Abstract

Radiance, color, and polarization of the light in forests combine to create complex optical patterns. Earlier sporadic polarimetric studies in forests were limited by the narrow fields of view of the polarimeters used in such studies. Since polarization patterns in the entire upper hemisphere of the visual environment of forests could be important for forest-inhabiting animals that make use of linearly polarized light for orientation, we measured 180° field-of-view polarization distributions in Finnish forests. From a hot air balloon we also measured the polarization patterns of Hungarian grasslands lit by the rising sun. We found that the pattern of the angle of polarization α of sunlit grasslands and sunlit tree canopies was qualitatively the same as that of the sky. We show here that contrary to an earlier assumption, the α-pattern characteristic of the sky always remains visible underneath overhead vegetation, independently of the solar elevation and the sky conditions (clear or partly cloudy with visible sun's disc), provided the foliage is sunlit and not only when large patches of the clear sky are visible through the vegetation. Since the mirror symmetry axis of the α-pattern of the sunlit foliage is the solar-antisolar meridian, the azimuth direction of the sun, occluded by vegetation, can be assessed in forests from this polarization pattern. Possible consequences of this robust polarization feature of the optical environment in forests are briefly discussed with regard to polarization-based animal navigation.

© 2007 Optical Society of America

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References

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  1. I. A. Kong, Polarimetric Remote Sensing (Elsevier, 1990).
  2. K. L. Coulson, Polarization and Intensity of Light in the Atmosphere (A. Deepak Publishing, 1988).
  3. M. L. Brines and J. L. Gould, "Skylight polarization patterns and animal orientation," J. Exp. Biol. 96, 69-91 (1982).
  4. V. C. Vanderbilt and L. Grant, "Polarization photometer to measure bidirection reflectance factor R(55°, 0°, 55°, 180°) of leaves," Opt. Eng. 25, 566-571 (1985).
  5. V. C. Vanderbilt, L. Grant, and C. S. T. Daughtry, "Polarization of light scattered by vegetation," Proc. IEEE 73, 1012-1024 (1985).
    [CrossRef]
  6. V. C. Vanderbilt, L. Grant, L. L. Biehl, and B. F. Robinson, "Specular, diffuse and polarized light scattered by two wheat canopies," Appl. Opt. 24, 2408-2418 (1985).
    [CrossRef] [PubMed]
  7. L. Grant, C. S. T. Daughtry, and V. C. Vanderbilt, "Polarized and non-polarized leaf reflectances of Coleus blumei," Environ. Exp. Bot. 27, 139-145 (1987).
    [CrossRef]
  8. L. Grant, C. S. T. Daughtry, and V. C. Vanderbilt, "Variations in the polarized leaf reflectance of Sorghum bicolor," Remote Sens. Environ. 27, 333-339 (1987).
    [CrossRef]
  9. L. Grant, C. S. T. Daughtry, and V. C. Vanderbilt, "Polarized and specular reflectance variation with leaf surface features," Physiol. Plant. 88, 1-9 (1993).
    [CrossRef]
  10. N. Shashar, T. W. Cronin, L. B. Wolff, and M. A. Condon, "The polarization of light in a tropical rain forest," Biotropica 30, 275-285 (1998).
    [CrossRef]
  11. G. Horváth, J. Gál, T. Labhart, and R. Wehner, "Does reflection polarization by plants influence colour perception in insects? Polarimetric measurements applied to a polarization-sensitive model retina of Papilio butterflies," J. Exp. Biol. 205, 3281-3298 (2002).
    [PubMed]
  12. J. A. Endler, "The color of light in forests and its implications," Ecol. Monogr. 63, 1-27 (1993).
    [CrossRef]
  13. G. Horváth, B. Bernáth, B. Suhai, A. Barta, and R. Wehner, "First observation of the fourth neutral polarization point in the atmosphere," J. Opt. Soc. Am. A 19, 2085-2099 (2002).
    [CrossRef]
  14. J. Gál, G. Horváth, V. B. Meyer-Rochow, and 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]
