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\begin{document}

 \title{What is light pollution, \\
        and how do we quantify it?}
 \author{Jan Hollan, \\
 N. Copernicus Observatory and Planetarium, Brno}
 \date{December 2006, January 2007, July-October 2008 }
 \maketitle

\centerline{(this pdf document with inline hyperlinks is available as
 \href{http://amper.ped.muni.cz/light/lp\_what\_is.pdf}
{http://amper.ped.muni.cz/light/lp\_what\_is.pdf})}

 
\tableofcontents

 \subsection*{Introduction to Pollution}
 \addcontentsline{toc}{subsection}{Introduction to Pollution}

Generally, pollution is an impairment of the purity of the environment
\myref{whatis}{[10]}. As a pure, reference state  of the environment, 
its natural state is to be considered, if applicable and adequate -- 
as in the case of the air, and natural water bodies outside buildings. 
Then, for example, even an anthropogenic change of its temperature is considered
to be pollution: a 
\href{http://en.wikipedia.org/wiki/Thermal\_pollution}{thermal one}. 

For indoor air, the reference state may be the same, especially if the interior
is generously connected to the exterior by windows -- surely in the case of 
chemical or particulate air pollution. 

In many cases indoors, other reference values may
represent purity, i.e.\ absence of pollution. 
We adapt buildings' interiors deliberately to make them non-natural. Even particles naturally
present outdoors, like pollen,  may be filtered by a ventilation system
to achieve purer air indoors.  Evidently, the pollen-laden indoor air in
spring or summer might be considered as polluted by allergic persons;
the reference value when he or she considers the air as pure might be
far lower than that occurring outdoors naturally. The same might apply 
to
a sound-reducing barrier represented by walls and windows against natural 
outdoor sounds -- people sleeping after night-work might rightly
consider loudly singing birds as 
\href{http://en.wikipedia.org/wiki/Noise\_pollution}{noise pollution} 
of their artificial bedroom environment, meant to remain silent even in
the morning. Another example is indoor air being warmer in winter
than the outdoor air but not being considered as thermally polluted.
Rather than a pure state, the  desirable state of the artificial indoor
environment is the reference. Of course, the definition of
the desirable state is a subjective one.  An opposite example from the thermal domain
might be a pleasant cold morning air, entering the windows of an office which
have just been  opened at the beginning of a sunny summer's day -- if the office
has not been massively ventilated during the night and remained hot from
the afternoon, the outdoor air entering the room becomes thermally
polluted soon, losing the refreshing effect...

So, for indoor environments adapted by people on purpose, the reference 
values representing an unpolluted state might be not the ones which
would be present in surrounding nature, but another ones preferred by
people within the interior -- whenever they are not compromising their
health. Odour, sound, and light which diminishes human health, 
even if welcomed (by others, possibly), are surely to be considered 
as \myref{Pfootnote}{\mylabel{pollutant}{pollutants}}. 

Pollution as an impairment of purity (natural purity outdoors  or
desired purity indoors) is a qualitative category. Quantitatively, it
might be unmeasurable, imperceptible, just noticeable, negligible,
insignificant, tolerable,  very apparent, serious, dangerous, even
life-threatening... This classification is very observer-dependent. How 
the amount of pollution should be classified and whether it should be
tolerated further, is a matter of culture, research, agreement and
development of knowledge. 
We pollute our environment and cannot eliminate most kinds
of pollution. However, we should mitigate them.

Pollution can be further categorised according to the environment being
polluted (air, water, soil, landscape) and by the type of pollutant
(biological, chemical, physical). The usual physical pollutants causing air
pollution are: solid or liquid particles; energy in form of sound;
electromagnetic waves/quanta or heat; and radionuclides.

If the natural state is the reference frame, pollution is exclusively 
anthropogenic, assuming situations where people are not just a minor, 
barely noticeable part of the larger environment nature (as in very ancient times).
However, as the reference frame, a \emph{usual} natural state can be 
taken instead, or even
a usual state of human-influenced environment. Then an alteration of
that usual state by natural forces, for example volcanic eruptions, storms, or floods
might be called pollution too. See the French wikipedia item
\href{http://fr.wikipedia.org/wiki/Pollution}{pollution} for more info. 

In the case of light, the polluting agents are photons of corresponding
energy, or electromagnetic waves of corresponding frequency, artificially
added to the environment. The reference state (as some light is always
present) varies with the natural day/night cycle. The non-polluted
state  outdoors is an environment with only natural sources of light. 
Does it sound too restrictive? Then remember an example of another kind 
of pollution, that of inland waters. Any river should ideally be 
without serious biological, chemical and physical pollution: in a state
where its water would be drinkable -- as was the case in most of Europe
even 200 years ago. 

The very term \emph{pollution} was used predominantly in a religious 
context before modern times. It began to acquire its current 
scientific meaning from the 19th to the 20th century, increasingly
after 1950, e.g.\ in case of 
\href{http://en.wikipedia.org/wiki/Eutrophication}{eutrophication} of waters.
This was accompanied by an emerging and rising awareness that 
emitting wastes into the environment degrades it, and restricts its
use, and is dangerous to human health. Pesticides became 
considered to be a real source of pollution only after Rachel Carson's 
\href{http://en.wikipedia.org/wiki/Silent\_spring}{Silent Spring} book
had been published in 1962, and in the case of artificial fertilisers, even later.

