How is colour measured?
Reading the colour of a light as chromaticity coordinates, then checking that the point falls inside the permitted colour region.
Candela values say how bright a light is, not what colour it is. Colour measurement records the light's colour as a pair of chromaticity coordinates (x, y), using an instrument that reads the spectrum of the light.
The measured point is plotted on a chromaticity chart and compared with the colour boundaries set for aeronautical ground lights. If it lies inside the region for that colour, the light is the right colour; if it lies outside, the colour fails, however bright the light is. Red, yellow, green, blue and white each have their own region.
Frequently Asked Questions
Q1What is colour measurement?
Measuring the colour of a light as chromaticity coordinates. An instrument reads the light's spectrum and gives an x, y point that can be compared with the permitted colour region.
Q2Why is colour measured separately from intensity?
Intensity gives brightness in candela and says nothing about colour. A light can be bright enough and still be the wrong colour, so both are checked.
Q3What are chromaticity coordinates?
Two numbers, x and y, that fix a colour's position on a chromaticity chart independent of how bright it is. They allow a measured colour to be compared with a boundary.
Q4What is a colour boundary?
The edge of the region on the chromaticity chart inside which a light of a given colour is acceptable. Each aeronautical ground light colour has its own region in the standard.
Q5What happens if the measured point falls outside the region?
The light does not meet the colour requirement. The cause, such as the filter, the LED or ageing, is investigated and the fixture is corrected or replaced.
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