In the world of coding, Brightness is Max(RGB). That is, you compare R, to G, to B, and take the highest of the three.
Example:
R = 156
G = 10
B = 122
Brightness = 156
With those numbers, if you go Brightness += 10, you then get: R = 166
G = 20
B = 132
You can derive Brightness with some savvy use of Apply Image and Lighten. You can even manipulate Brightness with certain other blending modes.