Table Of ContentContributions of artificial lighting sources on light pollution in
Hong Kong measured through a night sky brightness monitoring
network
Chun Shing Jason Pun1
4 [email protected]
1
0
2 Chu Wing So1
n
[email protected]
a
J
4 Wai Yan Leung1
] [email protected]
M
and
I
.
h
p Chung Fai Wong
-
o
r
t
s
a
[ ABSTRACT
1 Light pollution is a form of environmental degradation in which excessive artificial outdoor
v lighting, such as street lamps, neon signs, and illuminated signboards, affects the natural
0 environment and the ecosystem. Poorly designed outdoor lighting not only wastes energy,
0
money, and valuable Earth resources, but also robs us of our beautiful night sky. Effects of light
2
pollution on the night sky can be evaluated by the skyglow caused by these artificial lighting
1
. sources, through measurements of the night sky brightness (NSB). The Hong Kong Night Sky
1
Brightness Monitoring Network (NSN) was established to monitor in detail the conditions of
0
4 light pollution in Hong Kong. Monitoring stations were set up throughout the city covering a
1 wide range of urban and rural settings to continuously measure the variations of the NSB. Over
: 4.6 million night sky measurements were collected from 18 distinct locations between May 2010
v
i and March 2013. This huge dataset, over two thousand times larger than our previous survey
X
(Pun & So 2012), forms the backbone for studies of the temporal and geographical variations
r of this environmental parameter and its correlation with various natural and artificial factors.
a
The concepts and methodology of the NSN were presented here, together with an analysis of
the overall night sky conditions in Hong Kong. The average NSB in Hong Kong, excluding data
affected by the Moon, was 16.8 mag arcsec−2, or 82 times brighter than the dark site standard
established by the International Astronomical Union (IAU) (Smith 1979). The urban night sky
was on average 15 times brighter than that in a rural location, firmly establishing the effects of
artificial lighting sources on the night sky.
Subject headings: light pollution, night sky brightness, skyglow, Moon radiation, urban lighting
1
1. Introduction ferred to as NSB hereafter). The NSB is a com-
bination of the natural sky glow due to celestial
Outdoorlightingisanindispensableelementof
objects (Sun, Moon, planets, stars, Milky Way,
modern civilized societies for safety, recreation,
galaxies, etc.), and the sky glow due to direct
and decorating purposes. However, poorly de-
or reflected light from artificial lighting sources.
signedoutdoorlightingsystemsandexcessiveillu-
Apart from the effects of the Sun and the Moon,
mination levels can lead to light pollution (Smith
the chief contributor to the NSB for a highly
2009;Mar´ın&Jafari2007). Thescatteringofarti-
populated metropolitan city is the artificial light-
ficiallightbycloud,aerosol,andpollutantssuchas
ing source. The level of the NSB had mostly
suspended particulates in the atmosphere spread
been studied at professional astronomical obser-
the effects to distances beyond the position of the
vatories using the traditional astronomical tech-
lighting source and can brighten the entire night
nique of photometry — the amount of light de-
sky (Benn & Ellison 1998). Light pollution is a
tected in star-free regions on the CCD images
form of environmental degradation in which ex-
was extracted to estimate the NSB at different
cessive artificial outdoor lighting affect the natu-
wavelengthbands(Pedani2009;Stalinetal.2008;
ral environment and the ecosystem. It not only
Patat 2008; Sanchez et al. 2007; Krisciunas et al.
represents a waste of energy, money, and valuable
2007;Tayloretal.2004;Liuetal.2003). Thiskind
Earth resources, but also indirectly contributes to
ofobservationhelpedastronomerstorevealpoten-
the global environmental problems. Last but not
tial light pollution threats to the observatory and
least, the skyglow due to these artificial lighting
aidintheirsearchforanewpotentialdarkobserv-
sources leads to the degradation of the quality of
ing location. These observations would normally
night sky and reduces the number of observable
require skilled personnels using delicate experi-
stars, robbing us the beautiful night sky on dark
mental setup (telescope/lens/camera/mounting).
nights when it would otherwise be visible.
Thegeographicalandtemporalcoveragesofthose
The energy wasted by artificial lighting sources studies were usually limited (limited to a single
can be directly monitored by night-time images site, and only a relatively few observations per
of the Earth. Satellite imagery taken from the night).