The term `pollution' remains rather emotive. How it could be that somebody 
is polluting, when he/she behaves properly, and in what is perhaps the only possible or 
known way? It could be argued that native people living in the wilderness, 
or even poor, uneducated people living in overpopulated areas do not 
pollute, as they are just a part of nature, struggling for survival and
happiness. It could be further argued that
the notion of pollution becomes appropriate only after
people develop or gain an awareness that some of their activities have
negative consequences and that better alternatives exist, with less damaging
consequences for wildlife, their own health and the well-being of future
generations. In this sense it would be inappropriate to speak about
CO$_2$ from fossil fuels as about a pollutant, if it is  emitted by
people knowing nothing about increasing the greenhouse effect. This was 
the case with all humanity (with the exception of 
\href{http://en.wikipedia.org/wiki/Svante_Arrhenius}{Svante Arrhenius} 
and a couple of others) a century ago. However, rising carbon pollution of 
the atmosphere is a sad reality, even if many people still do not know that they 
are seriously polluting when burning fossil fuels. This shows that it 
is hardly tenable to limit the term pollution just to activities whose
originators know `there is a problem'.

Light and noise may be the most recent examples of this difficulty. If
somebody  believes it necessary to cut grass as often as possible
with a very loud  motor-driven strimmer, or that there should be
dusk-to-dawn lights on his house at night, blinding everyone who
approaches (and attracting insects from large distances), it might
be difficult to explain to him that he produces unacceptable pollution which
should and can be easily mitigated. Those who light roads, he
might agree that it is not really necessary to light the  surrounding
treetops or mountains as well as the road, but would insist that the
road lighting itself cannot be regarded a pollution, as `it is needed and
does not harm anybody'... Evidently, it is difficult to use
\emph{pollution} as a neutral term, assuming no `sin'  when
unconsciously created, and no `horror' if not very large or damaging (see 
\myref{AdverseFoot}{footnote 3}).


 \subsection*{Definition}
 \addcontentsline{toc}{subsection}{Definition}

 \subsubsection*{-- outdoors}

 \begin{description} \item[\textbf{(Outdoor) Light pollution} ]{ \textbf{is the
 alteration of light levels in the
 outdoor environment  (from those
 present naturally) due to \myref{manmade}{\mylabel{man-made}{man-made}}
 sources of light.}} \end{description}

 A peer-reviewed article (2000) where the definition was published with slightly
different wording is \myref{mnras}{[1]}. Before that, contradictory
concepts of light pollution had been common, limiting it just to 
`\myref{AdverseFoot}{\mylabel{adverse}{adverse effects}}', `skyglow'
etc. This practice continues even now, unfortunately. Some consequences
of such pollution (\emph{by} light and \emph{of} natural light) are
welcomed by some and disliked by others; some consequences are considered
adverse by all. Many consequences are not yet known: the relevant
discipline (scotobiology) \myref{scotobiology}{[2]} is just emerging.  The
most general consequence is a disruption of the day-night cycle as it has
existed on Earth for billions of years. 

 \subsubsection*{-- indoors}
 
Natural indoor lighting conditions due to windows and outdoor
environment with its natural light sources offer an analogous baseline.
This is adequate for times when people indoors don't wish to alter
their environment by artificial lighting, as during sleeping time or when
enjoying looking out through windows. Then a slightly
adapted version of the above definition is adequate:

 \begin{description} \item[\textbf{(Bedroom) Light pollution} ]{ \textbf{is the
 alteration of light levels in the room
 (from those
 present naturally) due to man-made sources of light.}} \end{description}

However, a significant part of our indoor life includes use of
artificial lighting. Then another definition is to be applied:


 \begin{description} \item[\textbf{Indoor light pollution} ]{ \textbf{is such
 alteration of light levels in the
 indoor environment due to sources of light,
 which compromises human health}} \end{description}

For \emph{health}, the official World Health Organisation definition is to be used:
\emph{`Health is a state of complete physical, mental and social well-being   
and not merely the absence of disease or infirmity.'} (see the
\href{http://en.wikipedia.org/wiki/Health}{Wikipedia's item Health 
quoting WHO}). 

Lights found obtrusive by somebody should be definitely considered
as polluting. But even if nobody finds the indoor light conditions obtrusive
or unpleasant (e.g.\ because of perceiving them as necessary, common or
even desirable), the environment might be still regarded as
light-polluted whenever some metabolic parameters of the organism are
shifted, due to artificial light, outside the range which is
considered as  being proper for a healthy function of the organism -- a
typical parameter is the concentration of the hormone melatonin, which
should follow its natural course driven by the natural daily light and
darkness cycle. In our epoch of electric lighting it does not follow 
that course. Outside
the season of naturally short nights, the production of melatonin in
humans does not last the  whole `civil night', up to 15 hours in
central Europe in December. Lack of melatonin has been hypothesised to
have profound health consequences, and further and further studies
support this hypothesis. See 
\myref{indoors}{footnote 2}.

