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-The '''spectral G-Index''' is a variable that was developed to quantify the amount of short [[wavelength]] [[light]] in a [[List_of_light_sources|visible light source]]. The smaller the G-index, the more [[blue]], [[violet]], or [[ultraviolet]] light a lamp emits. It is used in order to select outdoor [[lamps]] that minimize [[skyglow]] and [[ecological light pollution]]. The G-index was originally proposed by David Galadí Enríque, an astrophysicist at [[Calar Alto Observatory]].<ref name="Gdef">{{cite techreport |last1=Junte de Andalucia |title=Índice espectral G |date=2018 |url=http://www.juntadeandalucia.es/medioambiente/portal_web/web/temas_ambientales/atmosfera/contaminacion_luminica/5_iluminacion/spectralindexdescription_280518.pdf |accessdate=12 February 2019}}</ref><ref name="DGE">{{cite journal |last1=Galadí-Enríquez |first1=D. |title=Beyond CCT: The spectral index system as a tool for the objective, quantitative characterization of lamps |journal=Journal of Quantitative Spectroscopy and Radiative Transfer |date=February 2018 |volume=206 |pages=399–408 |doi=10.1016/j.jqsrt.2017.12.011}}</ref>
-== Definition ==
-The G-index is grounded in the system of [[Photometry (astronomy)|astronomical photometry]], and is defined as follows:
-:<math>
-G=-2.5\mathrm{log}_{10}
-\frac{\sum\limits _{\lambda=380 \mathrm{nm}} ^{500\mathrm{nm}} E(\lambda)}
-{\sum\limits _{\lambda=380 \mathrm{nm}} ^{780\mathrm{nm}} E(\lambda)V(\lambda)}
-</math>
-where
-* G is the spectral G-index;
-* ''λ'' is the wavelength in nanometers;
-* ''E'' is the [[spectral power distribution]] of the lamp;
-* ''V''(''λ'') is the [[luminosity function]]
-The sums are to be taken using a step size of 1 nm.<ref name="Gdef" /> For lamps with absolutely no emissions below 500 nm (e.g. Low Pressure Sodium or PC Amber LED), the G-index would in principle be undefined. In practice, measurement errors would mean that such lamps could be defined as having G greater than some value. The [[Regional Government of Andalusia]] has developed a [[spreadsheet]] to allow calculation of the G-index for any lamp for which the spectral power distribution is known (see External Links below).
-The G-index does not directly measure light pollution, but rather says something about the color of light coming from a lamp. If twice as many lamps are used, the G-index would not change, because it is a measure of fractional light, not total light. Similarly, the G-index is not related to the direction in which a lamp shines, so it is not directly related to skyglow.
-== Rationale ==
-The ongoing global switch from (mainly) orange high pressure sodium lamps for street lighting to (mainly) white LEDs has resulted in a shift towards broad spectrum light, with greater short wavelength (blue) emissions.<ref name="Davies">{{cite journal |last1=Davies |first1=Thomas W. |last2=Smyth |first2=Tim |title=Why artificial light at night should be a focus for global change research in the 21st century |journal=Global Change Biology |date=10 November 2017 |volume=24 |issue=3 |pages=872–882 |doi=10.1111/gcb.13927}}</ref> This switch is problematic from the perspective of increased astronomical and ecological light pollution. Short wavelength light is more likely to [[scattering|scatter]] in the [[atmosphere]], and therefore produces more artificial skyglow than an equivalent amount of longer wavelength light.<ref name="PNNL">{{cite techreport |last1=Kinzey |first1=Bruce |last2=Perrin |first2=Tess |last3=Miller |first3=Naomi |last4=Kocifaj |first4=Miroslav |last5=Aubé |first5=Martin |last6=Solano Lamphar |first6=Héctor |title=An Investigation of LED Street Lighting's Impact on Sky Glow |institution=Pacific Northwest National Lab |date=2017 |number= PNNL-26411 |url=https://www.osti.gov/biblio/1418092 |accessdate=12 February 2019}}</ref><ref name="Aube_2015">{{cite journal |last1=Aube |first1=M. |title=Physical behaviour of anthropogenic light propagation into the nocturnal environment |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |date=16 March 2015 |volume=370 |issue=1667 |pages=20140117–20140117 |doi=10.1098/rstb.2014.0117}}</ref><ref name="Luginbuhl">{{cite journal |last1=Luginbuhl |first1=Christian B. |last2=Boley |first2=Paul A. |last3=Davis |first3=Donald R. |title=The impact of light source spectral power distribution on sky glow |journal=Journal of Quantitative Spectroscopy and Radiative Transfer |date=May 2014 |volume=139 |pages=21–26 |doi=10.1016/j.jqsrt.2013.12.004}}</ref> Additionally, both broad spectrum light and short wavelength light tend to have a greater overall ecological impacts than narrow band and long wavelength visible light. <ref name="NPS_2016">{{cite techreport |last1=Longcore |first1=Travis |last2=Rich |first2=Catherine |last3=DelBusso |first3=Leigha |title=Artificial Night Lighting and Protected Lands / Ecological Effects and Management Approaches |date=2016 |number= NPS/NRSS/NSNS/NRR--2016/1213 |url=https://irma.nps.gov/DataStore/DownloadFile/582058 |accessdate=12 February 2019}}</ref><ref name="Longcore_2018">{{cite journal |last1=Longcore |first1=Travis |last2=Rodríguez |first2=Airam |last3=Witherington |first3=Blair |last4=Penniman |first4=Jay F. |last5=Herf |first5=Lorna |last6=Herf |first6=Michael |title=Rapid assessment of lamp spectrum to quantify ecological effects of light at night |journal=Journal of Experimental Zoology Part A: Ecological and Integrative Physiology |date=October 2018 |volume=329 |issue=8-9 |pages=511–521 |doi=10.1002/jez.2184}}</ref>  For this reason, lighting guidelines, recommendations, norms, and [[legislation]] frequently place limits on blue light emissions. For example, the "[[Light fixture|fixture]] seal of approval" program of the [[International Dark-Sky Association]] limits lights to have a [[Color Temperature|correlated color temperature]] (CCT) below 3000 [[Kelvin|K]], while the national [[France|French]] light pollution [[law]] restricts CCT to maximum 3000 K in most areas, and 2400 K or 2700 K in [[protected area|protected areas]] such as [[nature reserve|nature reserves]]. <ref name="IDA_FSA">{{cite web |title=Fixture Seal of Approval |url=https://www.darksky.org/our-work/lighting/lighting-for-industry/fsa/ |publisher=International Dark-Sky Association |accessdate=12 February 2019}}</ref><ref name="France_law">{{cite web |title=Arrêté du 27 décembre 2018 relatif à la prévention, à la réduction et à la limitation des nuisances lumineuses {{!}} Legifrance |url=https://www.legifrance.gouv.fr/eli/arrete/2018/12/27/TREP1831126A/jo/texte |website=www.legifrance.gouv.fr |accessdate=12 February 2019}}</ref>
-However, CCT is not perfectly correlated with blue light emissions; lamps with identical CCT can have quite different fractional blue light emissions.<ref name="DGE" /><ref name="JRC_GPP">{{cite techreport |last1=Donatello |first1=Shane |last2=Rodríguez Quintero |first2=Rocío |last3=Gama Caldas |first3=Miguel |last4=Wolf |first4=Oliver |last5=Van Tichelen |first5=Paul |last6=Van Hoof |first6=Veronique |last7=Geerken |first7=Theo |title=Revision of the EU Green Public Procurement Criteria for Road Lighting and traffic signals |number= EUR 29631 EN |institution=Joint Research Centre |date=2019 |url=http://publications.jrc.ec.europa.eu/repository/bitstream/JRC115406/jrc115406_eugpp_road_lighting_technical_report.pdf |accessdate=12 February 2019}}</ref> This is because CCT is based upon comparison to a [[blackbody]] light source, which is a poor approximation for [[LED|LEDs]] and [[gas-discharge lamp|vapor discharge lamps]] such as [[high pressure sodium]].<ref name="aube_2013">{{cite journal |last1=Aubé |first1=Martin |last2=Roby |first2=Johanne |last3=Kocifaj |first3=Miroslav |last4=Yamazaki |first4=Shin |title=Evaluating Potential Spectral Impacts of Various Artificial Lights on Melatonin Suppression, Photosynthesis, and Star Visibility |journal=PLoS ONE |date=5 July 2013 |volume=8 |issue=7 |pages=e67798 |doi=10.1371/journal.pone.0067798}}</ref> The G-index was therefore developed for use in decision making for the purchase of outdoor lamps and in lighting regulations as an improved alternative to the CCT metric.
-== Use ==
-In 2019, the [[European Commission|European Commission's]] [[Joint Research Centre]] incorporated the G-index into their guidelines for the [[Sustainable procurement|Green Public Procurement]]  of road lighting.<ref name="JRC_GPP" /> Specifically, in areas needing protection for astronomical or ecological reasons, they recommend the use of the G-index instead of CCT in making lighting decisions, because the G-index more accurately quantifies the amount of blue light.<ref name="JRC_GPP" /> In their "core criteria", they recommend that "in parks, gardens and areas considered by the procurer to be ecologically sensitive, the G-index shall be ≥1.5". In the case that G-index could for some reason not be calculated, they suggest that CCT≤3000 K is likely to satisfy this criteria. In the more strict "comprehensive criteria", they recommend that parks and ecologically sensitive areas or areas at specified distances from optical astronomy observatories have a G-index greater than or equal to 2.0. Again, in this case if calculating the G-index is not possible, CCT≤2700 K is suggested.
-The G-index is planned to be used by the Regional Government of Andalusia, specifically for the purpose of protecting the night sky.<ref name="Gdef" /> Depending on the "environmental zone", the regulation requires lighting to have a G value above 2, 1.5, or 1. In areas where astronomical activities are ongoing, it is expected that only monochromatic or quasi-monochromatic lamps will be used, with G>3.5 and in principle only emissions in the interval 585-605 nm.<ref name="Gdef" />
-== External links ==
-* [http://www.juntadeandalucia.es/medioambiente/site/portalweb/menuitem.7e1cf46ddf59bb227a9ebe205510e1ca/?vgnextoid=1278accde3f33610VgnVCM1000001325e50aRCRD&vgnextchannel=60172de6577ba410VgnVCM1000001325e50aRCRD Spreadsheet for calculating G-index], Regional Government of Andalusia (note: Spanish language page).
-* [http://susproc.jrc.ec.europa.eu/Street_lighting_and_Traffic_signs/documents.html Information on Green Public Procurement of Street Lighting in the EU], Joint Research Centre
-== References ==
-{{Reflist}}
-[[Category:Radiometry]]
-[[Category:Color]]
-[[Category:Lighting]]