That's a question solved by looking at physical units.

The **irradiance** (watt per square metre) on the whole object determines it's illumination, this unit varies with distance between object and light because the "subtended surface" diminishes by `1/r²` with the distance.  
(The radiant flux (W) of the light being constant)

For simplicity let's imagine a case where the light is at the camera position and we are looking at a disc. The **radiance** of the disk is the emitted light by its surface considering our vision angle : in watt per steradian per square metre.

The radiance is necessarily less than its irradiance if the material is diffuse. Why is that ? because the camera only subtends a tiny angle of the total re-emission directions; while the disc re-emits its energy at `2π` steradian (hemisphere).

So the radiance seen by the camera is `irradiance / 2π`. As you can see it does not depend on the distance between the object and the camera. Now, radiance is a unit per square meter, which means it defines "light intensity" per area, so when discretized it means that is the pixel value :)

I hope I'm right, this is always confusing.