Under closer inspection it appears that the last definition and the middle
one have considerable overlap: intrusion of light from outdoor man-made sources 
into a 
room at night may compromise the health of people if they find it disagreeable
(i.e.\ being not in a state of complete mental well-being anymore), not to mention
cases when their sleep is disturbed.

 \subsection*{How it can be quantified}
 \addcontentsline{toc}{subsection}{How it can be quantified}

The text in this section deals with the case that the reference 
unpolluted situation is an environment with light of natural origin 
only. For indoor spaces with artificial lighting, the reference unpolluted 
state would be that one when the light is not compromising health; 
these levels may be difficult to find with any certainty. Some guideline 
values are proposed within 
\myref{indoors}{\mylabel{outdoors}{footnote 2}}.

 \subsubsection*{-- absolutely}
 \addcontentsline{toc}{subsubsection}{-- absolutely}

  As for all pollutants, light added artificially to the environment 
  can be measured by \begin{description} \item[concentration ]{of the
  pollutant in some volume of the environment (of air or water). For
  light, a convenient unit for this purpose is the
  \href{http://en.wikipedia.org/wiki/lumen\_\%28unit\%29}{lumen} per
  square metre (it can be further divided by the speed of light to get a
  true volume concentration). It can be computed from a pair of fish-eye
  images (integrating the luminance over the full space angle). Seldom
  used for light. }

 \item[emissions ]{ -- amount of pollutant released to the environment
 within a time interval by some source. For light expressed most easily
 as lumen seconds. On packaging of lamps (incandescent, fluorescent and
 other discharge ones), their nominal luminous flux (rate of emissions)
 in lumens is usually given. }
\end{description}

  For light as a pollutant, two additional properties are relevant: its
\textbf{direction} and \textbf{spectrum}. Complete characterisation of
rate of emissions (from any surface) or of pollutant influx (to any
irradiated volume element) is given by a quantity called
\href{http://en.wikipedia.org/wiki/Spectral\_radiance}{spectral
radiance}. It is a function of both direction and wavelength (or,
alternatively, wavenumber, frequency or photon energy). Watts per
steradian per square metre per nanometre
(W.sr\ensuremath{^{-1}}m\ensuremath{^{-2}}.nm\ensuremath{^{-1}}) belongs
to its common units. From this quantity, the parameter  (`how bright is
this spot') called
\href{http://en.wikipedia.org/wiki/Luminance}{luminance} can be
computed, integrating spectral radiance \ensuremath{\times} spectral
sensitivity of human daylight perception (photopic vision); its unit is
the candela per square metre.

  In general, any radiometric or photometric quantity can be employed as
  a measure of light pollution in some situation. For example, any
  \textbf{source of pollution} (e.g., a single luminaire) can be
  characterised integrally (as a whole) by specifying how large
  \href{http://en.wikipedia.org/wiki/Luminous\_intensity}{ luminous
  intensity} it emits in each direction. This is an adequate parameter
  for very distant sources appearing as points. However, if a visible
  source is so close that it does not appear as a point, larger kinds
  (like long fluorescent tubes instead of small high-intensity discharge
  burners) with the same luminous intensity may be less polluting, as
  their luminance is lower (the same luminous intensity is produced by a
  larger surface). 

 Amount of pollutant hitting some surface is a useful integral measure
 in case of light. Per second and square metre, it it is measured
 directly by a luxmeter; the quantity is then 
 \href{http://en.wikipedia.org/wiki/Illuminance}{illuminance}, its unit
 is lux. The orientation of the surface element causes illuminance to
 differ from directionless pollutant concentration  (luminance is
 integrated just over the proper  half of the full space angle and
 multiplied by the cosine of the
 \href{http://en.wikipedia.org/wiki/Angle\_of\_incidence}{angle of
 incidence} to the  surface element  -- then light tangential to the
 surface makes no contribution to its illuminance). Per whole area and
 time interval,  it can be expressed in lumen seconds. 

 \subsubsection*{-- relatively}
 \addcontentsline{toc}{subsubsection}{-- relatively}

 Light is always present naturally. Because of this, a
 convenient way of expressing light pollution by giving the ratio is:

  $$\textit{man-made part of any photometric or radiometric quantity} 
  \over
  \textit{natural part of the same quantity} $$

  In most cases, it is preferable to express pollution this way: as a
dimensionless number rather than by photometric units. An amount of
light which causes serious pollution at night may be negligible at noon.
Even when there is a sharp boundary between the polluted and unpolluted
part of a visual scene, people do not notice the pollution if the
artificial contribution to the luminance of the polluted spot is below
one per cent of its natural luminance (assuming it holds true even for 
`\myref{Bfootnote}{\mylabel{blueLuminance}{blue luminance}}'). In many
cases, even a ten per cent increase of luminance over the natural value
may not be noticed (if there is no sharp boundary between areas of
differing luminance) -- because of this, pollution which is less than
10~\% of natural light level is often considered as insignificant.

