So I notice the first image has the intensity. What I don't get is how a light sources doesn't display and radiate in all directions in a specific amount. Like how could a light bulb show like Intensity of X at 5 meters away but Intensity X at 3 meters in a different direction? Isn't it all the same in any direction?
I'm not sure if this is correct, but from an implementation perspective, it seems like we can use radiant intensity to describe how light gets displayed in the environment. Irradiance can be describing the luminance on a surface point in the environment and radiance is the vessel in which we transfer the radiant intensity to irradiance?
We typically think about the "ideal" perspective, where a light shines equally in all directions. However, no lights actually behave like this: even a lightbulb has a dark spot below it. In the extreme case, think about a laser, where all the light shines in one direction.