Uniform diffuse distribution casts diffuse light in one hemisphere only, as if the light were emitted from a surface.
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Create panel > 
(Lights) > Photometric > Click Target Light or Free Light. > General Parameters rollout > Light Distribution (Type) group > Choose Uniform Diffuse from the drop-down list.
Uniform diffuse distribution obeys Lambert’s cosine law: the light has the same apparent intensity when viewed from any angle.
Uniform diffuse light distribution
Note: Uniform Diffuse distribution was known simply as Diffuse distribution in releases prior to 3ds Max 2009.