User:Anotherdoon/sandbox

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 selecting outdoor lamps that minimize skyglow, a form of 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

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. The G-index was developed for use in procurement of outdoor lamps and lighting regulations as an alternative to the correlated color temperature (CCT) metric. While CCT is somewhat correlated with blue light emissions, 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 like high pressure sodium.

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.

External links

• Spreadsheet for calculating G-index, Regional Government of Andalusia (note: Spanish language page).