arctic Circle,” Proc. R. Soc. London Ser. A 457, 1385–1399 (2001).
[CrossRef]

G. Horváth, J. Gál, I. Pomozi, R. Wehner, “Polarization portrait of the Arago point: video-polarimetric imaging of the neutral points of skylight polarization,” Naturwissenschaften 85, 333–339 (1998).
[CrossRef]

G. Horváth, I. Pomozi, J. Gál, “Neutral points of skylight polarization observed during the total eclipse on 11 August 1999,” Appl. Opt. (to be published).

Gonzalez, L.

J. L. Deuzé, C. Devaux, M. Herman, R. Santer, J. Y. Balois, L. Gonzalez, P. Lecomte, C. Verwaerde, “Photopolarimetric observations of aerosols and clouds from balloon,” Rem. Sens. Environ. 29, 93–109 (1989).
[CrossRef]

M. Herman, J. Y. Balois, L. Gonzalez, P. Lecomte, J. Lenoble, R. Santer, C. Verwaerde, “Stratospheric aerosol observations from a balloon-borne polarimetric experiment,” Appl. Opt. 25, 3573–3584 (1986).
[CrossRef] [PubMed]

Haiman, O.

Herman, M.

F. M. Bréon, J. L. Deuzé, D. Tanré, M. Herman, “Validation of space-borne estimates of aerosol loading from sun photometer measurements with emphasis on polarization,” J. Geophys. Res. D 102, 17187–17195 (1997).
[CrossRef]

J. L. Deuzé, F. M. Bréon, P. Y. Deschamps, C. Devaux, M. Herman, “Analysis of the POLDER (POLarization and Directionality of Earth’s Reflectances) airborne instrument observations over land surfaces,” Rem. Sens. Environ. 45, 137–154 (1993).
[CrossRef]

J. L. Deuzé, C. Devaux, M. Herman, R. Santer, J. Y. Balois, L. Gonzalez, P. Lecomte, C. Verwaerde, “Photopolarimetric observations of aerosols and clouds from balloon,” Rem. Sens. Environ. 29, 93–109 (1989).
[CrossRef]

M. Herman, J. Y. Balois, L. Gonzalez, P. Lecomte, J. Lenoble, R. Santer, C. Verwaerde, “Stratospheric aerosol observations from a balloon-borne polarimetric experiment,” Appl. Opt. 25, 3573–3584 (1986).
[CrossRef] [PubMed]

Horvath, G.

D. K. Lynch (David.K.Lynch@aero.org) wrote to G. Horvath on 26November2001: “The 4th neutral point has been a favorite topic of mine for some years, yet I have never published the results. I have observed it twice, once in 1983 and again in 1994 from aircraft, but I have not published the results because my calibrations were not too good. The existence, however, is certain.”

Horváth, G.

G. Horváth, A. Barta, J. Gál, B. Suhai, O. Haiman, “Ground-based full-sky imaging polarimetry of rapidly changing skies and its use for polarimetric cloud detection,” Appl. Opt. 41, 543–559 (2002).
[CrossRef] [PubMed]

I. Pomozi, G. Horváth, R. Wehner, “How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation,” J. Exp. Biol. 204, 2933–2942 (2001).
[PubMed]

J. Gál, G. Horváth, V. B. Meyer-Rochow, R. Wehner, “Polarization patterns of the summer sky and its neutral points measured by full-sky imaging polarimetry in Finnish Lapland north of the Arctic Circle,” Proc. R. Soc. London Ser. A 457, 1385–1399 (2001).
[CrossRef]

J. Gál, G. Horváth, A. Barta, R. Wehner, “Polarization of the moonlit clear night sky measured by full-sky imaging polarimetry at full moon: comparison of the polarization of moonlit and sunlit skies,” J. Geophys. Res. D 106(D19), 22647–22653 (2001).
[CrossRef]

J. Gál, G. Horváth, V. B. Meyer-Rochow, “Measurement of the reflection–polarization pattern of the flat water surface under a clear sky at sunset,” Rem. Sens. Environ. 76, 103–111 (2001).
[CrossRef]

