What is red light?
A narrow region at the red end of the chromaticity chart, closed by a yellow limit and a purple limit. A lamp behind a red filter and a red LED look very different inside, yet both can land in it.
Red light is an aeronautical ground light colour defined by a region at the red end of the chromaticity chart. A light is red if its measured (x, y) point lies inside that region. Red is the colour of “stop” and “danger”: it is shown by runway end lights, by stop bars at the holding position, by the below-slope signal of a PAPI, and by obstacle lights.
Unlike white, red sits at the edge of the chart, so it needs only two limits: a yellow limit that keeps it from drifting to orange, and a purple limit that keeps it from drifting to pink or purple. The edge of the chart itself closes the third side, because no real colour lies beyond it. Red can be made two ways. A filtered lamp starts with white light and a red filter absorbs everything except the red end, which wastes most of the lamp’s light. A red LED emits a narrow band of red 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, and because the eye is less sensitive at the red end, a red light needs more radiant power than a white one for the same candela.
A wedge at the edge of the chart, closed by a yellow limit and a purple limit. Any point inside both is red.
A patch in the middle of the chart, bordered by four colours. Red is the opposite case: one edge, two limits.
- Find red on the chart. It sits at the long-wavelength end, a narrow wedge at the edge of the chromaticity chart.
- Name its two limits. A yellow limit on the orange side and a purple limit on the pink side. The edge of the chart closes the third 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 orange it crosses the yellow limit; toward purple it crosses the purple limit.
- Check brightness separately. Colour passes or fails on the chart. Candela is checked on its own.
Frequently Asked Questions
Q1What is red light in airfield ground lighting?
Red is the aeronautical ground light colour whose chromaticity point lies inside the red region set in the standard. It is the colour of stop and danger signals: runway end lights, stop bars, the below-slope signal of a PAPI and obstacle lights.
Q2Why does red have only two limits?
Red sits at the edge of the chromaticity chart, where the pure spectral colours lie. No real colour exists beyond that edge, so the edge closes the region on its own. The standard needs only two limits: a yellow limit against orange, and a purple limit against purple and pink.
Q3How is a red light made?
Either a red filter is placed over a white lamp, or a red LED is used. The filter absorbs the rest of the lamp's light and passes only the red end, so most of the lamp's output is lost. A red LED emits a narrow band of red directly. Both can land inside the same red region.
Q4Why does a red light need more power than a white one?
The eye is less sensitive at the red end of the spectrum than in the middle. A red light therefore needs more radiant power than a white or green light to give the same candela. In a lamp-based fixture the filter loss adds to this.
Q5How can a red light fail its colour?
It can leave the region by either limit. A light that drifts toward orange crosses the yellow limit, and a light with blue mixed in drifts toward purple and crosses the purple limit. A fixture can be bright enough and still fail on colour, because colour and candela are checked separately.
Q6How is red light checked?
The light's chromaticity is measured with a spectroradiometer or colorimeter, giving an x, y point. The point is compared with the red limits in the standard, Appendix 1 of ICAO Annex 14 Volume I. Check which standard and edition applies to the fixture, such as ICAO Annex 14 or the national equivalent.
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