Editing Light pollution (section)

===Adjusting types of light sources===

Several different types of light sources exist, each having a variety of properties that determine their appropriateness for different tasks. Particularly notable characteristics are efficiency and spectral power distribution. It is often the case that inappropriate light sources have been selected for a task, either due to ignorance or because more appropriate lighting technology was unavailable at the time of installation. Therefore, poorly chosen light sources often contribute unnecessarily to light pollution and energy waste. By updating light sources appropriately, it is often possible to reduce energy use and pollutive effects while simultaneously improving efficiency and visibility.

Some types of light sources are listed in order of energy efficiency in the table below (figures are approximate maintained values), and include their visual skyglow impact, relative to LPS lighting.<ref>{{cite journal|last1=Luginbuhl|first1=C.|title=The impact of light source spectral power distribution on sky glow|journal=Journal of Quantitative Spectroscopy and Radiative Transfer|date=2014|volume=139|pages=21–26|doi=10.1016/j.jqsrt.2013.12.004|bibcode = 2014JQSRT.139...21L |doi-access=free}}</ref><ref>{{cite journal|last1=Aubé|first1=M.|last2=Roby|first2=J.|last3=Kocifaj|first3=M.|title=Evaluating Potential Spectral Impacts of Various Artificial Lights on Melatonin Suppression, Photosynthesis, and Star Visibility|journal=PLOS ONE|date=2013|doi=10.1371/journal.pone.0067798|pmid=23861808|pmc=3702543|volume=8|issue=7|pages=e67798|bibcode = 2013PLoSO...867798A |author1-link=:fr:Martin Aubé|doi-access=free}}</ref>

{| class="wikitable" style="width:60%;"
|-
!Type of light source
!Color
!Luminous efficiency<br />(in [[lumen (unit)|lumens]] per [[watt]])
!Sky glow impact<br />(relative to LPS)
|-
|[[LED street light]] (white)
| style="background:#fcfcff;"| warm-white to cool-white
|120
|4–8
|-
|[[Sodium vapor lamp#Low-pressure sodium|Low Pressure Sodium]] (LPS/SOX)
| style="background:#fff04f;"|yellow/amber
|110
|1.0
|-
|[[Sodium vapor lamp#High-pressure sodium|High Pressure Sodium]] (HPS/SON)
| style="background:#ffdc9f;"|pink/amber-white
|90
|2.4
|-
|[[Metal halide lamp|Metal Halide]]
| style="background:#fcfcff;"| warm-white to cool-white
|70
|4–8
|-
|[[Incandescent light bulb|Incandescent]]
| style="background:#ffffdf;"| yellow/white
|8–25
|1.1
|-
|PCA-LED
| style="background:#ffdc9f;"|amber
|
|2.4
|}

Many astronomers request that nearby communities use low-pressure sodium lights or amber [[Aluminium gallium indium phosphide]] LED as much as possible because the principal wavelength emitted is comparably easy to work around or in rare cases filter out.<ref>
{{cite conference
| title = Why Astronomy Needs Low-Pressure Sodium Lighting
| conference = IAU Symposium No. 196—Preserving the Astronomical Sky
| author = Luginbuhl, C.B.
| editor = Cohen, R. J.
| editor2 =Sullivan, W. T.
| issue = 196
| pages = 81–86
| publisher = PASP, San Francisco, USA
| date = 2001}}</ref> The low cost of operating sodium lights is another feature. In 1980, for example, [[San Jose, California]], replaced all street lamps with [[low pressure sodium lamp]]s, whose light is easier for nearby [[Lick Observatory]] to filter out. Similar programs are now in place in [[Arizona]] and [[Hawaii]]. Such yellow light sources also have significantly less visual [[skyglow]] impact,<ref name=fdsc-lslp>{{cite web|last1=Flagstaff Dark Skies Coalition|title=Lamp Spectrum and Light Pollution|url=http://www.flagstaffdarkskies.org/for-wonks/lamp-spectrum-light-pollution/|website=Lamp Spectrum and Light Pollution|access-date=10 April 2016|archive-date=23 September 2018|archive-url=https://web.archive.org/web/20180923164443/http://www.flagstaffdarkskies.org/for-wonks/lamp-spectrum-light-pollution/|url-status=live}}</ref> so reduce visual sky brightness and improve star visibility for everyone.

