What is white light?
A mixture of wavelengths whose chromaticity point falls inside the white region of the chart. White is a patch, not a point, so different lamps and LEDs can all be white.
White light is an aeronautical ground light colour defined by a region on the chromaticity chart. A light is white if its measured (x, y) point lies inside that region. White is not one wavelength but a mixture, and it is the colour of most runway edge, centreline, touchdown zone and approach lights. A PAPI also shows white as one of its two signals.
Because white sits in the middle of the chart, it has four neighbours: a light that is not quite white can be too yellow, too blue, too green or too purple, and each side of the region is a limit against one of them. A filament lamp and a white LED have very different spectra yet can land inside the same region. Dimming a filament lamp lowers its filament temperature and moves its point toward yellow, which is one reason colour is checked as well as brightness. Colour passes or fails on the chart; candela is checked separately.
A region on the chart, bordered by four colours. Any point inside it is white.
Red, yellow, green or blue: its own region, away from the middle. A filter removes part of white light to make it, or a coloured LED emits it directly.
- Find white on the chart. It is a patch near the middle of the chromaticity chart, not a single point.
- Name its four sides. Yellow on one side, blue on the opposite, green above and purple below. Each limit borders one of them.
- Measure the light. A spectroradiometer or colorimeter gives the light's x, y point.
- Plot the point. Mark the x, y point on the chart, whatever the lamp or LED underneath.
- Check the side it leaves by. A point outside the region says which way the light has drifted: yellow, blue, green or purple.
- Check brightness separately. Colour passes or fails on the chart. Candela is checked on its own.
Frequently Asked Questions
Q1What is white light in airfield ground lighting?
White is the aeronautical ground light colour whose chromaticity point lies inside the white region set in the standard. It is a mixture of wavelengths, not a single one, and it is the colour shown by most runway edge, centreline, touchdown zone and approach lights.
Q2Why is white a region and not a single point?
No real light is exactly one white. Lamps and LEDs differ slightly, and they drift with temperature, current and age. The standard therefore allows a region on the chromaticity chart, and a light is white if its measured point falls inside it.
Q3Can two white lights have different spectra?
Yes. A filament lamp gives a smooth spectrum that rises toward the red. A white LED gives a blue peak plus a broad hump from a phosphor. The two spectra look very different but can land on points inside the same white region, so the eye sees both as white.
Q4Why does a dimmed filament lamp look more yellow?
Dimming lowers the temperature of the filament, and a cooler filament shifts its light toward yellow and red. The chromaticity point slides along the chart toward the yellow side and can leave the white region. An LED dims by a different mechanism and moves differently.
Q5Why does white have four neighbouring colours?
White sits in the middle of the chart, so a light that is not quite white can fail in four ways: too yellow, too blue, too green or too purple. Each side of the white region is bounded by a limit against one of those colours.
Q6How is white light checked?
The light's chromaticity is measured with a spectroradiometer or colorimeter, giving an x, y point. The point is compared with the white limits in the standard. Appendix 1 of Annex 14 includes a white specification for LEDs, so check which definition applies to the fixture.
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