Why does the chromaticity diagram look like that?

I thought this might be a useful article because I've often had a similar question. But there's a diagram that has text:

> More simply put: imagine that you have red, green, and blue light sources. What is the intensity of each one so that the resulting light matches a specific color on the spectrum?

> ...

> The CIE 1931 color space defines these RGB color matching functions. The red, green, and blue lines represent the intensity of each RGB light source:

This seems very oddly phrased to me. I would presume that what that chart is actually showing is the response for each color of cone in the human eye?

In which case it's not a question of "intensity of the light source" but more like "the visual response across different wavelengths of a otherwise uniform intensity light source"?

... fwiw, I'm not trying to be pedantic, just trying to see if I'm missing the point or not.

The wording on the article is correct, despite being confusing. The CIE 1931 RGB primaries each stimulate multiple types of cone in human eyes, so the RGB Color Matching Functions (CMFs) don't represent individual cone stimulations.

However, the CMFs for LMS space[1] do directly represent individual cone stimulations over. Like the CIE RGB CMFs, the LMS CMFs can also be thought of as the required intensities of three primariy colors required to reproduce the color of a given spectrum. The reason these two definitions coorespond for LMS space is that each primary would stimulate only one type of cone. However, unlike CIE RGB, no colors of light which stimulate only one type of cone physically exist.

Finally, CIE RGB and LMS space are linear transformations of each other, so the CIE RGB CMFs are linear combinations of the LMS CMFs, so each CIE RGB CMF can be though of as representing a specific linear combination of cone stimulations (the combination excited by the primary color).

I often find it easiest to reason about these color spaces in terms of LMS space, since it's the most physically straightforward.

[1]: https://en.m.wikipedia.org/wiki/LMS_color_space