I. Pomozi, J. Gál, G. Horváth, R. Wehner, “Fine structure of the celestial polarization pattern and its temporal change during the total solar eclipse of 11 August 1999,” Rem. Sens. Environ. 76, 181–201 (2001).
[CrossRef]

G. Horváth, R. Wehner, “Skylight polarization as perceived by desert ants and measured by video polarimetry,” J. Comp. Physiol. A 184, 1–7 (1999).
[CrossRef]

G. Horváth, J. Gál, I. Pomozi, R. Wehner, “Polarization portrait of the Arago point: video-polarimetric imaging of the neutral points of skylight polarization,” Naturwissenschaften 85, 333–339 (1998).
[CrossRef]

G. Horváth, I. Pomozi, J. Gál, “Neutral points of skylight polarization observed during the total eclipse on 11 August 1999,” Appl. Opt. (to be published).

Können, G. P.

G. P. Können, Polarized Light in Nature (Cambridge U. Press, Cambridge, UK, 1985).

G. P. Können, Royal Netherlands Meteorological Institute, P.O. Box 201, NL-3730 AE De Bilt, The Netherlands; e-mail: konnen@knmi.nl (personal communication, November2001).

Lecomte, P.

J. L. Deuzé, C. Devaux, M. Herman, R. Santer, J. Y. Balois, L. Gonzalez, P. Lecomte, C. Verwaerde, “Photopolarimetric observations of aerosols and clouds from balloon,” Rem. Sens. Environ. 29, 93–109 (1989).
[CrossRef]

M. Herman, J. Y. Balois, L. Gonzalez, P. Lecomte, J. Lenoble, R. Santer, C. Verwaerde, “Stratospheric aerosol observations from a balloon-borne polarimetric experiment,” Appl. Opt. 25, 3573–3584 (1986).
[CrossRef] [PubMed]

Lenoble, J.

Leroy, M.

P. Y. Deschamps, F. M. Bréon, M. Leroy, A. Podaire, A. Bricaud, J. C. Buriez, G. Séze, “The POLDER mission: instrument characteristics and scientific objectives,” IEEE Trans. Geosci. Rem. Sens. 32, 598–615 (1994).
[CrossRef]

Lynch, D. K.

D. K. Lynch (David.K.Lynch@aero.org) wrote to G. Horvath on 26November2001: “The 4th neutral point has been a favorite topic of mine for some years, yet I have never published the results. I have observed it twice, once in 1983 and again in 1994 from aircraft, but I have not published the results because my calibrations were not too good. The existence, however, is certain.”

Meyer-Rochow, V. B.

J. Gál, G. Horváth, V. B. Meyer-Rochow, “Measurement of the reflection–polarization pattern of the flat water surface under a clear sky at sunset,” Rem. Sens. Environ. 76, 103–111 (2001).
[CrossRef]

J. Gál, G. Horváth, V. B. Meyer-Rochow, R. Wehner, “Polarization patterns of the summer sky and its neutral points measured by full-sky imaging polarimetry in Finnish Lapland north of the Arctic Circle,” Proc. R. Soc. London Ser. A 457, 1385–1399 (2001).
[CrossRef]

Neuberger, H.

H. Neuberger, “Arago’s neutral point: a neglected tool in meteorological research,” Bull. Am. Meteorol. Soc. 31, 119–125 (1950).

Podaire, A.

P. Y. Deschamps, F. M. Bréon, M. Leroy, A. Podaire, A. Bricaud, J. C. Buriez, G. Séze, “The POLDER mission: instrument characteristics and scientific objectives,” IEEE Trans. Geosci. Rem. Sens. 32, 598–615 (1994).
[CrossRef]

Pomozi, I.

