We use the standard fixed-function Blinn-Phong model. You can read up on all of the equations here at MSDN.
If you have a Tier 2 card, we actually do our lighting in a vertex shader. If you don’t have a Tier 2 card, we do the lighting on the CPU. Why not just use D3D9’s fixed-function APIs you might ask? We wanted to be able to blend a texture (SpecularMaterial.Brush) only with the specular component and it turns out the D3D9 fixed-function APIs can’t do that.
An important thing to know is all Material Brushes are treated as textures even though they might not be implemented as such (e.g. SolidColorBrush). This means if the amount of light hitting a vertex is more than 1.0, it will be capped at 1.0 before being blended with the texture. This is an unfortunate behavior of the fixed function pipeline. I discussed this more in the color knobs post.
There is one thing that may be unique to out lighting model. Since we are a retained system, we factor in a Light’s transform to all of its properties. We approximate the scale being applied to the light by taking the cube root of the absolute value of the determinant of the upper 3x3 part of the Light’s transform. For a uniform scale this will be the actual scale but for non-uniform it’s good enough (the real solution is too costly for something uncommon). Internally, this number is factored into the range and attenuation values so things make sense. For example, if the Light undergoes a uniform scale of 2.0, we’ll multiple its range by 2.0.