  For example, there is concern about the man-made increase of light in
clear night skies, because of the loss of visibility of the stars (a
typical reported quantity is sky   luminance in zenith, or the man-made
increase of that luminance   divided by natural luminance).  Relative
pollution may be however much higher under an overcast   sky,  implying a
huge disruption of the natural environment. 

 Let us compute this. Under an overcast moonless night sky, natural
 amounts of light are about ten times less than under a clear sky. If the
 absolute amount of artificially added light is the same in both
 cases, in relative terms it would therefore cause ten times more
 pollution under an overcast sky, locally. However, far from emission
 sources, even the intruding absolute light amounts can rise, as the
 light cannot escape to space and is reflected back to the ground by
 clouds. This can be further amplified by snow. Then the same emissions,
 which cause \emph{increase} of incident light amounts say by 100~\%
 under a clear sky (meaning that the man-made and natural parts are about the
 same, pollution is 1 in relative terms, very \emph{significant and
 conspicuous}), cause relative pollution at least a hundred times
 greater than under a clear sky! (In the remote areas of the largest Czech
 national park the relative concentration of the pollutant (light) under
 an overcast sky was three hundred times larger than under clear sky, as
 \href{http://amper.ped.muni.cz/noc/krnap}{measured in winter 2005}.)



  The \emph{radiometric or photometric quantity can be itself a ratio}, 
  a number with no unit, like
  \href{http://en.wikipedia.org/wiki/Contrast\_\%28formula\%29}{contrast}.
  Then the pollution by man-made light can be expressed even as a 
  \emph{decrease} of a quantity from its natural value. It is sufficient
  to say that \emph{stars get dimmer due to light pollution}: it means
  their contrast to the surrounding sky becomes lower, when the
  luminance of that sky is increased due to man-made light. To compute
  it exactly, each star can be ascribed an element of a space angle,
  which is perceived as a point by our vision (depending on acuity, sky
  luminance and brightness of the star,  it can be a circle with a
  diameter of 1\ensuremath{^\prime} to 5\ensuremath{^\prime} for young
  people with good eyesight). So we can speak about the luminance of a star,
  like the luminance of any target which we aim to see. Then the
  (Weber's) contrast of target is 

  $$\textit{luminance of the target} - \textit{luminance of the background} 
  \over
  \textit{luminance of the background}
  $$

 \subsubsection*{-- doubly relatively}
 \addcontentsline{toc}{subsubsection}{-- doubly relatively}

 \emph{Ratio of contrasts in polluted and unpolluted situation} is the
 best measure of pollution, as regards the visibility of faint lights. They
 include stars and another celestial phenomena, fireflies and glow-worms,
 or plentiful faint sparkles of light
 (\href{http://en.wikipedia.org/wiki/Bioluminescent}{bioluminescent} 
 plankton) which are so wonderful in the sea, but mostly unknown to
 populations in areas where strong artificial lighting is ubiquitous. 

  It is not just a matter of their visibility for people. In the case of
  luminous insects, they fail to recognise each other over long
  distances in polluted environments,  do not find mates, cease to
  reproduce and  their populations eventually collapse  (unfortunately,
  a common occurrence in towns and cities). For marine life, the
  ecosystem consequences of  loss of visibility of bioluminescent
  signals on moonless or overcast nights are not yet understood, but can
  hardly be negligible. 

  Reduction of contrast can wipe out whole terrestrial panoramas, even the very outlines of giant mountains  (\myref{studnicni}{[3]}).

 \subsubsection*{-- with special regard to the physiology of vision}
 \addcontentsline{toc}{subsubsection}{-- with special regard to the physiology of vision}

  Vision poorly registers absolute levels; due to
  \href{http://en.wikipedia.org/wiki/Adaptation\_\%28eye\%29}{adaptation}, 
  it adjusts itself to the prevailing light levels. For example, stars are
  perceived as dimmer if some other, additional light comes to our eyes.
  This added light may come from spots of high luminance, e.g.\ from
  luminaires, windows of lit rooms, vehicle lights and outdoor surfaces
  purposely illuminated. Stars become invisible in urbanised
  environments not just because of increased sky luminance, but also
  because of glare (including veiling luminance produced by the dispersion
  of light inside the eye) and mostly because of changed adaptation of
  vision due to increased light levels. In fact, the sky may be
  perceived as black or very dark from such heavily polluted sites,
  unlike in nature where vision adapts to its luminance during twilight
  and night: natural clear night sky is never dark between the stars,
  being the main source of illumination of the terrestrial landscape. 

  So, light pollution not only diminishes contrasts, but due to animal
  and human vision adapting to artificially increased light
  levels, light pollution \emph{reduces the number of photons registered
  on the retina} from natural sources. Physiologically, such sources
  \emph{become fainter} not just in relative, but even in absolute
  terms. This further reduces their visibility, as more contrast is
  needed if they are to be noticed (contrast sensitivity is worse at the
  bottom end of the span of perceived luminances than at its middle).



