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The spectral G-Index is a variable that was developed to quantify the amount of short wavelength light in a 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.

Definition

The G-index 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;
• 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.

The Regional Government of Andalusia has developed a spreadsheet to allow calculation of the G-index for any lamp who's spectral power distribution is known (see External Links below).

Rationale

The ongoing 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. This switch is problematic from the perspective of increased astronomical and ecological light pollution. Short wavelength light is more likely to scatter in the atmosphere, and therefore produces more artificial skyglow than an equivalent amount of longer wavelength light. Additionally, broad spectrum light and short wavelength light tend to have a greater overall ecological impact than narrow band and long wavelength visible light. For this reason, lighting guidelines, recommendations, norms, and legislation frequently place limits on blue light emissions. For example, the "fixture seal of approval" program of the International Dark-Sky Association limits lights to have a correlated color temperature (CCT) below 3000 K, while the national French light pollution law restricts CCT to maximum 3000 K in most areas, and 2400 K or 2700 K in protected areas such as nature reserves.

However, CCT is not perfectly correlated with blue light emissions, and therefore lamps with identical CCT can have very different fractional blue light emissions. The reason for this is that CCT is based upon comparison to a blackbody light source, which is a poor approximation for LEDs and vapor discharge lamps such as high pressure sodium. The G-index was therefore developed for use in decision making for the purchase of outdoor lamps and in lighting regulations as an alternative to the CCT metric.

Use

In 2019, the European Commission's Joint Research Centre incorporated the G-index into their guidelines for the Green Public Procurement of road lighting.

For lamps with no blue emissions (e.g. Low Pressure Sodium or PC Amber LED), the G-index is undefined.

External links

• Spreadsheet for calculating G-index, Regional Government of Andalusia (note: Spanish language page).
• Information on Green Public Procurement of Street Lighting in the EU, Joint Research Centre