This is pretty tricky. The reason you get different levels of illumination is because that's how diffuse reflection works in real life. The reflected diffuse intensity is going to be in proportion to the light energy per unit of area.

Imagine a pencil thin circular beam of light striking a surface at 90 degrees. The lit spot is exactly the same circular area as the cross section of the beam. However, if you tilt the light to a lower angle, then the lit area becomes spread out, which means the same amount of light energy is spread out over a larger area (an ellipse in this case), thus lower energy per unit area.

To counteract this, we need to do a non-photoreal (NPR) shader (material) on the wall and floor.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)