 \subsection*{Where the direction or a spectrum plays a role}
 \addcontentsline{toc}{subsection}{Where the direction or a spectrum plays a role}

 With closed eyelids, we barely perceive the direction of the incoming
 light, but it is still important: if it comes from one side only, we
 can turn towards the other side 
 (thereby reducing the illuminance of the face).
 Similarly, trees affected by acid deposits brought by a wind coming
 from the west are less affected on their east-facing sides. 
 With open eyes however,
 we notice even tiny spots with increased luminance. Any artificially
 lit terrain on a distant slope spoils the natural appearance of the
 scene, any directly visible light becomes a conspicuous detail of it (and
 very prominent pollution), even if it contributes just a tiny
 fraction of total light input to our eyes. Directly visible lights are
 also the most harmful for wildlife, security (through an effect called
 glare) and aesthetics, or even the very visibility of the true
 landscape including the sky.

We should take into account not just the mere (photopic) amount of
light, but also its spectral composition (perceived often as a colour by
us and many other  organisms). For example, pure blue light (with no
green or red component) contributes little to the usual photopic
quantities, as  expressed in lumens or candelas, but can still be
conspicuous (altering, i.e.\ polluting the natural scenery). Moreover,
such shortest-wavelength  light is the signal to us, and to other
animals, that it is day or night, and it controls the animal's
metabolism. An established way of somehow taking into account the
spectral composition of light is by also reporting its  `scotopic
luminous flux' (it is relevant for deep-night vision, when photopic
luminances do not exceed  1~mcd/m\ensuremath{^{-2}}), computed using a
spectral sensitivity function which culminates at 500~nm instead of at
555~nm as for photopic vision. In analogy, `renormalised red, green and  
\myref{Bfootnote}{\mylabel{blueLumens}{luminous flux}}' or luminous 
intensities might be introduced. Renormalisation might consist in
considering them to be all equal for summer daylight (they can be
measured easily by using RGB digital imaging; `B luminous flux' is a good
measure of metabolism-relevant light \myref{B}{[5]}).

  If we need to light something at night to see its details, we can
  suppress the blue component (or ideally the whole short-wave half of
  the light spectrum) to safeguard our health and reduce the harmful impact
  of artificial lighting to wildlife. Avoiding ultraviolet is important
  too, to protect wildlife. Low-pressure sodium lighting is an old
  (outdoor) example; yellow light indoors (blue being mostly filtered out
  by a yellow filter gel or transparent glass paint) is very comfortable
  (the author's family has had such a separate lighting system for evenings and
  nights for four years now). For occasional blue-demanding tasks such as
  proofs of colour magazines, a small non-filtered spot light may be
  used for a limited time  (just for a central part of the field of
  view). Some caution is needed  with very blue-deficient lighting if
  people with certain types of
  \href{http://en.wikipedia.org/wiki/Color-blindness}{colour blindness} 
  use it.

  Imposing obligatory geometric and spectral limits on artificial  
lighting can reduce some relevant measures of pollution (and its  
harmful consequences) by orders of magnitude, even if the total  
emissions (measured as photopic luminous flux) remain similar to before. This
makes light easier to handle than most other pollutants. Of course,
even the growth of total emissions should be halted and reversed
towards a steady decline, to get pollution levels back to values which
may be tolerable, taking into consideration all adverse effects of
lighting (for light, those of the late 19th century might be  
almost surely regarded as sustainable, while an interim goal might be a  
decline to the levels of 1970).

 \subsection*{References and further links}
 \addcontentsline{toc}{subsection}{References and further links}

 \begin{enumerate} 
 
 \item{} \mylabel{mnras}{\emph{`alteration of natural light levels in
 the outdoor environment owing to artificial light sources'}} -- 
 the first published scientific definition, as it appeared in: Cinzano et
 al. 2000, \emph{Monthly Notices of Royal Astron. Soc.}, 318, 64\\
 (online:
 \href{http://www.lightpollution.it/cinzano/download/mnras\_paper.pdf}{
 The artificial night sky brightness mapped from DMSP Operational
 Linescan System measurements}, 1MB pdf).
 
 \item{}
 \mylabel{scotobiology}{
 \href{http://muskokaheritage.org/ecology-night/scotobiology.asp}{definition
 of Scotobiology}} see at 2003
\emph{Ecology Of The
 Night} conference pages: \\
 \href{http://muskokaheritage.org/ecology-night}{http://muskokaheritage.org/ecology-night}

 \item{} \mylabel{studnicni}{ Brychtová J, Hollan J, Krause J:}  \emph{Evaluation of the influence of artificial illumination of selected ski resorts to nature and landscape of Giant Mts. Nat. Park.} July 2005,  in Czech. Available at \href{http://amper.ped.muni.cz/noc/krnap/}{http://amper.ped.muni.cz/noc/krnap} (some English excerpts are within a  \href{http://amper.ped.muni.cz/light/ida/2005report.html\#SECTION00042000000000000000}{2006 report for IDA}).



 \item{} \mylabel{photometry}{ Stockman} A, Sharpe LT, 2000: Spectral sensitivities of the middle- and long-wavelength sensitive cones derived from measurements in observers of known genotype. \emph{Vision Research}, \textbf{40}, 1711--1737. The photopic sensitivity data, together with the standard CIE  scotopic data, can be downloaded at  \href{http://cvision.ucsd.edu/}{http://cvision.ucsd.edu}.

