9 light pollution 2010 - University of Hong Kongwork6002/09 light pollution 2010...LIGHT POLLUTION The function of all outdoor lighting is to enhance the visibility or the aesthetics

UNIVERSITY OF HONG KONG Department of Electrical & Electronic Engineering

M.Sc.(Eng) in Building Services Engineering MEBS6002 2010 Lighting Engineering

K.F. Chan (Mr.) Page 1 of 22 Jan., 2010 All rights reserved

LIGHT POLLUTION

The function of all outdoor lighting is to enhance the visibility or the aesthetics in the

night-time environment. The light should go to where it is needed. If not, it is

‘spilled’, causing light pollution.

The National Park Service defined light pollution as any illumination of the night sky

caused obtrusive artificial light, whereas the International Dark Sky Association refers

light pollution to any “adverse effect of artificial light including sky glow, glare, light

trespass, light clutter, decreased visibility at night and energy wastage”

Besides light trespasses, a major form of light pollution is the glow extending over the

night sky, usually called sky glow. Sky glow presents itself as a background

luminance over the sky, against which the astronomical objects are to be observed.

The interference of astronomical observations is caused by the resulting reduction of

their luminance contrast.

One of the earliest legislation regarding light pollution was the ‘Lombardy Law’,

which was enacted in the Lombardy region of Italy after 25,000 citizens signed

petitions demanding action against obtrusive outdoor lighting. In early 2002, the

Czech Republic became the first country to introduce national legislation that includes

measures for the control of light pollution. Known as the “Protection of the

Atmosphere Act”, the law defines light pollution as “every form of illumination by

artificial light which is dispersed outside the areas it is dedicated to, particularly if

directed above the level of the horizon”. The Czech public is expected to employ full

cut off luminaires that emit no light above the horizontal direction.




Forms of light pollution

Light pollution shows itself in several different ways: -

1. Direct light entering the premises - Usually this is called light intrusion or light

trespass. Residents suffer most when the light invades their private life.

This is also called light immission. In Hong Kong, there are increasing reports

by the media and complaints received by the Environmental Protection

Department about lives of people living in the immediate vicinity of neon

signboard and poorly designed and installed floodlights – they were annoyed

by the excessive illumination.

2. Sky glow – this is the brightening of the night sky, or glow extending over the

night sky. It presents itself as a background luminance over the sky, against

which the astronomical objects are to be observed. Sky glow is the major

obstruction for optical astronomy. Obtrusive light can come from light emitted

by luminaires above the horizontal, but it can also come from light first

‘The Sporting Life’ by Patricia HUNT

LD+A August 2005 “The Sporting Life” by Patricia HUNT

LD+A August 2005




emitted by luminaires downwards then reflected upwards by the surfaces, both

intentionally or unintentionally, illuminated.

3. Horizon pollution. A major part of the light spilled by outdoor lighting is

emitted more or less straight upward. This light is scattered in the

atmosphere, and forms a haze over the light source. It is particularly

conspicuous when the light stems from large, concentrated sources like city

centers, airports, sports stadia, or industrial premises. From a distance, the

scattered light looks like a ‘light blob’ directly over the source, easily visible

from a great distance – up to 200 km or more. As the blob is the result of the

light emitted almost vertically upwards and scattered in the atmosphere, it

cannot be higher than the thickness of the atmosphere. If we take that as

being about 8km, above which the atmospheric pressure and the amount of

aerosols are too low to cause much scatter, the elevation of the top of the blob

will be at 50 km distance about 9°, and about 4.5° at 100 km. Because few

astronomical observations are made at elevations smaller than some 15°, the

blob represents ‘horizon pollution’ but it is no threat to astronomical

observations.

4. Light clutter – groups of strong light positioned together will cause confusion,

distract the viewers from obstacles and potentially contribute to traffic

accidents. This is particularly noticeable on roads where the street lights and

signs are interfered by excessively lit advertising signboards.

5. Glare – improperly positioned and designed road lights will distract and even

blind drivers or pedestrians and may contribute to accidents. In astronomy

direct light mainly refers to light shining directly into the telescope, thus

causing glare.

6. Decreased visibility at night – a major concern about light pollution is the

decrease of number of stars visible in the night sky.

7. Energy wastage – poorly designed lighting results in only a small portion of

the lamp flux generated is directed to where it is intended with a major portion

directed upwards. This not only causes light pollution but also wastes energy

and indirectly increases air pollution and global warming because of the extra




electricity consumption incurred.

Conspicuity of point sources

There are two further points that deserve some attention when discussing light

pollution. The first is the visibility of point light sources, and more in particular

the conspicuity of far-away lighting equipment. The second is what is

sometimes called the flat-glass controversy.

First the detection of point sources. The colloquial description of a point source

is simply a source that is so small that no extension can be seen. Stars are point

sources. However, the retinal image is not a point. As a result of diffraction, it

measures about 0.002mm, corresponding to about 0.2 minute of arc.

Many people, astronomers and naturalists alike, complain that distant luminaires

are conspicuous and that they cause horizon pollution. This effect is most

noticeable at a distance between some 3 and 10 km. At such distances, the direct

light from the outdoor lighting luminaires is clearly visible and may cause

considerable disturbance.




