What are colour coordinates?
The pair of numbers, x and y, that pin a light's colour to a single point on the chromaticity chart.
Colour coordinates are the two numbers, x and y, that locate a light’s colour on the CIE chromaticity chart. Where chromaticity is the idea of colour apart from brightness, the coordinates are the values that put it on paper: read x along one axis, y along the other, and the pair (x, y) is a single point.
Only two are needed because the third CIE value is fixed by them, z = 1 − x − y. For AGL, the standard sets the permitted colour of each signal light as a region on the chart, bounded by straight lines written in terms of x and y. Checking a light then becomes simple: measure its (x, y) and see whether the point falls inside the limits for its colour. Brightness plays no part in this, which is why candela is checked separately.
Frequently Asked Questions
Q1What are colour coordinates?
The pair of numbers, x and y, that give the position of a light's colour on the CIE chromaticity chart. They turn a colour into a point that can be written down and compared.
Q2What is the difference between chromaticity and colour coordinates?
Chromaticity is the idea of colour apart from brightness. Colour coordinates are the actual numbers, x and y, that express it for a particular light.
Q3Why only x and y?
The three CIE values add up to one, so z equals one minus x minus y. Once x and y are known the third value is fixed and adds no information.
Q4How are colour coordinates used to check a light?
The measured (x, y) is compared with the boundary of the region set for that colour. If the point is inside the limits the colour is acceptable; if it is outside, the light fails on colour.
Q5Do colour coordinates say anything about brightness?
No. Brightness is carried by candela. Two lights of very different intensity can have the same colour coordinates.
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