What is green light?
A region on the upper-left edge of the chromaticity chart, closed by yellow, white and blue limits. A lamp behind a green filter and a green LED look very different inside, yet both can land in it.
Green light is an aeronautical ground light colour defined by a region on the upper-left edge of the chromaticity chart. A light is green if its measured (x, y) point lies inside that region. It is the colour of threshold lights and wing bars, of taxiway centreline lights and of lead-on lights.
Green is larger than red and has three limits: a yellow limit against yellowish green, a blue limit against bluish green and a white limit against pale green. The edge of the chart closes the fourth side, because no real colour lies beyond it. Green can be made two ways. A filtered lamp starts with white light and a green filter passes only a band of it, which wastes most of the lamp’s light. A green LED emits a band of green directly. The two spectra look very different but can land inside the same region. Colour passes or fails on the chart; candela is checked separately. The eye is most sensitive near green, so green needs less radiant power than red for the same candela.
A region on the chart edge with three limits: yellow, white and blue. Any point inside all three is green.
A narrow wedge at the opposite end of the chart with only two limits, yellow and purple.
- Find green on the chart. It is a region on the upper-left edge of the chromaticity chart, larger than red and set well away from the middle.
- Name its three limits. A yellow limit on one side, a blue limit on the other and a white limit towards the middle. The chart edge closes the fourth side.
- Measure the light. A spectroradiometer or colorimeter gives the light's x, y point.
- Plot the point. Mark it on the chart, whatever the lamp, filter or LED underneath.
- Check the side it leaves by. Toward yellow, blue or white it crosses that limit, and the side says which way the light has drifted.
- Check brightness separately. Colour passes or fails on the chart. Candela is checked on its own.
Frequently Asked Questions
Q1What is green light in airfield ground lighting?
Green is the aeronautical ground light colour whose chromaticity point lies inside the green region set in the standard. It is the colour of threshold lights and wing bars, taxiway centreline lights and lead-on lights.
Q2Why does green have three limits?
Green sits on the edge of the chromaticity chart, and the edge closes one side of the region. The other sides border yellow, white and blue, so the standard sets three limits: a yellow limit against yellowish green, a white limit against pale green, and a blue limit against bluish green.
Q3How is a green light made?
Either a green filter is placed over a white lamp, or a green LED is used. The filter passes a band of the lamp's light and absorbs the rest, so most of the lamp's output is lost. A green LED emits a band of green directly. Both can land inside the same green region.
Q4Why is green the efficient colour?
The eye is most sensitive to light near the middle of the visible spectrum, in the green and yellow-green. A green light therefore needs less radiant power than a red or blue light to give the same candela. Colour and candela are still checked separately.
Q5How can a green light fail its colour?
It can leave the region by any of its three limits. A light that drifts toward yellow crosses the yellow limit, one that turns bluish crosses the blue limit, and one that turns pale, for example when white light leaks past a damaged filter, crosses the white limit. A fixture can be bright enough and still fail on colour.
Q6How is green light checked?
The light's chromaticity is measured with a spectroradiometer or colorimeter, giving an x, y point. The point is compared with the green limits in the standard, Appendix 1 of ICAO Annex 14 Volume I. Check which definition applies to the fixture, because limits can differ by light source.
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