diffuse lighting calculates the amount of light that a vertex receives based on the angle between the vertex normal and the light vector (which points in the direction of the light source). The smaller the angle, the more light the vertex receives, and the larger the angle, the less light the vertex receives. If the angle is greater than or equal to 90 degrees, the vertex receives no light.

from Introduction to 3D game programming with DirectX 9.0

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