Chromaticity is determined by plotting the initial light colour on the CIE 1976 colour space diagram.
At intervals the colour of the light emitted is measured and plotted on the colour space diagram. As the graph here shows, over time the colour of the light emitted gradually shifts away from its initial value. With chromaticity shift there are at least two different factors at work – temperature and time.
The temperature at which an LED operates affects the structure of the silicon at the heart of the LED. At higher temperatures the space between the lattices in the crystalline structure of the silicon increases causing a shift to the red end of the spectrum. This explains the difference between the chromaticity of LEDs at different temperatures in the graph.
Difference between the chromaticity of LED's at different temperatures over time.