How does Blender lights work in Cycles regarding intensity? The scene:  


Just a plane, a camera, and a light.  
The plane is facing the camera, the lamp is facing the plane, all 3 completely aligned.  
The plane is just a diffuse, R,G,B=1,1,1 which means the material bring back the light it received with the same intensity.  
The light has a size of 10cm, R,G,B=1,1,1 and strength of 10, I've tried each type (spot, area...) put a fall off node in constant (which for me and blender manual means, light intensity remain the same regardless the distance it travels).  


Render result:  

So when I hit render and analyze the scene linear value, I expect to have a value of 10 for the lighted pixels, but I've got pixels around R,G,B=0.25,0.25,0.25 or 0.7,0.7,0.7 depending on the type.

Even worst I've constated that the light is stronger when set to quadratic than to constant, but not close to 10 neither.  

I've tried with only the world background R,G,B=1,1,1 and strength to 10, this time the result is as I expected, analyzed lighted pixels R,G,B=10,10,10  

Question:  

So the question is how do these lights work in cycles, and are they broken?



Edit :  

Look Mike's answer and my comment below to see why there's nothing to worry about these lights in Cycles.  

Edit2 :  

I offert the bounty to Bertmoog because he provide a good in deep answer, in fact, if it was possible, I would have split the bounty 50/50 with Mike, because he point me to the fact that W/m² aren't the same as pixels values, which was an important part to understand why I haven't got the same values for the source and for the mesured pixel (ray intensity).
Thanks to both of you.
The light energy in Cycles are defined using the following unit: According to the doc: "Sun lamps are specified in Watts/m^2, other lamps are specified in Watts, and Emission shaders on meshes are also in Watts/m^2. I am not sure how the world energy is defined.

Because the energy are specified in watts (over an area when applicable), I don't see any reason why a perfectly diffuse surface should reflect the same numerical RGB value as the lamp's energy.