I got an email back from Adrian Marsh, Technical Development Manager, LEE Filters UK.........
"Thank you for your enquiry regarding the x and y chromaticity co-ordinates that we list for our lighting filter products.
A system was developed in colour science some time back by the Commission International Eclairage (CIE) as a way of expressing colours mathematically – I won’t go into the details of how it was developed or the calculations involved, but the result of their work was an x-y graph and curve on which theoretically, all colour can be expressed. (The calculations also take into account the light source being used (Tungsten, Daylight, etc.) to observe / measure the colours as this obviously has an effect on how the colour is viewed / perceived.)
When we manufacture our products, the individual colour spectrums are measured using a spectrophotometer and the results are fed into a calculation which produces the x and y chromaticity values. We can then plot these values on our own small reproduced section of the CIE graph, together with tolerance boxes, to ensure that the colours we produce are consistent. The x and y values we list in our literature can be used to distinguish different colours from each other.
I have attached an image of the CIE 1931 chromaticity space which I hope illustrates what I have been trying to explain – there are also some notes on the diagram itself which may help with the explanation.
I hope this helps with your enquiry – let me know if you need any further information.
Adrian Marsh – Technical Development Manager, LEE Filters UK"
One can read a bit about chromaticity and the meaning of the x,y values here...
His graph is looks the same as the one linked to, withh added notes. I'll try and attach that...maybe in another post
EDIT: fixed the formatting in my added text...
Edited by Gregg MacPherson, 17 May 2018 - 12:53 AM.