US Air Force Defence Meteorological Satellite
Large-scale surveys of the sky conditions had
Program (DMSP) Operational Linescan System
also been carried out by campaigns which re-
(OLS)1 sampledlargeareasoflandmassatamod-
cruited ordinary people to conduct visual studies
erate spatial resolution (∼ 1 km), making studies
of the night sky following simple procedures with
of the extent and degree of night time radiation
minimal technical expertise required, such as the
over a metropolis or even globally possible (Kyba GLOBEatNight3 andtheGreatWorldWideStar
et al. 2013; Small & Elvidge 2013; Cinzano & Count4 projects. Without the need for special-
Elvidge 2004; Cinzano et al. 2001). The night-
ized or expensive equipment, this kind of citizen-
time photographs taken by astronauts onboard
scienceprojectencourageda largenumberof peo-
the International Space Station (ISS)2 provided
ple from around the world to take part in studies
evenhigherspatialresolution(at∼6mperpixel)
of light pollution, yielding results with broad ge-
for selected locations on Earth. While the light
ographical coverage and high spatial resolution if
intensityontheISSphotographofHongKongwas
the number of observers were large enough. Fur-
found to be positively correlated with population
thermore,manyobservationswereconductednear
densityingeneral(Liuetal.2011), thescarcityof
places where the participants lived, such as city
these photographs was insufficient for studies of
centersorsuburbanregions,oratplacesthatthey
nightly variations of light pollution in the city.
could gain access to, such as country parks. Re-
The extent of light pollution can also be mon- cent comparison of a subset of the GLOBE at
itored by studying the night sky brightness (re- Night results with the night-time satellite images
1DepartmentofPhysics,TheUniversityofHongKong, 3http://www.globeatnight.org/
PokfulamRoad,HongKong 4http://www.windows2universe.org/citizen_science/
1http://ngdc.noaa.gov/eog/ starcount/
2http://eol.jsc.nasa.gov/
2
illustratedthatthiskindofstudyhashugepoten- light pollution. However, the dataset collected
tialforglobal scalestudiesof thenightsky (Kyba was limited by its geographic distribution (volun-
et al. 2013). teers made measurements usually within or near
Hong Kong is a populous metropolitan city urban population areas), temporal resolution (a
(mid-2012 population 7,154,600 (Census and majority of volunteers made measurements usu-
Statistics Department 2013)) with a high pop- ally once or twice every several nights), a short
ulation density (6,620 per km2), known for its monitoring time (volunteers were swapped every
spectacular night lights. The mountainous city few months to allow for more participation), and
has a complex geographical landscape, leading to possible human-related errors (volunteers might
a short supply of habitable land. A photograph make mistakes during measurements and/or data
taken from the ISS in March 2003 (Figure 1) re- reporting).
vealed the huge amount of upward shooting light WelaunchedthesucceedingprojectHongKong
atnight. Asseeninthefigure,thereweresubstan- Night Sky Brightness Monitoring Network (re-
tial variations in the level of night-time emission ferredtoasNSNhereafter)in2010tocomprehen-
from the city, mostly due to differences in pop- sively study the properties of the NSB in Hong
ulation and land utilization. It seems obvious Kong and its dependence on time, location, and
that the degree of light pollution in Hong Kong variousatmosphericandmeteorologicalconditions
should be strongly dependent on human activity, with the support of the Environment and Con-
in particular, how and where people use external servation Fund of the Hong Kong SAR govern-
lighting. ment. The range, depth, and accuracy of data
The availability of low-cost light sensors orig- collection were optimized by setting up automatic
inally targeted for the astronomical community NSB measurement stations in 18 distinct urban
allowed for detailed and comprehensive studies and rural locations around Hong Kong. All these
of the NSB. Between 2007 and 2009, we con- stations were designed so that on-site NSB mon-
ducted a citizen-science survey of light pollution itoring for over a year was possible after secur-
inHongKongbyinvitingstudents,astronomyen- ing long-term commitments from our collaborat-
thusiasts, and campsite employers to measure the ing partners (refer to acknowledgment section for
NSBusingonesuchdevice,theSkyQualityMeter the full list). The temporal resolution of data
(SQM) (Pun & So 2012). Compared to projects collection was vastly improved and human errors
such as GLOBE at Night, the use of a standard were eliminated through the use of the ethernet
measuring device in this study reduced the uncer- version of the SQM. In Section 2, details of the
tainties in night sky measurements due to varia- survey are discussed, including the setup of the
tions in observers’ eyesight and experience. From automatic NSB measuring stations, site selection,
the over 2,000 measurements taken at almost 200 data selection, data quality control, and calibra-
locations by over 170 volunteers, it was concluded tion of the measurement devices. The resulting
that light pollution in Hong Kong is severe, with database forms the backbone for a study of the
largebrightnesscontrastbetweentheobservedur- overall light pollution conditions in Hong Kong.