I. Pomozi, J. Gál, G. Horváth, R. Wehner, “Fine structure of the celestial polarization pattern and its temporal change during the total solar eclipse of 11 August 1999,” Rem. Sens. Environ. 76, 181–201 (2001).
[CrossRef]

I. Pomozi, G. Horváth, R. Wehner, “How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation,” J. Exp. Biol. 204, 2933–2942 (2001).
[PubMed]

G. Horváth, J. Gál, I. Pomozi, R. Wehner, “Polarization portrait of the Arago point: video-polarimetric imaging of the neutral points of skylight polarization,” Naturwissenschaften 85, 333–339 (1998).
[CrossRef]

G. Horváth, I. Pomozi, J. Gál, “Neutral points of skylight polarization observed during the total eclipse on 11 August 1999,” Appl. Opt. (to be published).

Rao, N. C. R.

N. C. R. Rao, “Balloon measurements of the polarization of the light diffusely reflected by the earth’s atmosphere,” Planet. Space Sci. 17, 1307–1309 (1969).
[CrossRef]

Rozenberg, G. V.

G. V. Rozenberg, Twilight (Plenum, New York, 1966).

Santer, R.

J. L. Deuzé, C. Devaux, M. Herman, R. Santer, J. Y. Balois, L. Gonzalez, P. Lecomte, C. Verwaerde, “Photopolarimetric observations of aerosols and clouds from balloon,” Rem. Sens. Environ. 29, 93–109 (1989).
[CrossRef]

M. Herman, J. Y. Balois, L. Gonzalez, P. Lecomte, J. Lenoble, R. Santer, C. Verwaerde, “Stratospheric aerosol observations from a balloon-borne polarimetric experiment,” Appl. Opt. 25, 3573–3584 (1986).
[CrossRef] [PubMed]

Séze, G.

P. Y. Deschamps, F. M. Bréon, M. Leroy, A. Podaire, A. Bricaud, J. C. Buriez, G. Séze, “The POLDER mission: instrument characteristics and scientific objectives,” IEEE Trans. Geosci. Rem. Sens. 32, 598–615 (1994).
[CrossRef]

Soret, M. J. L.

M. J. L. Soret, “Influence des surfaces d’eau sur la polarisation atmosphérique et observation de deux points neutres à droite et à gauche de Soleil,” C. R. 107, 867–870 (1888).

Suhai, B.

Tanré, D.

F. M. Bréon, J. L. Deuzé, D. Tanré, M. Herman, “Validation of space-borne estimates of aerosol loading from sun photometer measurements with emphasis on polarization,” J. Geophys. Res. D 102, 17187–17195 (1997).
[CrossRef]

Verwaerde, C.

J. L. Deuzé, C. Devaux, M. Herman, R. Santer, J. Y. Balois, L. Gonzalez, P. Lecomte, C. Verwaerde, “Photopolarimetric observations of aerosols and clouds from balloon,” Rem. Sens. Environ. 29, 93–109 (1989).
[CrossRef]

M. Herman, J. Y. Balois, L. Gonzalez, P. Lecomte, J. Lenoble, R. Santer, C. Verwaerde, “Stratospheric aerosol observations from a balloon-borne polarimetric experiment,” Appl. Opt. 25, 3573–3584 (1986).
[CrossRef] [PubMed]

Wehner, R.

I. Pomozi, J. Gál, G. Horváth, R. Wehner, “Fine structure of the celestial polarization pattern and its temporal change during the total solar eclipse of 11 August 1999,” Rem. Sens. Environ. 76, 181–201 (2001).
[CrossRef]

I. Pomozi, G. Horváth, R. Wehner, “How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation,” J. Exp. Biol. 204, 2933–2942 (2001).
[PubMed]

J. Gál, G. Horváth, V. B. Meyer-Rochow, R. Wehner, “Polarization patterns of the summer sky and its neutral points measured by full-sky imaging polarimetry in Finnish Lapland north of the Arctic Circle,” Proc. R. Soc. London Ser. A 457, 1385–1399 (2001).
[CrossRef]

