Spectral G-index

For the author-level metric, seeg-index.

Thespectral G-Indexis a variable that was developed to quantify the amount of shortwavelengthlightin avisible light sourcerelative to its visible emission (it is a measure of the amount of blue light perlumen). The smaller the G-index, the moreblue,violet,orultravioletlight a lamp emits relative to its total output. It is used in order to select outdoorlampsthat minimizeskyglowandecological light pollution.The G-index was originally proposed byDavid Galadí Enríquez,anastrophysicistatCalar Alto Observatory.

Definition

Lamps with very different color temperatures will usually also have different G-indexes. Lamps with higher color temperatures have more blue light, and will therefore usually have lower G-index.

The G-index is grounded in the system ofastronomical photometry,and is defined as follows:

$G=-2.5\mathrm {log} _{10}{\frac {\sum \limits _{\lambda =380\mathrm {nm} }^{500\mathrm {nm} }E(\lambda )}{\sum \limits _{\lambda =380\mathrm {nm} }^{780\mathrm {nm} }E(\lambda )V(\lambda )}}$

where

• G is the spectral G-index;
• λis the wavelength in nanometers;
• Eis thespectral power distributionof the lamp;
• V(λ) is theluminosity function

The sums are to be taken using a step size of 1 nm.For lamps with absolutely noemissionsbelow 500 nm (e.g. Low Pressure Sodium or PC AmberLED), the G-index would in principle be undefined. In practice, such lamps would be reported as having G greater than some value, due to the limits of measurement precision. TheRegional Government of Andalusiahas developed aspreadsheetto allow calculation of the G-index for any lamp for which the spectral power distribution is known, and it can also be calculated in the "Astrocalc" softwareor the f.luxometer web app.

The G-index does not directly measure light pollution, but rather says something about the color of light coming from a lamp. For example, since the equation defining G-index is normalised to total flux, if twice as many lamps are used, the G-index would not change; it is a measure of fractional light, not total light. Similarly, the definition of G-index does not include the direction in which light shines, so it is not directly related to skyglow, which depends strongly on direction.

Rationale

The ongoing global switch from (mainly) orangehigh pressure sodiumlamps for street lighting to (mainly) white LEDs has resulted in a shift towards broad spectrum light, with greater short wavelength (blue) emissions.This switch is problematic from the perspective of increased astronomical and ecological light pollution. Short wavelength light is more likely toscatterin theatmosphere,and therefore produces more artificial skyglow than an equivalent amount of longer wavelength light.Additionally, both broad spectrum (white) light and short wavelength light tend to have greater overall ecological impacts than narrow band and long wavelength visible light.For this reason, lighting guidelines, recommendations, norms, andlegislationfrequently place limits on blue light emissions. For example, the "fixtureseal of approval "program of theInternational Dark-Sky Associationlimits lights to have acorrelated color temperature(CCT) below 3000K,while the nationalFrenchlight pollutionlawrestricts CCT to maximum 3000 K in most areas, and 2400 K or 2700 K inprotected areassuch asnature reserves.

The problem with these approaches is that CCT is not perfectly correlated with blue light emissions. Lamps with identical CCT can have quite different fractional blue light emissions.This is because CCT is based upon comparison to ablackbodylight source, which is a poor approximation forLEDsandvapor discharge lampssuch as high pressure sodium.The G-index was therefore developed for use indecision makingfor the purchase of outdoor lamps and in lighting regulations as an improved alternative to the CCT metric.

Use

In 2019, theEuropean Commission'sJoint Research Centreincorporated the G-index into their guidelines for theGreen Public Procurementof road lighting. 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.In their "core criteria", they recommend that "inparks,gardensand 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 becalculated,they suggest that CCT≤3000 K is likely to satisfy this criterion. In the stricter "comprehensive criteria", they recommend that parks and ecologically sensitive areas or areas at specifieddistancesfromopticalastronomy 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 thenight sky.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 onlymonochromaticor quasi-monochromatic lamps will be used, with G>3.5 and in principle only emissions in the interval 585-605 nm.

Questionable Use Warning

The G-index has not been evaluated or adopted by a standards development organization (SDO), such as the CIE. Generally, for a specification to be used in a regulation or tender, it must go through the rigorous process of evaluation and adoption by an SDO. It is thus questionable for the EC Joint Research Center and the Andalusian Regional Government (and others) to suggest or prescribe mandatory requirements based on the G-index.

A measure focused solely on reducing blue light will not provide ecological protection. Because the intensity of light plays a role as strong or stronger than spectrum, putting the light in the right places (on road surfaces and sidewalks) and avoiding spillage into ecological regions is likely to be more effective than manipulating the spectrum of the light. Spectrum does play a role, but in order to prevent disturbance to sensitive animals, changes must be made to the spectrum which cannot be described by the G-index. Those changes are also species dependent. A specific (red-dominant) spectrum has been proven to be as good as darkness for many (but not all) light sensitive insect and bat species.An amber spectrum is proven to be less eco-friendly than a red spectrum for some species, although both have negligible blue content and ‘favorable’ G index. Therefore the use of spectral G-index is overly simplistic and may do more harm than good. The use of the G-index is therefore strongly discouraged for use in lighting specifications or regulations.

External links

• More information, including a spreadsheet for calculating G-index,Regional Government of Andalusia (note: Spanish language page; English language handbook on index G, and LibreOffice spreadsheet in English for index computation, are linked from the lower part of the page.)
• Information on Green Public Procurement of Street Lighting in the EU,Joint Research Centre
• f.luxometer web tool,Online calculator for g-index and other indexes
• Astrocalc software,Carlos Tapia Ayuga (note: Spanish language page, English instructions at bottom)