ban versus rural locations. Moreover, later night Results of the analyses can be found in Section 3,
skies (at 23:30, local time (UTC+8) hereafter) focusing on the contributions of artificial lighting
were generally darker than at an earlier times (at sourcesontheobservedNSBinanurbanenviron-
21:30), which could be attributed to some public ment. Discussion of the results from this study
andcommerciallightsourcesbeingturnedofflate and possible potentials of this NSB database are
at night. presented in Section 4.
This survey not only provided the first glimpse
of the light pollution situation in Hong Kong, but
also spread the message of dark sky conservation
and energy saving among students and the gen-
eralpublicthroughparticipationsinthehands-on
sky brightness measurements and first-hand ob-
servations of the environmental consequences of
3
Fig. 1.— Image taken from the International Space Station (ISS) when it flew over Hong Kong in orbit at
altitude of ∼ 300 km. The image was taken at 00:51 (UTC+8) on 11 March 2003 local time. Landmass
boundaries and locations of NSN monitoring stations obtained from Google Map are overlaid on the im-
age. Refer to Table 1 for station codes and Section 2.1.2 for details. (credit: Image Science and Analysis
Laboratory, NASA-Johnson Space Center, The Gateway to Astronaut Photography of Earth)
2. Methodology mag arcsec−2. The SQM-LE is manufactured by
the Canadian company Unihedron6 and each unit
2.1. Data collection
was calibrated by the manufacturer before ship-
ment. The light sensor of SQM-LE is the TAOS
2.1.1. Measurement instrumentation
TSL237High-SensitivityLight-to-FrequencyCon-
In the current study, the lensed version of the verter covered by a near-infrared blocking filter
SQM, the Sky Quality Meter - Lens Ethernet Hoya CM-500. The combined filter-sensor sys-
(SQM-LE), was used to collect the night sky ob-
servationdata. BoththeSQMandSQM-LEmea- thecelestialsphere. 1”=1/3,600◦. Supposethemeasured
sure the brightness of the night sky in units5 of NSBatsiteAis20.0magarcsec−2,thenthebrightnessof
the sky is equivalent to a celestial object of 20 mag filling
up a patch of sky of area 1 arcsec × arcsec. Suppose the
5Magnitude (mag)isalogarithmscaleinternationalunitto
measuredNSBatsiteBis19.0magarcsec−2,thenthesky
measure the brightness of astronomical objects. A differ-
atsiteBis2.512timesbrighterthanthatatsiteA.
enceof1magreferstoanobservedlightfluxratioof100.4
= 2.512. Arc second (arcsec or ”) is the unit of length on 6http://unihedron.com/projects/sqm-le/
4
tem has a central wavelength of 540 nm and a SQM-LE,theconfigurationandappearanceofthe
Full Width Half Maximum (FWHM) of 240 nm setup were shown in Figure 2. The enclosure for
(Cinzano2013a),andisdesignedtohaveasimilar eachmodulealsoincludedapowersupplyadapter
sensitivity to that of the human eye. Compared and a network device for data transmission (to be
to the earlier version of the SQM, the SQM-LE discussedindetailinSection2.1.3). ExceptSQM-
has a narrower field of view of ∼ 20◦ in FWHM. LE, almost all components of the observing mod-
The detector sensitivity drops sharply away from ule employed readily available commercial prod-
the zenith direction, with sensitivities at factors uctstoreducethecostforconstructionandmain-
15 and 100 respectively, lower than that at the tenance. The entire housing was supported on
zenith at 20◦ and 40◦ off-axis (Cinzano 2013b). a tailor-made stainless steel frame mounted on a
The SQM-LE was preferred over the original un- poleattachedusuallytotherailingsontherooftop
lensedversionoftheSQMforaNSBstudyinHong of buildings. The entire housing and frame could
Kong because the measurements would be less af- easily be dismounted so that the entire unit could
fectedbyeffectsofstraylightingdirectlyreaching be collected from an observing site to our labora-
the light sensor in this dense urban environment. tory for regular checking and maintenance. The
The stated accuracy of the SQM-LE by the man- modulardesignalsoallowedforswappingofhard-
ufactureris0.1magarcsec−2,orroughly±10%of ware between stations.
