$rimlight
From Valve Developer Community
The $rimlight
VMT command adds a rim light to the model that appears to come indirectly from the environment. The color of the rim light is directly influenced by Ambient light.

To do: Explain similarities and differences of Phong Fresnel ranges.
VMT syntax
$rimlight <bool>
VertexLitGeneric { $phong 1 $bumpmap <texture> $phongexponent 5 $phongexponenttexture <texture> $phongboost 1.0 $phongfresnelranges "[0 0.5 1]" $rimlight 1 $rimlightexponent 2 $rimlightboost .2 }
Additional parameters
-
$rimexponent <int>
(New with Half-Life 2: Episode Two / Source 2007) - Exponent for phong component of rim lighting. The rim exponent defines the "tightness" of the highlight. A higher exponent results in a smaller, tighter highlight while a lower exponent results in a broader flatter one.
-
$rimboost <int>
(New with Half-Life 2: Episode Two / Source 2007) - Boost for ambient cube component of rim lighting.
$rimmask <boolean>
(New with Half-Life 2: Episode Two / Source 2007)- Uses the alpha channel of the $phongexponenttexture to mask rimlighting.