J. Gál, G. Horváth, A. Barta, R. Wehner, “Polarization of the moonlit clear night sky measured by full-sky imaging polarimetry at full moon: comparison of the polarization of moonlit and sunlit skies,” J. Geophys. Res. D 106(D19), 22647–22653 (2001).
[CrossRef]

G. Horváth, R. Wehner, “Skylight polarization as perceived by desert ants and measured by video polarimetry,” J. Comp. Physiol. A 184, 1–7 (1999).
[CrossRef]

G. Horváth, J. Gál, I. Pomozi, R. Wehner, “Polarization portrait of the Arago point: video-polarimetric imaging of the neutral points of skylight polarization,” Naturwissenschaften 85, 333–339 (1998).
[CrossRef]

Whitehead, V. S.

K. L. Coulson, V. S. Whitehead, C. Campbell, “Polarized views of the earth from orbital altitude,” in Ocean Optics VIII, M. A. Blizard, ed., Proc. SPIE637, 35–41 (1986).
[CrossRef]

Appl. Opt. (2)

Bull. Am. Meteorol. Soc. (1)

H. Neuberger, “Arago’s neutral point: a neglected tool in meteorological research,” Bull. Am. Meteorol. Soc. 31, 119–125 (1950).

C. R. (2)

M. J. L. Soret, “Influence des surfaces d’eau sur la polarisation atmosphérique et observation de deux points neutres à droite et à gauche de Soleil,” C. R. 107, 867–870 (1888).

M. A. Cornu, “Observations relatives a la couronne visible actuellement autour du Soleil,” C. R. 99, 488–493 (1884).

C. R. Acad. Sci. Paris (1)

J. Babinet, “Sur un nouveau point neutre dans l’atmosphère,” C. R. Acad. Sci. Paris 11, 618–620 (1840).

IEEE Trans. Geosci. Rem. Sens. (1)

P. Y. Deschamps, F. M. Bréon, M. Leroy, A. Podaire, A. Bricaud, J. C. Buriez, G. Séze, “The POLDER mission: instrument characteristics and scientific objectives,” IEEE Trans. Geosci. Rem. Sens. 32, 598–615 (1994).
[CrossRef]

J. Comp. Physiol. A (1)

G. Horváth, R. Wehner, “Skylight polarization as perceived by desert ants and measured by video polarimetry,” J. Comp. Physiol. A 184, 1–7 (1999).
[CrossRef]

J. Exp. Biol. (1)

I. Pomozi, G. Horváth, R. Wehner, “How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation,” J. Exp. Biol. 204, 2933–2942 (2001).
[PubMed]

J. Geophys. Res. D (2)

F. M. Bréon, J. L. Deuzé, D. Tanré, M. Herman, “Validation of space-borne estimates of aerosol loading from sun photometer measurements with emphasis on polarization,” J. Geophys. Res. D 102, 17187–17195 (1997).
[CrossRef]

J. Gál, G. Horváth, A. Barta, R. Wehner, “Polarization of the moonlit clear night sky measured by full-sky imaging polarimetry at full moon: comparison of the polarization of moonlit and sunlit skies,” J. Geophys. Res. D 106(D19), 22647–22653 (2001).
[CrossRef]

Naturwissenschaften (1)

G. Horváth, J. Gál, I. Pomozi, R. Wehner, “Polarization portrait of the Arago point: video-polarimetric imaging of the neutral points of skylight polarization,” Naturwissenschaften 85, 333–339 (1998).
[CrossRef]

Philos. Mag. J. Sci. (1)

D. Brewster, “On the polarisation of the atmosphere,” Philos. Mag. J. Sci. 31, 444–454 (1847).

Planet. Space Sci. (1)

N. C. R. Rao, “Balloon measurements of the polarization of the light diffusely reflected by the earth’s atmosphere,” Planet. Space Sci. 17, 1307–1309 (1969).
[CrossRef]

Proc. R. Soc. London Ser. A (1)