CIE limits

In 1997 CIE published a report on the different aspects of sky glow.

The first general recommendation refers to zoning. The idea is that light pollution

does not cause the same amount of disturbance at all places. More in particular, in

brightly lit city centers or industrial estates, the requirements regarding the limits of

sky glow can be lower than in a Natural Park or in a Protected Landscape. The

British IES introduced in the 1990s a system of four zones. This system is adopted

by and subsequently used by a number of national organizations. The zones are

summarized in the below table.

Zone Surroundings Lighting environment Examples

E1 Natural Intrinsically dark National parks or

protected sites

E2 Rural Low district brightness Agricultural or

residential rural areas

E3 Suburban Medium district brightness Industrial or residential

suburbs

E4 Urban High district brightness Town centers and

commercial areas

Description of the environmental zones, adapted from CIE, 2003,table 2.1

The second general recommendation refers to the fact that light pollution does not

cause the same amount of disturbance at all times. A major part of the social

activities that did begin in the day will continue in the evening. Therefore the strict

requirements concerning the limits of sky glow that can be imposed in the middle of

the night cannot be used in the evening. For this, the term curfew is often used,

while introducing the evening regime and the night regime. The moment when the

evening regime ends and the night regime begins is mostly laid down in national or

regional regulations.




The third general recommendation refers to the upward light emission of outdoor

lighting installations. It has been mentioned earlier that CIE introduced the Upward

Light ratio ULR. This ratio refers only to individual luminaries. Being the ratio

between the upward flux and the lamp flux it is a relative value only. However it

does not give any information about what is the effect of a complete lighting

installation.

The fourth general recommendation refers to the colour of the light. It is generally

accepted that to use the monochromatic light of low-pressure sodium lamps is one of

the most effective ways to reduce light pollution as regards astronomical observations.

The reason is that it is not too difficult to filter out the one sodium spectral line.

Furthermore, they are the most efficient light sources available at present.




Remedial measures

a. Switching off the lights – time control

The most obvious solution to reduce light pollution is of course to switch off

lights when they are not needed. However, this cannot be done in full as other

equally important aspects of our social life require night-time activities and

consequently artificial light at night. Nevertheless, a fair number of light sources

can be switched off without any damage. Furthermore, in many cases light sources

can be replaced by smaller sources emitting less light. An important way to

improve the situation is to guarantee that most lights, particularly those that

contribute most to light pollution like sports stadium lighting, are not used after a

certain time. Many overseas laws and ordinances impose time restrictions in the

use of such lighting installations. Modern electronic ballasts allow for a less

drastic solution, as almost all gas-discharge lamps can be equipped with a dimmer.

b. Limiting sky glow

Basically there are two ways to avoid or at least to reduce the light pollution, more

in particular the disturbance for astronomical observations :

- avoid the light by switching and gated viewing, or

- reduce the disturbance by light control and spectral selection.

Gated viewing

Electric lamps emit light essentially only when electric current is passed

through them. This means that (on a 50 Hz grid) lamps are extinguished

100 times per second. This gives the possibility to apply the principle of

gated viewing : the shutter of the photo apparatus at the telescope, or of its

electronic equivalent, is opened 100 times a second, and only during the

periods that the lamp does not emit light. In this way, the observer will not

notice the presence of the light.

However, gated viewing does not reduce the overall sky glow.

The principle of gated viewing is not new. However, in public lighting it

has not been put into practice at any considerable scale. The first aspect that

must be taken into account is the lamp type. Clearly, it is essential that

during the ‘off’ period the lamp should emit no light for the method to have

any appreciable effect. This automatically rules out incandescent lamps and

all lamps that use fluorescence. Many types of gas-discharge lamps can be




used, notably when they are equipped with dimmers that allow light emission

only in a small part of the period of the alternating current. These dimmers

are standard equipment, and can be used for most types of high-pressure

sodium lamps. More convenient are, of course, Light Emitting Diodes or

LEDs. A LED will emit light only when the voltage is appropriate.

The main problem for the application of the gated viewing system is that is

has not been tested on any large scale. It will cost some money, particularly

in the first stages of design and construction; it is not feasible in large cities

with existing elaborate outdoor lighting installations; it is not feasible in

places where are large part of the outdoor lighting is privately owned; and

finally, it does not reduce the overall sky glow. It seems, however, to be a

very promising system for small villages, that until now have no electric

lighting, and are near to major astronomical observatories. In this case, the

financial burden is light, and it can easily be carried out by those agencies

that promote clear, dark skies.

Light control

Light control means, in simple terms, preventing light being emitted above

the horizon; more precisely, it means that the light is directed to the objects to

be illuminated, downwards, ways include:




a. Precise photometric control – sharp cut off preventing spills and direct

flux to only where it is needed.




‘Staying in Bounds’ by Nancy Clanton and Randy Burkett, LD+A August 2005

Bright gas station

Lower glare gas station



Low glare gas station





High glare parking lot


Good lighting in parking lot

Documents

9 light pollution 2010 - University of Hong Kongwork6002/09 light pollution 2010...LIGHT POLLUTION The function of all outdoor lighting is to enhance the visibility or the aesthetics