  15. M. V. Berry, M. R. Dennis, and R. L. Jr. Lee, "Polarization singularities in the clear sky," New J. Phys. 6, doi: (2004).
    [CrossRef]
  16. J. H. Hannay, "Polarization of sky light from a canopy atmosphere," New J. Phys. 6, doi: (2004).
    [CrossRef]
  17. R. Hegedüs, S. Åkesson, and G. Horváth, "Anomalous celestial polarization caused by forest fire smoke: why do some insects become visually disoriented under smoky skies?" Appl. Opt. 46, 2717-2726 (2007).
    [CrossRef] [PubMed]
  18. R. Hegedüs, S. Åkesson, R. Wehner, and G. Horváth, "Could Vikings have navigated under foggy and cloudy conditions by skylight polarization? On the atmospheric optical prerequisites of polarimetric Viking navigation under foggy and cloudy skies," Proc. R. Soc. London , Ser. A 463, 1081-1095 (2007).
    [CrossRef]
  19. R. Hegedüs, S. Åkesson, and G. Horváth, "Polarization patterns of thick clouds: overcast skies have distribution of the angle of polarization similar to that of clear skies," J. Opt. Soc. Am. A (in press) (2007).
    [CrossRef]
  20. J. T. Woolley, "Reflectance and transmittance of light by leaves," Plant Physiol. 47, 656-662 (1971).
    [CrossRef] [PubMed]
  21. L. Grant, "Diffuse and specular characteristics of leaf reflectance," Remote Sens. Environ. 22, 309-322 (1987).
    [CrossRef]
  22. V. P. Rvachev and S. G. Guminetskii, "The structure of light beams reflected by plant leaves," J. Appl. Spectrosc. 4, 415-421 (1966).
    [CrossRef]
  23. G. P. Können, Polarized Light in Nature (Cambridge University Press, 1985).
  24. R. M. A. Azzam and N. M. Bashara, Ellipsometry and Polarized Light (North-Holland, 1992).
  25. J. Gál, G. Horváth, A. Barta, and 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, 22647-22653 (2001).
    [CrossRef]
  26. I. Pomozi, G. Horváth, and 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]
  27. B. Suhai and G. Horváth, "How well does the Rayleigh model describe the E-vector distribution of skylight in clear and cloudy conditions? A full-sky polarimetric study," J. Opt. Soc. Am. A 21, 1669-1676 (2004).
    [CrossRef]
  28. A. Barta and G. Horváth, "Why is it advantageous for animals to detect celestial polarization in the ultraviolet? Skylight polarization under clouds and canopies is strongest in the UV," J. Theor. Biol. 226, 429-437 (2004).
    [CrossRef] [PubMed]
  29. R. Hegedüs, Á. Horváth , and G. Horváth, "Why do dusk-active cockchafers detect polarization in the green? The polarization vision in Melolontha melolontha is tuned to the high polarized intensity of downwelling light under canopies during sunset," J. Theor. Biol. 238, 230-244 (2006).
    [CrossRef]
  30. E. O. Wilson, Insect Societies (Harvard University Press, 1971).
  31. G. Horváth and D. Varjú, Polarized Light in Animal Vision-Polarization Patterns in Nature (Springer-Verlag, 2003).

2007 (3)

R. Hegedüs, S. Åkesson, R. Wehner, and G. Horváth, "Could Vikings have navigated under foggy and cloudy conditions by skylight polarization? On the atmospheric optical prerequisites of polarimetric Viking navigation under foggy and cloudy skies," Proc. R. Soc. London , Ser. A 463, 1081-1095 (2007).
[CrossRef]

R. Hegedüs, S. Åkesson, and G. Horváth, "Polarization patterns of thick clouds: overcast skies have distribution of the angle of polarization similar to that of clear skies," J. Opt. Soc. Am. A (in press) (2007).
[CrossRef]

R. Hegedüs, S. Åkesson, and G. Horváth, "Anomalous celestial polarization caused by forest fire smoke: why do some insects become visually disoriented under smoky skies?" Appl. Opt. 46, 2717-2726 (2007).
[CrossRef] [PubMed]

2006 (1)

R. Hegedüs, Á. Horváth , and G. Horváth, "Why do dusk-active cockchafers detect polarization in the green? The polarization vision in Melolontha melolontha is tuned to the high polarized intensity of downwelling light under canopies during sunset," J. Theor. Biol. 238, 230-244 (2006).
[CrossRef]