the brightness of the sky in terms of emitted flux. The summer temperature of Hong Kong can
While the convenience and the durability of reach a high of 35◦C and thus temperature con-
SQM-LEfavoredtheirusageforalong-termnight trolisimportantforprotectionofthevariouselec-
skymonitoringproject, afewproblemsofthisde- tronic devices, including the SQM-LE (maximum
vice had been identified. First, dropped trans- operating temperature 85◦C). Even though the
parency of the filter could occur due to frosting unit’s temperature compensation algorithm is ad-
developedwhenoperatedinahumidenvironment equateundertheexpectedoutdooroperatingcon-
foraprolongedperiod. Afilterwithanti-moisture ditions in Hong Kong (Schnitt et al. 2013), we in-
coatinghadbeenprovidedbythemanufacturerfor stalled ventilation fans at the bottom of the hous-
replacement(cfSection2.3). Second,theunithad ing and glued silver reflective linings to reduce
not been calibrated to accurately measure a night the heat accumulation in the interior of the unit.
sky darker than 23 mag arcsec−2. However, this We placed the linings inside the housing instead
is a non-factor for our study as the night sky in of outside to ensure that they would not be de-
HongKongwouldneverreachthatdarklevel. Fi- tached after long-term outdoor exposure, and it
nally, in earlier versions of the SQM-LE firmware had proved to serve its purpose. In each mod-
(before version 19), the precision of the meter ule, an electronic timer switch limited the power
for readings brighter than ∼ 17.0 mag arcsec−2 supply to 15:50 − 09:00 each day for both energy
suffered from digitization problems, causing the savingandforreducingheatgeneration. Thedaily
units to round off measurements between 15.2 power reset also occasionally helped to overcome
and 17.0 mag arcsec−2 to the nearest 0.15 mag systemfailuresduringthenight,andrecoveredop-
arcsec−2, and measurements brighter (smaller in erationofthemoduleinthenextdaytoreducethe
NSB) than 15.2 mag arcsec−2 to the nearest 0.18 number of required manned on-site service main-
mag arcsec−2. As this problem had been fixed by tenance.
asubsequentfirmwareupgrade, only∼ 8%ofthe Through regular maintenance of the observing
total data collected at 13 different stations from modulescollectedfromthemonitoringstations, it
14 individual units in the first 20 months of the wasdiscoveredthatthetransparentcoverwindow
survey could be affected by this error. of the polycarbonate housing suffered from obvi-
The SQM-LE was not originally designed for ous aging and turned visually yellow after ∼ 12
permanent outdoor installations. To protect it months of outdoor operations. To prevent fur-
from the wear and tear of the outdoor environ- ther degradation in data quality, a small portion
ments,eachunitwasfittedintoawaterproofpoly- ofthepolycarbonatehousingrightontopofSQM-
carbonate housing which has a transparent cover LE was replaced with a 25.4 mm in diameter and
for light to reach the SQM-LE sensor. A unit of 3 mm thick high efficiency circular glass window
5
Fig. 2.— A unit of SQM-LE is shown in (a) (credit: Unihedron). Its semi-conductor light sensor is located
under the filter in greenish blue. The view in the bottom shows the power inlet, the ethernet inlet, and
the switch for calibration purpose. The dimension of a SQM-LE unit is about 91 × 66 × 28 mm. The
internal hardware configuration and appearance of the iObs NSN monitoring station are shown in (b) and
(c) respectively. The dimension of this model of housing is 278 × 278 × 100 mm. As discussed in the main
text, a small piece of the polycarbonate surface where the SQM-LE points towards the zenith had been
replaced by a piece of a thin glass 25.4 mm in diameter. For each complete module, hardware costed about
USD770in2010, weights∼5kg, andrequireslessthan∼16WACpowersupplyduringnormaloperation.