 \item{} \mylabel{B}{ Hollan J}: \emph{Metabolism-influencing light:
 measurement by digital cameras}. Poster at Cancer and Rhythm
 conference, Graz 2004. \\ Online as
 \href{http://amper/noc/english/canc\_rhythm/g\_camer.pdf}
 {http://amper/noc/english/canc\_rhythm/g\_camer.pdf}.

 \item{} \href{http://savethenight.eu/Home.html}{Save the Night in
 Europe}, a comprehensive source aimed not just at clear sky
 luminance. \href{http://savethenight.eu}{http://savethenight.eu}

 \item{} \href{http://urbanwildlands.org}{www.urbanwildlands.org}, a
 site of the Ecological Consequences of Artificial Night Lighting
 project (contains links to an excellent article, Ecological Light
 Pollution, and to conference proceedings).

 \item{} Clark B.A.J:  \emph{A Rationale for the Mandatory Limitation of
 Outdoor Lighting}. Document Version 2.4, 29 February 2008. Online as
 \href{http://amper.ped.muni.cz/bajc/lp181.pdf}
 {http://amper.ped.muni.cz/bajc/lp181.pdf}.

 \item{}
Hollan J: \emph{Digital imaging photometry with common cameras --
results, methods and  perspectives}. Lecture slides
 for a meeting of
 \href{http://www.ctio.noao.edu/light\_pollution/iau50/}{ IAU commission
 50}, Prague, August 2006. Online as 
 \href{http://amper.ped.muni.cz/light/lectures/06IAU50t\_small.pdf}
{http://amper.ped.muni.cz/light/lectures/06IAU50t\_small.pdf} (8 MB).

 \item{} \mylabel{whatis}{Cinzano P}: \emph{What is light pollution}. 
Online as
 \href{http://www.savethenight.eu/What\%20is.html}
{http://www.savethenight.eu/What\%20is.html}.
Retrieved on July 10, 2008.

 \end{enumerate}


 \subsection*{Changes:} 
 \addcontentsline{toc}{subsection}{Changes} 

 \begin{enumerate}

 \item{}The term \emph{immissions} has been used in the first version of
the text. This is however not common in English, unlike in many other 
European languages (even if 
\href{http://iaspub.epa.gov/trs/trs\_proc\_qry.alphabet?p\_term\_nm=I\&p\_reg\_auth\_id=1\&p\_data\_id=11605\&p\_version=1}{EPA
lists it}), perhaps due to the problem that it could not be distinguished
from \emph{emissions} in English pronunciation, as the beginning
\emph{e}  is read as \emph{i} there.  This  is, fortunately, not the
case for a similar pair of terms  emigrate - immigrate.

 \item{}\emph{Alteration} instead of Increase used in the definition, 
 to be fully consistent with general definition of any pollution.  Of
 course, in the case of added light (and most other pollutants) the
 alteration of \emph{absolute} levels is mostly \emph{increase}. 
 Generally, decreases from natural absolute levels of occurrence can
 result from reactions of primary pollutants with the environment (e.g.,
 daylight can be diminished by emissions of  sulphur oxides, due to
 induced decreased transparency of the air,  but this is no light
 pollution, but say a sulphate and aerosol one). However, light
 pollution leads to an absolute decrease of signal registered by the eyes from 
 faint lights at night, due to visual adaptation.  Very
 conspicuously, it can cause a \emph{decrease} of the 
 quantity important for visibility: of 
\href{http://en.wikipedia.org/wiki/Contrast\_\%28vision\%29}{\emph{contrast}}.

 \item{}\emph{Non-visual photometry} and adverse effects footnotes added,  double relative (ratio of contrasts) pollution measure included, examples of  a decrease of quantities (contrast and number of registered photons) from their natural levels due to light pollution added 
in January 2007

 \item{}\emph{Indoor light pollution} definitions included in July 
2008, 
an introduction on pollution in general and a footnote on man-made sources of 
light.
The footnote of indoor light has been shortened.

 \item{}\emph{Pollutants} footnote added in August 2008.

 \item{}\emph{Acknowledgements:} Major language correction made by Bob Mizon
 on September 15, 2008.  Michael Tabb and Christopher Baddiley read the text
as well. I am very  thankful to these
\href{http://www.britastro.org/dark-skies}{CfDS} personalities. My
thanks are also due to Barry Clark, Steve Willner and Pierantonio
Cinzano for advice concerning the early 2006 and 2007 drafts of the
text. Mat\v{e}j Hollan helped me with clarifying various meanings of
`pollution' in August 2008. None of them is responsible for 
the contents of the present paper of course.

 \item{}\emph{Photometry:} The bad jargon of having various 
lumens etc. has been replaced by having various quantities (depending 
on spectral luminous efficiency functions and on the normalisation) 
but just one standard set of units (October 2008).