2004 (4)

B. Suhai and G. Horváth, "How well does the Rayleigh model describe the E-vector distribution of skylight in clear and cloudy conditions? A full-sky polarimetric study," J. Opt. Soc. Am. A 21, 1669-1676 (2004).
[CrossRef]

M. V. Berry, M. R. Dennis, and R. L. Jr. Lee, "Polarization singularities in the clear sky," New J. Phys. 6, doi: (2004).
[CrossRef]

J. H. Hannay, "Polarization of sky light from a canopy atmosphere," New J. Phys. 6, doi: (2004).
[CrossRef]

A. Barta and G. Horváth, "Why is it advantageous for animals to detect celestial polarization in the ultraviolet? Skylight polarization under clouds and canopies is strongest in the UV," J. Theor. Biol. 226, 429-437 (2004).
[CrossRef] [PubMed]

2002 (2)

G. Horváth, J. Gál, T. Labhart, and R. Wehner, "Does reflection polarization by plants influence colour perception in insects? Polarimetric measurements applied to a polarization-sensitive model retina of Papilio butterflies," J. Exp. Biol. 205, 3281-3298 (2002).
[PubMed]

G. Horváth, B. Bernáth, B. Suhai, A. Barta, and R. Wehner, "First observation of the fourth neutral polarization point in the atmosphere," J. Opt. Soc. Am. A 19, 2085-2099 (2002).
[CrossRef]

2001 (3)

J. Gál, G. Horváth, A. Barta, and 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, 22647-22653 (2001).
[CrossRef]

I. Pomozi, G. Horváth, and 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, and 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]

1998 (1)

N. Shashar, T. W. Cronin, L. B. Wolff, and M. A. Condon, "The polarization of light in a tropical rain forest," Biotropica 30, 275-285 (1998).
[CrossRef]

1993 (2)

J. A. Endler, "The color of light in forests and its implications," Ecol. Monogr. 63, 1-27 (1993).
[CrossRef]

L. Grant, C. S. T. Daughtry, and V. C. Vanderbilt, "Polarized and specular reflectance variation with leaf surface features," Physiol. Plant. 88, 1-9 (1993).
[CrossRef]

1987 (3)

L. Grant, C. S. T. Daughtry, and V. C. Vanderbilt, "Polarized and non-polarized leaf reflectances of Coleus blumei," Environ. Exp. Bot. 27, 139-145 (1987).
[CrossRef]

L. Grant, C. S. T. Daughtry, and V. C. Vanderbilt, "Variations in the polarized leaf reflectance of Sorghum bicolor," Remote Sens. Environ. 27, 333-339 (1987).
[CrossRef]

L. Grant, "Diffuse and specular characteristics of leaf reflectance," Remote Sens. Environ. 22, 309-322 (1987).
[CrossRef]

1985 (3)

V. C. Vanderbilt and L. Grant, "Polarization photometer to measure bidirection reflectance factor R(55°, 0°, 55°, 180°) of leaves," Opt. Eng. 25, 566-571 (1985).

V. C. Vanderbilt, L. Grant, and C. S. T. Daughtry, "Polarization of light scattered by vegetation," Proc. IEEE 73, 1012-1024 (1985).
[CrossRef]

V. C. Vanderbilt, L. Grant, L. L. Biehl, and B. F. Robinson, "Specular, diffuse and polarized light scattered by two wheat canopies," Appl. Opt. 24, 2408-2418 (1985).
[CrossRef] [PubMed]

1982 (1)

M. L. Brines and J. L. Gould, "Skylight polarization patterns and animal orientation," J. Exp. Biol. 96, 69-91 (1982).

1971 (1)

J. T. Woolley, "Reflectance and transmittance of light by leaves," Plant Physiol. 47, 656-662 (1971).
[CrossRef] [PubMed]

1966 (1)

V. P. Rvachev and S. G. Guminetskii, "The structure of light beams reflected by plant leaves," J. Appl. Spectrosc. 4, 415-421 (1966).
[CrossRef]

Appl. Opt. (2)

Biotropica (1)