J. Gál, G. Horváth, V. B. Meyer-Rochow, R. Wehner, “Polarization patterns of the summer sky and its neutral points measured by full-sky imaging polarimetry in Finnish Lapland north of the Arctic Circle,” Proc. R. Soc. London Ser. A 457, 1385–1399 (2001).
[CrossRef]

Rem. Sens. Environ. (4)

J. Gál, G. Horváth, V. B. Meyer-Rochow, “Measurement of the reflection–polarization pattern of the flat water surface under a clear sky at sunset,” Rem. Sens. Environ. 76, 103–111 (2001).
[CrossRef]

I. Pomozi, J. Gál, G. Horváth, R. Wehner, “Fine structure of the celestial polarization pattern and its temporal change during the total solar eclipse of 11 August 1999,” Rem. Sens. Environ. 76, 181–201 (2001).
[CrossRef]

J. L. Deuzé, C. Devaux, M. Herman, R. Santer, J. Y. Balois, L. Gonzalez, P. Lecomte, C. Verwaerde, “Photopolarimetric observations of aerosols and clouds from balloon,” Rem. Sens. Environ. 29, 93–109 (1989).
[CrossRef]

J. L. Deuzé, F. M. Bréon, P. Y. Deschamps, C. Devaux, M. Herman, “Analysis of the POLDER (POLarization and Directionality of Earth’s Reflectances) airborne instrument observations over land surfaces,” Rem. Sens. Environ. 45, 137–154 (1993).
[CrossRef]

Rep. Brit. Assoc. Adv. Sci. (1)

D. Brewster, “On the existence of a new neutral point and two secondary neutral points,” Rep. Brit. Assoc. Adv. Sci. 2, 13–25 (1842).

Other (9)

G. P. Können, Polarized Light in Nature (Cambridge U. Press, Cambridge, UK, 1985).

K. L. Coulson, Polarization and Intensity of Light in the Atmosphere (A. Deepak, Hampton, Virginia, 1988).

M. J. A. Barral, Oeuvres de Francois Arago I–V (Gide, Paris, 1858).

G. Horváth, I. Pomozi, J. Gál, “Neutral points of skylight polarization observed during the total eclipse on 11 August 1999,” Appl. Opt. (to be published).

T. Gehrels, ed. Planets, Stars, and Nebulae Studied with Photopolarimetry (University of Arizona Press, Tucson, Ariz., 1974).

G. V. Rozenberg, Twilight (Plenum, New York, 1966).

D. K. Lynch (David.K.Lynch@aero.org) wrote to G. Horvath on 26November2001: “The 4th neutral point has been a favorite topic of mine for some years, yet I have never published the results. I have observed it twice, once in 1983 and again in 1994 from aircraft, but I have not published the results because my calibrations were not too good. The existence, however, is certain.”

K. L. Coulson, V. S. Whitehead, C. Campbell, “Polarized views of the earth from orbital altitude,” in Ocean Optics VIII, M. A. Blizard, ed., Proc. SPIE637, 35–41 (1986).
[CrossRef]

G. P. Können, Royal Netherlands Meteorological Institute, P.O. Box 201, NL-3730 AE De Bilt, The Netherlands; e-mail: konnen@knmi.nl (personal communication, November2001).

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Figures (8)

Fig. 1
Fig. 1

A, B, schematic diagram showing the normal positions of the Arago (AR), Babinet (BA), and Brewster (BR) neutral points of skylight polarization in the vertical plane including the ground-based observer (OB), sun (SU), zenith (ZE), anti-solar point (AS), and nadir (NA). From the ground, only two neutral points are visible simultaneously: either the Arago and Babinet points (A, for lower solar elevations), or the Babinet and Brewster points (B, for higher solar elevations). From the ground, the fourth neutral point (4TH) is not visible. The insets are portraits of Dominique Francois Jean Arago (1786–1853), Jacques Babinet (1794–1872) and David Brewster (1781–1868), the discoverers of the neutral points. C, at an appropriately high altitude, all four neutral points can be observed simultaneously. Then the Arago and Babinet points are the neutral points of downwelling polarized skylight, while the Brewster and fourth neutral points are the neutral points of upwelling polarized earthlight. D, geometry of the space-borne observation of the Arago and fourth neutral points.

