# Calculate RGB color ratio for a color space given a white point?

I am new to the field of displays and color science. I am trying to calculate RGB color ratio for NTSC color space using D65 white point. Can anyone point me to a good reference on it or how I can do it? I found this article but I don't understand the logic he uses to calculate the color ratio. I understood up till the point he starts calculating blue to red ratio using purple point and green to purple ratio using white point.

• Hi, Thanks for your reply. I do understand how the article goes about using line equation to calculate the purple point but what I don't understand is how the ratio of red to blue is (yr/yb)*(yr-yp)/(yp-yb). I get that y coordinate represents intensity but why is the ratio using purple point just not (yr-yp)/(yp-yb)? Jul 19, 2020 at 1:08
• Oh. Sounds like a different question than I'd first assumed. You are asking someone to review that article for you, confirm it is correct, and then explain to you why that author came to correct conclusions (and formulas) and why your approach isn't correct. My mistake, not yours. I'll remove my comment as unhelpful.
– jonk
Jul 19, 2020 at 1:46

Here is the chart from https://en.wikipedia.org/wiki/CIE_1931_color_space#/media/File:CIE1931xy_blank.svg

This is what the formulas do.

Imagine you have an RGB LED that contains LEDs of the three colors in the three black circles.

You want to produce white light with those three LEDs.

Turn on only the red LED to around 1/2 brightness.

Turn on the blue LED and adjust the brightness of the blue LED until you see the purple color in the gray circle.

Turn on the green LED and adjust the brightness of the green LED until you see the desired white light.