N. Shashar, T. W. Cronin, L. B. Wolff, and M. A. Condon, "The polarization of light in a tropical rain forest," Biotropica 30, 275-285 (1998).
[CrossRef]

Ecol. Monogr. (1)

J. A. Endler, "The color of light in forests and its implications," Ecol. Monogr. 63, 1-27 (1993).
[CrossRef]

Environ. Exp. Bot. (1)

L. Grant, C. S. T. Daughtry, and V. C. Vanderbilt, "Polarized and non-polarized leaf reflectances of Coleus blumei," Environ. Exp. Bot. 27, 139-145 (1987).
[CrossRef]

J. Appl. Spectrosc. (1)

V. P. Rvachev and S. G. Guminetskii, "The structure of light beams reflected by plant leaves," J. Appl. Spectrosc. 4, 415-421 (1966).
[CrossRef]

J. Exp. Biol. (3)

G. Horváth, J. Gál, T. Labhart, and R. Wehner, "Does reflection polarization by plants influence colour perception in insects? Polarimetric measurements applied to a polarization-sensitive model retina of Papilio butterflies," J. Exp. Biol. 205, 3281-3298 (2002).
[PubMed]

M. L. Brines and J. L. Gould, "Skylight polarization patterns and animal orientation," J. Exp. Biol. 96, 69-91 (1982).

I. Pomozi, G. Horváth, and 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 (1)

J. Gál, G. Horváth, A. Barta, and 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, 22647-22653 (2001).
[CrossRef]

J. Opt. Soc. Am. A (3)

J. Theor. Biol. (2)

A. Barta and G. Horváth, "Why is it advantageous for animals to detect celestial polarization in the ultraviolet? Skylight polarization under clouds and canopies is strongest in the UV," J. Theor. Biol. 226, 429-437 (2004).
[CrossRef] [PubMed]

R. Hegedüs, Á. Horváth , and G. Horváth, "Why do dusk-active cockchafers detect polarization in the green? The polarization vision in Melolontha melolontha is tuned to the high polarized intensity of downwelling light under canopies during sunset," J. Theor. Biol. 238, 230-244 (2006).
[CrossRef]

New J. Phys. (2)

M. V. Berry, M. R. Dennis, and R. L. Jr. Lee, "Polarization singularities in the clear sky," New J. Phys. 6, doi: (2004).
[CrossRef]

J. H. Hannay, "Polarization of sky light from a canopy atmosphere," New J. Phys. 6, doi: (2004).
[CrossRef]

Opt. Eng. (1)

V. C. Vanderbilt and L. Grant, "Polarization photometer to measure bidirection reflectance factor R(55°, 0°, 55°, 180°) of leaves," Opt. Eng. 25, 566-571 (1985).

Physiol. Plant. (1)

L. Grant, C. S. T. Daughtry, and V. C. Vanderbilt, "Polarized and specular reflectance variation with leaf surface features," Physiol. Plant. 88, 1-9 (1993).
[CrossRef]

Plant Physiol. (1)

J. T. Woolley, "Reflectance and transmittance of light by leaves," Plant Physiol. 47, 656-662 (1971).
[CrossRef] [PubMed]

Proc. IEEE (1)

V. C. Vanderbilt, L. Grant, and C. S. T. Daughtry, "Polarization of light scattered by vegetation," Proc. IEEE 73, 1012-1024 (1985).
[CrossRef]

Proc. R. Soc. London (2)

R. Hegedüs, S. Åkesson, R. Wehner, and G. Horváth, "Could Vikings have navigated under foggy and cloudy conditions by skylight polarization? On the atmospheric optical prerequisites of polarimetric Viking navigation under foggy and cloudy skies," Proc. R. Soc. London , Ser. A 463, 1081-1095 (2007).
[CrossRef]

J. Gál, G. Horváth, V. B. Meyer-Rochow, and 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]

Remote Sens. Environ. (2)

L. Grant, "Diffuse and specular characteristics of leaf reflectance," Remote Sens. Environ. 22, 309-322 (1987).
[CrossRef]

L. Grant, C. S. T. Daughtry, and V. C. Vanderbilt, "Variations in the polarized leaf reflectance of Sorghum bicolor," Remote Sens. Environ. 27, 333-339 (1987).
[CrossRef]

