Shaders: Brightness of a colour?

I want to get a singular value for the brightness of a colour. Basically I want to take _Reflection.rgb and turn it into a single float value. Black would be 0, white would be 1.

Is there a function for this?

Hi, getting the luminance of a color is the same as converting to black and white (I assume brightness = luminance)

l = color.r * 0.3 + color.g * 0.59 + color.b * 0.11

Source: Luminance