 \end{enumerate}



   \hrulefill{}

 \subsection*{Footnotes}
 \addcontentsline{toc}{subsection}{Footnotes}


 \subsubsection*{\mylabel{manmade}{\myref{man-made}{1 On man-made sources of light}}}
 \addcontentsline{toc}{subsubsection}{On man-made sources of light}
 
 Predominantly, they are electricity-related sources, i.e.\ various 
kinds of lamps in developed
countries. Flame sources may prevail in non-electrified regions.
 However, in principle, even strong, conspicuous secondary sources of sunlight
 might be included, as sun reflections in car windscreens or in
 tilted windows of buildings. If perceived as glaring lights, considering them
 as causes of light pollution might be more valid than including them just
 in the broad category of
\href{http://en.wikipedia.org/wiki/Visual\_pollution}{visual pollution}
(containing three-dimensional artifacts altering the desired natural or
cultural landscape).
  
 \subsubsection*{\mylabel{indoors}{\myref{outdoors}{2 On light added indoors}}} 
 \addcontentsline{toc}{subsubsection}{On light added indoors}

 There is no doubt that indoor light is toxic in
 some cases, causing sleep and metabolism disturbances.  In this
 sense, common light levels produced by today's artificial  (electric)
 lighting have to be considered a significant pollution of indoor
 \emph{night} air,  similar to tobacco smoke at \emph{any time}. Keeping 
 \myref{Bfootnote}{B illuminance} of eyes below 1 lx during the time
 when we do not sleep at night is to be recommended; keeping it below
 0.25~lx can be considered a \emph{safe environmental  limit} for 
humans
 (this is the natural night-time outdoor maximum which can be
 tolerated by most organisms due to eons of evolution; some rodents  are
 still a hundred times more sensitive, probably due to the  fact they
 inhabit dark niches).

 A special kind of indoor light pollution might arise in a situation when  
 even the natural outdoor sources of light compromise the health of people
 indoors, altering the desired indoor light levels. People needing to sleep
 in the daytime are an example, and glare coming from windows is an even more
 usual one, especially for people working at computer screens. 
 This is the reason why the `man-made' attribute was omitted in the latter 
 definition. 

 \subsubsection*{\mylabel{AdverseFoot}{\myref{adverse}{3 On adverse effects}}} 
 \addcontentsline{toc}{subsubsection}{On adverse effects}

 Light has been so popular throughout the human history and prehistory, that
 it is still seen to be heresy, even by some leading proponents of light
 pollution abatement, to regard all man-made light outdoors as
 pollution. There is no other way, however. We may approve of some cases of
 pollution (like the decent lighting of a yard where we are having a party), 
and we
 may well tolerate other's light (e.g.\ from a neighbour...), but pollution has its
 definition which does not depend on what people like or dislike at 
 any particular
 moment, and what is the polluting agent. When it's light, it's light.

  Was the application of DDT, broadly used outdoors in the fifties and
  sixties, a pollution? Surely it was, even though it was considered
  entirely beneficial in those times. Is a single LED in the middle of
  the Sahara a pollution? Surely it is, at least as environmental
  `litter': only stones and sand should be there, and some (hardly
  visible) wildlife, in a non-polluted state. A metal beer-bottle cap
  would qualify as pollution too, as well as the bottle itself, broken
  or not. At night, a shining (ordinary, not 70lm) LED would be visible
  not just from an immediate vicinity, as in daylight. Even from 1 km 
away,
  it could be as bright as the star Vega... surely tolerable, if it is 
meant to be
  a vital reference light, but still a pollution. The mere existence of
  pollution does not depend on whether anybody is watching or objecting.

 We can live with some pollution, just as we can live with beer, wine and
 brandy. We can enjoy their positives, without forgetting their toxic
 aspects. We should bring their adverse effects under our control. And as
 with brandy and children, or beer and bears, we should be aware there
 are organisms which are much more vulnerable to damage by some of those
 agents we love so much. For example, there are nocturnal frogs which never cross a
 continuously lit road: once lit, it becomes a barrier dividing the
 previously contiguous habitat.



 \subsubsection*{\mylabel{Bfootnote}{\myref{blueLuminance}{4 On blue luminance}}}
 \addcontentsline{toc}{subsubsection}{On blue luminance}

 Traditional photometry deals with human daylight (photopic) vision,
 with human deep-night (scotopic) vision, and attempts to quantify
 visual performance at intermediate light levels (mesopic vision). In
 SI, these photometric quantities are fixed to radiometric ones just at
 a single wavelength of 555~nm. The
 \href{http://physics.nist.gov/cuu/Units/current.html}{SI definition} is 

 \begin{quotation} The candela is the luminous intensity, in a given
 direction, of a source that emits monochromatic radiation of frequency
 540\ensuremath{\times}10\ensuremath{^{12}} hertz and that has a radiant
 intensity in that direction of 1/683 watt per steradian.
 \end{quotation} 

 For light in the real world, which contains electromagnetic radiation of
 various wavelengths, there are but approximations of correspondence 
between 
 amounts perceived by human vision (true photometry) and those
 measured by instruments. This was established by investigations resulting
 in various
 \href{http://en.wikipedia.org/wiki/Luminosity\_function}{spectral 
luminous efficiency functions} of the two visual systems in humans \myref{photometry}{[4]}.
 The author of the present text has proposed a different approach, enabling
 us to introduce many analogues of these `luminosity functions'. One of them
 is the \href{http://en.wikipedia.org/wiki/Luminosity\_function}{action 
 spectrum} of the non-imaging visual system (a 
 \href{http://en.wikipedia.org/wiki/Melanopsin}{third one in humans})
 affecting melatonin production \myref{B}{[5]}. 


