I am looking for the mathematical equation that converts luminance values into Y Tristimulus values, or the other way around?

I am looking for the mathematical equation that converts luminance values into Y Tristimulus values, or the other way around?

Also, Tristimulus values are used to match paint or inks under a controlled light source. This CIE system is very accurate in matching the human visual experience. Are the luminance values just as accurate? If not, why are they used? If so, where can I read more information on the comparison of the two systems? Are they related to each other in their mathematical equations?