What is light pollution,
and how do we quantify it?
Jan Hollan,
N. Copernicus Observatory and Planetarium, Brno
December 2006, January 2007, July-October 2008, April 2009
(this pdf document with inline hyperlinks is available as http://amper.ped.muni.cz/light/lp what is.pdf)
Contents
Introduction to Pollution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Deﬁnition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
How it can be quantiﬁed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
– absolutely . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
– relatively . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
– doubly relatively . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
– with special regard to the physiology of vision . . . . . . . . . . . . . . . . . . 6
Where the direction or a spectrum plays a role . . . . . . . . . . . . . . . . . . . . . . 6
References and further links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
On man-made sources of light . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
On light added indoors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
On adverse eﬀects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
On blue luminance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
On pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Introduction to Pollution
Generally, pollution is an impairment of the purity of the environment [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 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 soundreducing
barrier represented by walls and windows against natural outdoor sounds – people
sleeping after night-work might rightly consider loudly singing birds as noise pollution of their
artiﬁcial bedroom environment, meant to remain silent even in the morning. Another example
1
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 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 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 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 nonpolluted
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 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 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 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
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be inappropriate to speak about CO2 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 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. Road lighting engineers
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 pollution as a
neutral term, assuming no ‘sin’ when unconsciously created, and no ‘horror’ if not very large
or damaging (see note 3).
Deﬁnition
– outdoors
(Outdoor) Light pollution is the alteration of light levels in the outdoor environment
(from those present naturally) due to man-made sources of light.
A peer-reviewed article (2000) where the deﬁnition was published with slightly diﬀerent
wording is [1]. Before that, contradictory concepts of light pollution had been common, limiting
it just to ‘adverse eﬀects’, ‘skyglow’ etc. This practice continues even now, unfortunately.
Some consequences of such pollution (by light and 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) [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.
– 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:
(Bedroom) Light pollution is the alteration of light levels in the room (from those
present naturally) due to man-made sources of light.
However, a signiﬁcant part of our indoor life includes use of artiﬁcial lighting. Then another
deﬁnition is to be applied:
Indoor light pollution is such alteration of light levels in the indoor environment
due to sources of light, which compromises human health
For health, the oﬃcial World Health Organisation deﬁnition is to be used: ‘Health is a state
of complete physical, mental and social well-being and not merely the absence of disease or
inﬁrmity.’ (see the Wikipedia’s item Health quoting WHO).
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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 lightpolluted
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 note 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.
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
note 2.
– absolutely
As for all pollutants, light added artiﬁcially to the environment can be measured by
concentration of the pollutant in some volume of the environment (of air or water). For
light, a convenient unit for this purpose is the 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.
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.
For light as a pollutant, two additional properties are relevant: its direction and 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 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−1
m−2
.nm−1
) belongs to
its common units. From this quantity, the parameter (‘how bright is this spot’) called luminance
can be computed, integrating spectral radiance × 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 source of pollution (e.g., a single luminaire)
can be characterised integrally (as a whole) by specifying how large 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
4
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
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 angle of incidence to the surface
element – e.g., light tangential to the surface makes no contribution to its illuminance). Per
whole area and time interval, it can be expressed in lumen seconds.
– relatively
Light is always present naturally. Because of this, a convenient way of expressing light pollution
by giving the ratio is:
relative light pollution =
man-made part of any photometric or radiometric quantity
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 ‘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
increase of incident light amounts say by 100 % under a clear sky (meaning that the manmade
and natural parts are about the same, pollution is 1 in relative terms, very signiﬁcant
and conspicuous), cause relative pollution at least a hundred times greater (10 000 %...) under
an overcast sky! Factor of 10 comes from the ten times lower natural light amounts due to
the clouds, another factor of 10 or more from reﬂection of man-made light from clouds, snow,
clouds... ‘chanelling’ that light eﬃciently even quite far from its source. An example: 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 measured
in winter 2005.
The radiometric or photometric quantity can be itself a ratio, a number with no unit, like
contrast. Then the pollution by man-made light can be expressed even as a decrease of a
quantity from its natural value. It is suﬃcient to say that 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
5
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 to 5 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
contrast =
luminance of the target − luminance of the background
luminance of the background
– doubly relatively
Ratio of two contrasts =
contrast polluted
contrast clean
which are determined for a polluted and an unpolluted situations, 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 (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 ([3]).