\noindent\centerline{\includegraphics[width=\textwidth]{16spTN}}

  If the particular action spectrum has at 555~nm, a value whose relative  
uncertainty is considerably larger than that at the maximum of the  
spectral curve, setting its fixed point just at  555~nm would lead to  
large uncertainty of the so-defined luminous flux etc. In some cases, the  
whole spectrum may be poorly known. Still, responses of a sensing  
system (animal or instrumental) having such a poorly known spectral 
sensitivity may be recalibrated by  
setting them to be the same as (easy to measure) photopic quantities for
some well known light spectrum taken as a reference. The author
proposes \myref{B}{[5]}   natural sunlight for that. On example of a
digital camera with R, G   and B pixels:  \emph{`By setting the B
luminance to a photopic one (measured by a luxmeter, or computed
astronomically), the camera is simply calibrated.'} To confine the
choice of a reference light composition still more, AM1.5 global
sunlight might be taken as a standard. Repeatability at the level of one
per cent could be achieved this way. 

  A similar system exists in astronomical photometry. It stems from true  
visual photometry, having been extended to photographic   light-sensing
tools, and finally to electric devices. All filter-based   measurements
(even the detector itself behaves as a filter, responding   differently
to different wavelengths) are calibrated on stars of a   spectral class
A0V: the reported values for any such star (Vega is used   as the
primary standard) should always be the same, regardless of  whether the  
detector records a portion of UV, visual or infrared domain. A0 stars  
are good standards for stellar photometry, but sunlight is easier to  
use for terrestrial multicolour photometry. Astronomical photometry  
results are mostly expressed as dimensionless logarithmic quantities  
(with a unit called magnitude, a difference of 2.5~mag corresponds to 10  
decibels in acoustic analogy, i.e.\ to a ratio of  luminous flux  
densities of 10; a star having just 0~mag gives a luminous flux density  
of 2.56~\ensuremath{\mu}lm/m\ensuremath{^{2}}), terrestrial photometry  
should use photometric units like lux to   make the reported values easy 
to understand. All photometry   concerning light-containing wavelengths
usual in nature  (daylight, airglow, moonlight)   could and should
become an analogue of  photometry based on human   photopic vision.
Instead of the proper SI radiometric normalisation at    
540\ensuremath{\times}10\ensuremath{^{12}}~Hz, which would result at 
vary different values for various spectral  luminous efficiency
functions applied to common light sources, photometric quantities
renormalised to AM1.5 clear sky global light might be recommended for
most purposes. For scotopic ones, the renormalisation would result in
dividing the SI values by 2.5. For another spectral  luminous efficiency
functions, such a coefficient may be still less well known. In spite of
that, properly renormalised photometric values can be taken  seriously. 

 \subsubsection*{\mylabel{Pfootnote}{\myref{pollutant}{5 On pollutants}}}
 \addcontentsline{toc}{subsubsection}{On pollutants}
 
 In a typical technical sense, pollution is due to a pollutant, some 
microscopic agent dispersed in the environment. The German word Schmutz 
(meaning dirt) indicates this view well, as well as the term 
Verschmutzung for pollution, not being charged with religious overtones.
However, macroscopic objects may represent pollution too. A common case 
is litter. Sometimes it may just spoil the beauty of the environment.
Sometimes it can even be fatal to animals, as in the case of plastic bags in the sea.
Even objects regarded as necessary or welcome by some may be regarded as 
spoiling or polluting the environment, or more precisely, our way of 
experiencing it -- power lines, roads,  billboards, cars, etc. See 
\href{http://en.wikipedia.org/wiki/Visual\_pollution}
{http://en.wikipedia.org/wiki/Visual\_pollution}. Visible non-natural
lights and lit objects at night belong to this category too, changing the
landscape profoundly. 

This brings us back towards the religious context: the world is damaged
by a presence of unwanted man-made phenomena. Night-time darkness can be
easily considered sacred, together  with its celestial lights. An
extreme notion of pollution, which has to be respected too, is purely
religious: something (permanently) polluted due to the mere past
presence of an intruding factor, for example a man stepping on top of a
sacred mountain. The  purity can eventually be restored by a proper
ritual...

I know of no such attitude to light. Pollution by light  can be cleaned
up without any such ceremony, simply by switching the light sources off. Still,
the consequences of past light pollution may  persist for decades: like
absent darkness-dependent animals, or people with a childhood
experience of starry heavens -- and therefore an interest in this 
phenomenon.
Subjecting whole populations to growing and living without the night,
without the heavens, is the largest social experiment ever.  Light
at night may be a far more serious pollutant than many others...


\end{document}