In the outdoor photograph, in addition to the night sky measurement module, two cloud sensors (narrow
cylinders mounted on the right side of the rectangular housing) were also mounted on the left-side pole,
while an automatic weather station installed by the Hong Kong Space Museum on the right-side pole was
also shown.
withanti-reflectioncoating(EdmundOpticsStock Apart from the demographic and geographic
No. #46-0987). The glass was glued directly considerations described above, each observing
above the SQM-LE sensor by drilling a small hole module should have a wide field-of-view (FOV)
on the housing cover. Procedures for corrections to the sky not obstructed by any lighting source.
of the data that suffered from the aging problem Thiscouldposeabigchallengeforoururbanmon-
will be described in detail in Section 2.3. itoring site selection due to the high density of
buildingsinthecityresidentialareas. Inalmostall
2.1.2. Monitoring stations cases the measurement module was placed on the
rooftop of a building for the widest FOV. More-
From our previous study of the light pollu-
over,theambientenvironmentneareachmonitor-
tion condition in Hong Kong (Pun & So 2012),
ing station was checked so that the top floor of
it was clear that the level of external lighting was
the nearest building was at least 40◦ away from
the dominating factor of the NSB level observed.
zenith. As discussed in Section 2.1.1, the sensitiv-
With a highly mixed land utilization within a
ity of the SQM-LE at that angle was only a fac-
small landmass, along with a complex landscape
tor of 0.01 of that at the zenith. All monitoring
and diverse forms of human activities in different
stations satisfy this criteria, except in the WTS
areas in Hong Kong, it was desirable to strategi-
where the rooftops of a handful of buildings were
callychoosemonitoringstationstoformanetwork
visible at an zenith angle of ∼ 30◦. Other fac-
coveringaswideageographicalcoverage, rangeof
tors for consideration included the availability of
population density, landscape type, and land use
power, and more importantly, steady and strong
as possible within the budget constraint.
mobileInternetnetworkattheobservinglocations
(cf Section 2.1.3). These could be demanding re-
7http://www.edmundoptics.com/
6
quirements for some of the rural locations in our Kong Observatory (HKO). Situated on the
survey. summit of a small hill at the middle of
With a hilly and complex terrain, the popula- Kowloon, it offers a location with a rela-
tion density of Hong Kong is highly uneven. Re- tively wide FOV in the middle of a densely
gions of the highest population density lie on ei- populatedmixedresidentialandcommercial
thersideoftheVictoriaHarborbetweentheHong area. A large variety of surface meteorologi-
Kong Island and Kowloon. Additional popula- calmeasurementsareconducteddailybythe
tioncentersspreadthroughouttherestofthecity many equipment operated by the HKO at
(known as the New Territories region), observable this station8.
as clusters of light in the ISS image in Figure 1.
TC This urban station was located in a satellite
On the other hand, large portions of the territory
residential town adjacent to the Hong Kong
are allocated as reserved park areas and remain
InternationalAirport. TheNSNmodulewas
sparselypopulated. Inanidealsituation,themon-
installed at the rooftop of a two-storey gov-
itoring network should cover as wide a geograph-
ernment building in the center of the town.
ical distribution and population density as possi-
In addition to shopping malls and high-rise
ble. However,withlimitationsofmanpower,fund-
residential buildings nearby, the major ar-
ing, and accessibility to suitable sites, we ended
tificial light source is the Airport passenger
up selecting 18 locations for construction of NSN
terminal at about 3 km away.
stations. Details of these stations were listed in
Table 1. In Figure 1, the station locations were HKn This urban station was located near the
overlaid on the night-time image taken from ISS astronomical observatory on the rooftop of
in 2003. Station codes in Table 1 will be used an environmental education center complex
to refer to specific stations hereafter. While this on the foothill of Tai Mo Shan, the high-
projectwasconductedbetween2010and2013,al- est peak in Hong Kong. While it has the
mostadecadeaftertheISSimageinFigure1was highest elevation (∼ 140 m above sea level)
taken, it is expected that the overall pattern or amongallstationsinthenetwork,itwasstill
distributionofpopulation, henceforththeamount affected by external lighting of even taller
of external lighting used, did not change by much highrisesnearbyandfromtheadjacenttown
foramajorityoftheregionsduringtheintervening center. The education center held regular
period. school group and public visits which could
Unlike many cities worldwide, the wide spread occasionally affect the data taken.