Disadvantages of low-pressure sodium lighting are that fixtures must usually be larger than competing fixtures, and that color cannot be distinguished, due to its emitting principally a single wavelength of light (see [[security lighting]]). Due to the substantial size of the lamp, particularly in higher wattages such as 135 W and 180 W, control of light emissions from low-pressure sodium luminaires is more difficult. For applications requiring more precise direction of light (such as narrow roadways) the native lamp efficacy advantage of this lamp type is decreased and may be entirely lost compared to [[sodium vapor lamp|high pressure sodium lamps]]. Allegations that this also leads to higher amounts of light pollution from luminaires running these lamps arise principally because of older luminaires with poor shielding, still widely in use in the UK and in some other locations. Modern low-pressure sodium fixtures with better optics and full shielding, and the decreased [[skyglow]] impacts of yellow light preserve the luminous efficacy advantage of low-pressure sodium and result in most cases is less energy consumption and less visible light pollution. Unfortunately, due to continued lack of accurate information,<ref>Section ''4.10 What Types of Lamps Are Used in Outdoor Lighting?'' in [http://www.nofs.navy.mil/about_NOFS/darksky/OLCHB1.14/lc-hb-v1-14.html ''Outdoor Lighting Code Handbook''] {{Webarchive|url=https://web.archive.org/web/20161212231714/http://www.nofs.navy.mil/about_NOFS/darksky/OLCHB1.14/lc-hb-v1-14.html |date=2016-12-12 }}. International Dark-Sky Association (2000)</ref> many lighting professionals continue to disparage low-pressure sodium, contributing to its decreased acceptance and specification in lighting standards and therefore its use. According to Narisada and Schrueder (2004), another disadvantage of low-pressure sodium lamps is that some research has found that many people find the characteristic yellow light to be less pleasing aesthetically, although they caution that this research isn't thorough enough to draw conclusions from.<ref>{{cite book |last1=Narisada |first1=Kohei |last2=Schreuder |first2=Duco |title=Light Pollution Handbook |date=2004 |publisher=Springer Science & Business Media |isbn=978-1-4020-2665-2 |page=605 |url=https://books.google.com/books?id=61B_RV3EdIcC&pg=PA605 |language=en |access-date=2023-03-19 |archive-date=2023-09-18 |archive-url=https://web.archive.org/web/20230918112521/https://books.google.com/books?id=61B_RV3EdIcC&pg=PA605 |url-status=live }} "When, however, people were asked whether they liked the lighting, the general opinion was that the low pressure sodium lighting was "ugly” or “eerie”. The size of the study does not, however, permit robust conclusions."</ref>

Because of the increased sensitivity of the human eye to blue and green wavelengths when viewing low-luminances (the [[Purkinje effect]]) in the night sky, different sources produce dramatically different amounts of visible [[skyglow]] from the same amount of light sent into the atmosphere. To reduce light pollution caused by blue light, it is necessary to adopt lamps that, while maintaining the same photopic luminous flux, produce minimal scotopic light, and at the same time establish specific restrictions on the emitted wavelengths, particularly by shifting the spectral flux towards the blue side of the scotopic band (below 440 nm), to protect star visibility and significantly reduce the impact of artificial night sky glow.<ref name=CIE2001>{{cite book |author=CIE |date=2001 |title=The CIE System of Physical Photometry Draft Standard 010.2/E}}
</ref>

The protected wavelength range, the "P-band", should focus between 440 and 540 nm to preserve star visibility and reduce light pollution in wavelengths harmful to scotopic vision, while using lamps that emit less light in this range can help minimize pollution without compromising star visibility and protect health and the environment.<ref name=Falchi2011articolo2>{{cite journal |last1=Falchi |first1=F. |date=2011 |title=Campaign of sky brightness and extinction measurements using a portable CCD camera |journal=Mon. Not. R. Astron. Soc. |volume=412 |issue=1 |pages=33–48|doi=10.1111/j.1365-2966.2010.17845.x |doi-access=free |bibcode=2011MNRAS.412...33F }}
</ref>