Fig. 2
Fig. 2

Main parameters of our two hot air balloon flights conducted on June 28 (local sunrise at 04:52=local summer time=UTC+2) and August 25 (sunrise at 05:56) 2001 to measure the polarization patterns of upwelling earthlight and the characteristics of the fourth neutral point of atmospheric polarization. A, launching site (Pákozd, 47°13N, 18°33E), landing site (Adony, 47°06N, 18°51E), and trajectory of the first flight (hovering above the launching site during the second flight) on the map of Hungary. B, altitude of the balloon as a function of time after sunrise for both flights. Triangles, altitudes and points of time at which polarimetric measurements were done; dashed vertical line, measurement, the results of which are shown in Fig. 4.

Fig. 3
Fig. 3

A, Geometry of our polarimetric measurements in the gondola of a hot air balloon. B, C, two-dimensional system of polar coordinates of the terrestrial and celestial hemisphere in which the maps of Figs. 4 and 5 are represented. The azimuth angle φ is measured clockwise from the solar meridian, and the zenith or nadir angle θ is measured radially so that for the zenith or nadir, θ=0° for the horizon θ=90°, and the radius from the zenith or nadir is proportional to θ. θs, solar zenith angle or nadir angle of the anti-solar point. Note that in map B the North, West, South, and East are represented on the compass rose as is usual in maps seen from above; but in map C East and West are transposed, because then we are looking up toward the celestial dome rather than down onto the ground.

Fig. 4
Fig. 4

180°-field-of-view color photograph of the landscape below the gondola of the hot air balloon (A) and the patterns of radiance I (B, E, H), degree of linear polarization p (C, F, I), and angle of polarization α (D, G, J) of upwelling earthlight. Measurements were taken by using 180°-field-of-view imaging polarimetry at an altitude of 3500 m and a solar elevation of 3° for wavelengths of 450, 550, and 650 nm immediately after local sunrise (05:12; local summer time=UTC+2; June 28, 2001). The position of the sun and the neutral points are indicated by dots. Time of exposure=1/60 s, aperture=2.8, color reversal film Fujichrome Sensia II, 100 ASA.

Fig. 5
Fig. 5

As Fig. 4 for the full clear sky measured from the ground at local sunrise (August 26, 1999, 06:00=local summer time=UTC+1, solar elevation=0°; Chott el Djerid, a salt pan, 10 km from Kriz, 33°52.mN, 8°22.mE, Tunisia). Note that on the compass rose East and West are transposed, because we are looking up toward the celestial dome rather than down toward the ground. Time of exposure=1/60 s, aperture=2.8, color reversal film Fujichrome Sensia II, 100 ASA.

Fig. 6
Fig. 6

Degree of linear polarization p of earthlight (A, C, E) and skylight (B, D, F) along the solar and anti-solar meridian measured in the red (650 nm), green (550 nm), and blue (450 nm) spectral ranges as a function of the nadir/zenith angle θ. Data for earthlight and skylight originate from the polarization patterns in Fig. 4 and Fig. 5, respectively. Gray stripes show the overexposed areas.

Fig. 7
Fig. 7

As Fig. 6 for angle of polarization α of earthlight and skylight measured from the solar/anti-solar meridian.

Fig. 8
Fig. 8

Three-dimensional spatial distribution of radiance and color (A), degree (B), and angle (C) of linear polarization as well as the Arago and fourth neutral points observable around a hot air balloon at 450 nm and at an altitude of 3500 m. Each sphere in the picture is the combination of the patterns of parts A, C, D in Figs. 4 and 5.

Tables (1)

Tables Icon

Table 1 Angular Distance of the Arago, Babinet, Brewster, and Fourth Neutral Points from the Nadir or the Zenith As Determined on the Basis of the Patterns of the Degree and Angle of Linear Polarization in Figs. 4 and 5 at 650, 550, and 450 nm

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