of population centers in Hong Kong made the ex-
HKU This urban station was installed near the
act classification of each monitoring locations as
teaching astronomical observatory on the
urban, suburban, and rural to be ambiguous. On
rooftop of the Department of Physics build-
the other hand, the ISS image did seem to justify
ing at the University of Hong Kong (HKU).
theclassificationofonlytwocategories,urbanand
HKU has an urban campus with high-rise
rural. The classifications were listed in Table 1
residentialbuildingsinthesurrounding. Me-
and shown in Figure 1. With this rough system,
teorological equipment including two cloud
a total of 10 monitoring stations were classified as
sensors, an all sky camera, and an auto-
urbanand6asrural. Intwoinstances,thestation
matic weather station were installed near
was neither classified as urban nor rural because
the night sky monitoring module for addi-
of their unique locations near the Hong Kong In-
tionalmeasurementsoftheatmosphericcon-
ternational Airport (GFS) and the River Trade
ditions. Occasional educational events were
Terminal (TMD).
held at the observatory which could affect
Briefdescriptionsofallthemonitoringstations
the operation of the monitoring station.
listed in Table 1, in their order of operation start
dates, are provided below. iObs This rural station was installed on the
rooftop of the building housing the robotic
KP This urban station was located at the King’s
Park Meteorological Station of the Hong 8http://www.hko.gov.hk/wxinfo/aws/kpinfo.htm
7
observatory of the Hong Kong Space Mu- observationsduringclearweathernightsand
seum inside a rural country park. The ob- theseoperationscouldbrieflyaffectthedata
servatoryispartofacomplexofbuildingsin collected.
a public camping village. The main sources
Cap Thisruralstationwaslocatedontherooftop
of artificial lighting include the external fix-
of the marine science research center oper-
tures of the camp cottages and recreational
atedbyHKUatoneofthesoutheasterntips
facilities within the complex. Moreover,
of the Hong Kong Island. The building lies
scattered lights from a town centre about
on the shore of a marine reserve area with
6 km away in southwest (Sai Kung) can be
minimalamountofartificiallightingfixtures
noticed from the observatory. Similar to
intheambientenvironmenttoallowforeco-
the HKU station, meteorological equipment
logical studies of the region. The main light
including two cloud sensors, an all sky cam-
sourceisalighthouse(ataheight20mabove
era, and an automatic weather station were
the observing module) located at ∼ 200 m
installednearthenightskymonitoringmod-
away.
ule (the latter two equipment were installed
by the Hong Kong Space Museum). As part
AP This rural station was located in the As-
of a public educational facility, stargazing
tropark, a designated stargazing park de-
events were regularly held as part of the
signed by the Hong Kong Space Museum.
Museum and village programs.
The AP station was located ∼ 3 km deeper
into thesamecountry park comparedto the
TST This urban station was installed on the
iObs station described earlier. No exter-
rooftop of the Hong Kong Space Museum.
nal lighting operates within the park, but
The museum is located in the waterfront of
scattered light from a water sports center at
the Victoria Harbor, which is the central
100mawaycouldaffectmeasurementstaken
commercial area of the city clustered with a
at the station. Since its opening in 2010,
large number of hotels, shopping malls, and
the park has been one of the most popular
touristattractions. TheMuseumbuildingis
stargazing venues in Hong Kong for astron-
relatively low in height (∼10 m) compared
omy enthusiasts.
to office buildings and hotel towers nearby
that can reach ∼100 m. In addition to the
TMD This station was located in a temporary
spilledlightfromtheinteriorsofthesebuild-
observatory setup near the Hong Kong Sci-
ings, other lighting sources include the large
ence Museum’s depot in western New Ter-
andbrightilluminatedbillboardsdecorating
ritories. While the nearest residential town
the building exterior, particularly near the
was relatively distant at ∼ 4 km away, the
rooftopofthebuildings. SimilartotheiObs
station’s lighting environment was mainly
station,stargazingandeducationalactivities
affected by the River Trade Terminal at
at the rooftop could occasionally affect the
about 0.5 km away. This container terminal
measurements.