– with special regard to the physiology of vision
Vision poorly registers absolute levels; due to 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 reduces the number of photons
registered on the retina from natural sources. Physiologically, such sources 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).
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
6
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−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 blue 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 [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 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).
References and further links
1. ‘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,
Monthly Notices of Royal Astron. Soc., 318, 64
(online: The artiﬁcial night sky brightness mapped from DMSP Operational Linescan
System measurements, 1MB pdf).
2. deﬁnition of Scotobiology see at 2003 Ecology Of The Night conference pages:
http://muskokaheritage.org/ecology-night
3. Brychtov´a J, Hollan J, Krause J: 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 http://amper.ped.muni.cz/noc/krnap (some English excerpts are
within a 2006 report for IDA).
7
4. Stockman A, Sharpe LT, 2000: Spectral sensitivities of the middle- and long-wavelength
sensitive cones derived from measurements in observers of known genotype. Vision Research,
40, 1711–1737. The photopic sensitivity data, together with the standard CIE
scotopic data, can be downloaded at http://cvision.ucsd.edu.
5. Hollan J: Metabolism-inﬂuencing light: measurement by digital cameras. Poster at Cancer
and Rhythm conference, Graz 2004.
Online as http://amper/noc/english/canc rhythm/g camer.pdf.
6. Save the Night in Europe, a comprehensive source aimed not just at clear sky luminance.
http://savethenight.eu
7. 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).
8. Clark B.A.J: A Rationale for the Mandatory Limitation of Outdoor Lighting. Document
Version 2.4, 29 February 2008. Online as http://amper.ped.muni.cz/bajc/lp181.pdf.
9. Hollan J: Digital imaging photometry with common cameras – results, methods and perspectives.
Lecture slides for a meeting of IAU commission 50, Prague, August 2006.
Online as http://amper.ped.muni.cz/light/lectures/06IAU50t small.pdf (8 MB).
10. Cinzano P: What is light pollution. Online as http://www.savethenight.eu/What%20is.html.
Retrieved on July 10, 2008.
Changes:
1. The term 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 EPA lists it),
perhaps due to the problem that it could not be distinguished from emissions in English
pronunciation, as the beginning e is read as i there. This is, fortunately, not the case for
a similar pair of terms emigrate - immigrate.
2. 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 absolute levels is mostly 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 decrease of the quantity important for visibility: of contrast.
3. Non-visual photometry and adverse eﬀects notes 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
4. Indoor light pollution deﬁnitions included in July 2008, an introduction on pollution in
general and a note on man-made sources of light. The note of indoor light has been
shortened.
5. Pollutants note added in August 2008.
8
6. 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 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.
Matˇej 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.
7. 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).
8. Relative and Doubly relative pollution: Explanations clariﬁed; wrong/missing words replaced
on 3 other pages, all thanks to proposals by Jan-Herbert Damm (April 2009).
Notes
1 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
visual pollution (containing three-dimensional artifacts altering the desired natural or cultural
landscape).
2 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 night air, similar to tobacco smoke at
any time. Keeping 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 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.
3 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 manmade
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.
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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.
4 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 SI deﬁnition is
The candela is the luminous intensity, in a given direction, of a source that
emits monochromatic radiation of frequency 540×1012
hertz and that has a radiant
intensity in that direction of 1/683 watt per steradian.
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 spectral luminous eﬃciency functions of the two visual systems in humans
[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 action spectrum of the
non-imaging visual system (a third one in humans) aﬀecting melatonin production [5].
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0
0.2
0.4
0.6
0.8
1
400 450 500 550 600 650 700 750 800
count,flux,sensitivity:arbitraryunits
wavelength / 1 nm
Fuji S5000 spectral sensitivity
CCD colours: R
G
B
photopic
scotopic
"metabolic" (proposed)
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 [5] natural sunlight for that. On example
of a digital camera with R, G and B pixels: ‘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 µlm/m2
), 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 540×1012
Hz, which would result in very 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 pur-
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poses. 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.
5 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 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...
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