What is yellow/amber light?
A narrow band on the edge of the chromaticity chart, between red and green, closed by red, white and green limits. A lamp behind a yellow filter and an amber LED look very different inside, yet both can land in it.
Yellow/amber light is an aeronautical ground light colour defined by a narrow band on the edge of the chromaticity chart, between red and green. A light is yellow if its measured (x, y) point lies inside that band. ICAO Annex 14 calls the colour yellow; amber is the everyday name for the same region. It is the colour of caution: runway edge lights in the caution zone, runway guard lights, intermediate holding position lights, rapid exit taxiway indicator lights, and the yellow half of alternating green and yellow taxiway centreline lights.
Like green, yellow has three limits: a green limit against greenish yellow, a red limit against orange and a white limit against pale yellow. The edge of the chart closes the fourth side. The band is thin, so a small shift in the light’s spectrum crosses a limit, and the orange between yellow and red belongs to neither colour. Yellow can be made two ways. A filtered lamp starts with white light and a yellow filter blocks the blue and green end and passes the rest. A filament lamp runs redder when it is dimmed, so the filtered light can tilt toward orange at low intensity steps. An amber LED emits one band directly, and its colour moves with temperature. Colour passes or fails on the chart; candela is checked separately.
A narrow band on the chart edge with three limits: red, white and green. Little room before a limit is crossed.
A narrow wedge at the long-wavelength end with only two limits, yellow and purple. Orange lies between it and yellow.
- Find yellow on the chart. It is a narrow band on the upper-right edge of the chromaticity chart, between red and green.
- Name its three limits. A green limit on one side, a red 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 orange it crosses the red limit, toward greenish the green limit, toward pale the white limit. The side says which way the light has drifted.
- Check the intensity steps and the brightness. A dimmed lamp tilts redder, so colour is checked at the steps in use. Candela is checked on its own.
Frequently Asked Questions
Q1What is yellow/amber light in airfield ground lighting?
Yellow is the aeronautical ground light colour whose chromaticity point lies inside the yellow band set in the standard. ICAO calls it yellow; amber is the everyday name for the same region. It is the colour of caution: runway edge lights in the caution zone, runway guard lights, intermediate holding position lights, rapid exit taxiway indicator lights and the yellow half of alternating green and yellow taxiway centreline lights.
Q2Why is the yellow band so narrow?
Yellow sits on the edge of the chromaticity chart between red and green, where the spectral colours change quickly with wavelength. The band is closed by a red limit on one side, a green limit on the other and a white limit towards the middle, and the chart edge closes the rest. A small change in the light's spectrum is enough to cross one of them.
Q3How is a yellow light made?
Either a yellow filter is placed over a white lamp, or an amber LED is used. The filter blocks the blue and green end of the lamp's light and passes the rest. An amber LED emits one band directly. Both can land inside the same yellow band.
Q4Why can a dimmed lamp turn orange?
A filament lamp runs redder as it is dimmed, so the light behind a yellow filter tilts toward red. At a low intensity step a filtered yellow light can cross the red limit and look orange. An amber LED has its own drift, because its colour moves with temperature. A light that is yellow at full brightness is not automatically yellow at every step.
Q5How can a yellow light fail its colour?
It can leave the band by any of its three limits. A light that turns orange crosses the red limit, one that turns greenish crosses the green limit, and one that turns pale crosses the white limit. Orange belongs to neither red nor yellow. A fixture can be bright enough and still fail on colour.
Q6How is yellow light checked?
The light's chromaticity is measured with a spectroradiometer or colorimeter, giving an x, y point. The point is compared with the yellow 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, and check the colour at the intensity steps in use, not only at full brightness.
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