for river barges has 49 berths and a large
number of lighting fixtures remained turned
SH This rural station was located on the rooftop
onthroughoutthenight. Thereforethissta-
of the building housing the observatory of
tion is classified as neither an urban nor a
theHongKongAstronomicalSociety, which
rural site.
was operated by local amateur astronomy
enthusiasts in a rural village setting, with
MWo This rural station was installed on the
onlyasmallnumberofhousesinthevicinity.
rooftop of a campsite at about 1 km away
While the Airport is located about 10 km
from a ferry pier, which was also the loca-
away to the North, this site is not expected
tionofaruraltowncenter. Thisstationwas
to be strongly affected by its strong lighting
closed permanently in August 2011 due to
due to the shielding effect from the Lantau
the closure of the campsite for major reno-
Peak(altitude934m)inbetween. Members
vation work.
of the Society might use the observatory for
8
TPo This urban station was installed on the ST This urban station was installed on the
rooftop of a government office building lo- rooftop of a school located in a population
catedwithinapopulationcenterintheNew center in southern New Territories. This
Territories. Public facilities and residential school is also surrounded by taller residen-
estates in form of high-rise apartments as tialbuildings,withtheclosestblockatabout
closeas30mawaycouldaffectthemeasure- 80 m away.
ments collected.
GFS This station was installed within the con-
TSW Thisurbanlocationwasestablishedwithin fines of the Hong Kong International Air-
the Hong Kong Wetland Park which is a port. This busy airport operates 24 hours
conservation,education,andeco-tourismfa- a day and most of the external lighting for
cility. The park comprises of a visitor cen- air traffic remains turned on throughout the
ter and 60 hectares of outdoor wetland pro- night. This station was installed on the
tected areas. The station was installed in a rooftop of a facility building on the south-
location inside the park that was not open western corner of the airport. The South
to the pubic. As an ecologically important runway and the airport passenger terminal
area, usage of outdoor lighting within the are located about 500 m in North and 3 km
park is kept to a minimum. On the other innortheast. BoththeHKOandtheairport
hand, a major population center is located operator(AirportAuthorityHongKong)in-
about 500 m away from our detector and stalledsuitesofadvancedmeteorologicaland
could contribute to the observed NSB. air quality monitoring systems at different
locations of the airport for air traffic opera-
WTS This urban station was installed on the
tionsandforenvironmentalmonitoring. We
rooftop of a school located in a densely
also installed cloud sensors adjacent to the
populated district with residential estates,
SQM-LE for further studies.
shopping malls, and transportation hubs in
Kowloon. At only nine storeys high, our de-
Thelightingenvironmentneareachmonitoring
tector was practically surrounded by taller
stationwererecordedatthebeginningofthestudy
buildings in all directions, with the closest
andmonitoredregularlythroughourmaintenance
one at about 70 m away extending up to
trips to ensure that the FOV of all the SQM-LE
∼ 30◦ in zenith angle.
werenotblockedbyanyobstaclessuchastreesor
artificialstructures. Moreover,weobservednoev-
TpM This rural station was installed on the
idence of drastic change of the lighting settings in
rooftop of a Police Post on a sparsely pop-
our monitoring stations throughout up to 3 years
ulated (population ∼ 100) island on the
of duration of survey.
northeastern corner of Hong Kong. The
entireislandwasinsideacountrypark,with
2.1.3. Data flow
limited artificial lighting sources apart from
public facilities. The remoteness of the site The NSB measurements taken from the SQM-
also led to our selection of a different data LE could be transferred by Internet directly via
acquisition mode for this station due to the a TCP connection to the survey computer server
limited mobile Internet strength (cf Sec- (the main server). With a majority of monitoring
tion 2.1.3). stationslocatedatthird-partyproperties,themo-
bile3GnetworkwasusedforInternetconnections
SSh This urban station was installed on the
toallthemonitoringstationstominimizethecost
rooftop of a school located in a population
and labor required.
centerinnorthernNewTerritories. Situated
For all but one of the monitoring stations, we
near the edge of town center, this observing
adopted the active mode of data transfer. Each
location has relatively wide FOV. This sta-
SQM-LE unit acted as a TCP client, linking first
tionwasthenorthernmostinourstudy, and
to a commercial 3G router, and then a USB 3G
its data could also be affected from the city
modem for Internet connection. Each unit would
of Shenzhen, China, at only 4 km away.
9
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