Sun lamp and Diffuse surface is not lambertian? I made simple scene with 1 sphere and 1 sun light. I set strength of sun lamp to 1.000 (emission node) and add diffuse BSDF surface with white(R1G1B1) to sphere. When I render this scene, sphere shows gray color(max 0.3) even at completly orthogonal with sun direction. 

I think diffuse surface reflect light using lambertian, so surface shows white color at orthogonal point. 

What's wrong in my scene?

(I used Cycles renderer and cut off shadow)
Assuming the sun light has size 0 and strength 1, the correct maximal value on the sphere is 1&frasl;pi, because of energy conservation and the units used.

Cycles uses physically based units for light strength, in this case Watts&frasl;m2 for sun lights. Other renderers and game engines (typically ones that were not designed with physically based rendering in mind) often use a light strength that is implicitly multiplied by pi, which would give the maximal value of 1 you expect.