Other (6)

I. A. Kong, Polarimetric Remote Sensing (Elsevier, 1990).

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

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

R. M. A. Azzam and N. M. Bashara, Ellipsometry and Polarized Light (North-Holland, 1992).

E. O. Wilson, Insect Societies (Harvard University Press, 1971).

G. Horváth and D. Varjú, Polarized Light in Animal Vision-Polarization Patterns in Nature (Springer-Verlag, 2003).

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Figures (7)

Fig. 1
Fig. 1

(Color online) Color photograph (A), and patterns of the degree of linear polarization p (B–D) and the angle of polarization α from the local meridian (E–G) of a clear sky (scene S43, Tables 1 and 2), measured by 180° field-of-view imaging polarimetry in the red, green and blue parts of the spectrum. The optical axis of the fisheye lens was vertical, thus the horizon is the perimeter and the center of the circular patterns is the zenith. At the perimeter of the circular color picture the dark silhouette of trees can be seen. The sun near the horizon was occluded by a small black disc placed on a thin wire, which is seen radially in the circular patterns.

Fig. 2
Fig. 2

(Color online) As Fig. 1 for scene S0 with grassland lit by the rising sun. The measurements were performed from a hot air balloon at an altitude of 100   m . The optical axis of the fisheye lens pointed toward the nadir, which is the center of the circular patterns.

Fig. 3
Fig. 3

(Color online) As Fig. 1 for scene S22 with the overhead vegetation of a forest composed of birch trees lit by the setting sun. In pattern B black shows the tree foliage and white indicates the sky.

Fig. 4
Fig. 4

(Color online) Color pictures and α-patterns of Finnish skies and tree canopies (scenes S1–S42, Tables 1 and 2), measured in the blue part of the spectrum. Quite similar α-patterns were obtained in the green and red spectral ranges.

Fig. 5
Fig. 5

Fig. 4.Continued.

Fig. 6
Fig. 6

Schematic representation of the polarization characteristics of the different components (SU, S-SK, S-LE, D-SU, D-SK, D-LE, T-SU, T-SK, T-LE) of light transmitted (A) and reflected (B) by a leaf in the foliage. Circles and ellipses with double-headed arrows represent unpolarized and partially linearly polarized light, respectively. SU: sunlight (unpolarized); SK: skylight (partially linearly polarized); LE: leaflight (partially linearly polarized); S-SU, S-SK and S-LE: specularly reflected sunlight, skylight and leaflight (partially linearly polarized); D-SU, D-SK and D-LE: diffusely reflected sunlight, skylight and leaflight (unpolarized); T-SU, T-SK and T-LE: transmitted sunlight, skylight and leaflight (unpolarized).

Fig. 7
Fig. 7

(Color online) Left and middle columns: Color pictures and the α-patterns of a clear sky (scene S7) and the tree canopy (scene S8), measured in the blue ( 450   nm ) part of the spectrum under the same sky conditions (Table 1). Right column: Theoretical α-patterns calculated on the basis of the single-scattering Rayleigh-model (for S7) and the model of Berry et al. (2004) (for S8), respectively. For the sake of easier comparisons, the circular pictures and patterns were rotated so that the solar-antisolar meridian became vertical in both cases.

Tables (2)

Tables Icon

Table 1 Date, Time, Solar Elevation Angle ε, Sky and Canopy Conditions During our Polarimetric Measurements Performed in Hungary (Scene S0) and Finland (Scenes S1–S43)

Tables Icon

Table 2 Optical Characteristics of the Hungarian (S0) and Finnish (S1–S43) Scenes Measured in the Red ( R , 650 nm), Green ( G , 550 nm), and Blue ( B , 450 nm) Parts of the Spectrum and Averaged over the Entire Hemispherical Field of View. The Definition of Noisiness n of α Is Given in the Materials and Methods Section. The Foliage Ratio f Gives the Percentage of the Overhead Vegetation of the Full Celestial Hemisphere (Not Applicable in Scenes with Open Skies). The Date, Time, Latitude, Longitude, Solar Elevation and Sky Conditions of Scenes S0–S43 are Given in Table 1

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