Download Degree of linear polarization

Lunar Skylight Polarization Signal
Polluted by Urban Lighting
Christopher Kyba1,2, Thomas Ruhtz1,
Jürgen Fischer1, Franz Hölker2
1Freie
Universität Berlin
2Leibniz-Institute of Freshwater Ecology and
Inland Fisheries
11th Dark Sky Symposium, Osnabruck
Oct 6, 2011
Verlust der Nacht
(Loss of the Night)
• Collaboration of 9
Universities /
Institutes
• 14 Integrated
Subprojects
• Many aspects of
light pollution
considered, from
measurement to
ecology to sociology
http://userpage.fu-berlin.de/~kyba/
Light pollution conferences
Twitter: @skyglowberlin
Youtube: skyglowberlin
Outline
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Polarization of light
Nocturnal insect navigation
Experiment setup
Results and interpretation
Ecological consequences
How does light become polarized?
• Polarization is a
characteristic of light
• Most sources produce
unpolarized light
• Horizontally polarized
light generated by
reflections
• Linearly polarized light
generated through
Rayleigh scattering
How does light become polarized?
Nocturnal Navigation
• Dung beetles navigate
using the polarized sky
light of the moon
• Signal strength seven
orders of magnitude
smaller than in
daytime
Dacke et al. 2003
Properties of sky polarization
• Degree of linear pol. is strongest at twilight
• Twilight band of maximum polarization
runs approximately North/South
• Moonlight polarization pattern almost
identical to sunlight p.p. (Gál et al. 2001)
• Polarization pattern is visible in partly
cloudy skies (Pomozi et al. 2001)
=> Polarization is a more robust directional
signal than the sun, moon, or stars
Experimental Setup
o
• Berlin is at 52 N
• North Star is 38o from zenith
• Moon’s max deviation from
ecliptic is 18o-28o
• Moon is always 62o to 118o
from North Star (66o to 114o
in 2010)
• North Star is always near
peak of Rayleigh polarization
• Geometry is independent of
position on Earth, and
changes a few degrees/day
Measurement Locations
N
• Compare lunar skylight
polarization at urban
and rural location
• Brandenburg is very
dark compared to
Berlin
50 km
Measurement equipment
Sigma 24mm f1.8
LPF
Blue
CFW-8
SC4022
Red
tripod
Note: Measure each LPF position (at least) 4x for each filter
Skyglow pollutes lunar
polarization signal!
Degree of linear polarization
Urban moonrise:
Urban with moon:
Rural moon:
Urban daytime:
Laboratory (LCD):
3.9 ± 0.2%
11.3 ± 0.3%
29.2 ± 0.8%
56.6 ± 1.0%
98.1 ± 1.2%
Skyglow pollutes lunar
polarization signal!
Degree of linear polarization
Urban no moon:
Urban moonrise:
Urban with moon:
Rural moon:
Urban daytime:
Laboratory (LCD):
8.6 ± 0.3%?!?!?
3.9 ± 0.2%
11.3 ± 0.3%
29.2 ± 0.8%
56.6 ± 1.0%
98.1 ± 1.2%
Should skyglow be polarized?
• Most sources of light
pollution are unpolarized
• Horizontally polarized
light scattered upwards
• Rayleigh scattering can
direct light downwards
• Sources of light pollution
are spatially distributed
• Light pollution is
generally uncollimated
Naively, no
What do simulations say?
• Skyglow simulations
describe propagation of
light from sources to
observer
• Most do not take
polarization into
account
• The simulation that does
(Kerola 2006) predicts
that skyglow is almost
unpolarized (~2%)
Aubé 2007
But skyglow can be polarized!
Degree of linear polarization
370-510 nm: 10.1 ± 0.5%
490-580 nm: 9.4 ± 0.7%
590-690 nm: 8.5 ± 1.4%
370-700 nm: 8.6 ± 0.3%
• Moonless, clear
sky observing
conditions
• Similar values
observed several
months apart
• Results for one
particular direction
in one city
How can this be?
Ecological consequences
• Nocturnal insect
navigation
–
–
–
–
dung beetles
crickets?
moths?
bees?
• Bird attraction to
searchlights?
Dacke et al. 2003
Conclusions
• Skyglow pollutes the natural polarization
signal of the moon
• Extremely likely to affect navigational
abilities of some nocturnal insects
• Skyglow itself can be weakly polarized
• Skyglow polarization could be used for
remote sensing of aerosols at night
Acknowledgements
Funding
BMBF 033L038A
MILIEU (FU Berlin)
Photo Credits
Table: Briho (Wikimedia Commons)
Sky: Christopher Kyba
Dragonfly: Andreas Trepte (WC)
Waggle dance: Jüppsche (WC)
Skyglow: Jeremy Stanley
Glacier National Park: Ray Stinson
New York City: CharlieBrown7034 (WC)
Light pollution map: WEW/FU Berlin
Light pollution model: Martin Aubé (2007)
Thank you!
Polarization of daytime skylight
Rayleigh scattering
(Pomozi et al. 2001)
Measuring Stokes Vector
When this is done for every pixel, you have imaging polarimetry
Searchlights
Visual brightness
(blue band)
Degree of
linear polarization
Animal use of polarized light:
Migration
Mulheim et al. 2006
Cochran et al. 2004
• Birds use polarization cues
to re-calibrate their
magnetic compass daily
• Average of sunset and
sunrise is true North, and
independent of latitude
and time of year
• Experiments hold birds in
altered magnetic field
during twilight
• Birds look for this cue
preferably at the horizon
Animal use of polarized light:
Material detection
• Water detection is most
well known use
• Leads to “polarized light
pollution” from artificial
surface reflections
(Horváth et al. 2009)
• Acquatic animals also
use it in hunting