$halflambert
…
$halflambert is a material shader parameter available in all 
Source games for VertexLitGeneric, Teeth, EyeRefract, VortWarp, Infected (shader). It is a boolean parameter that enables half-lambertian lighting, which wraps lighting further around a model to prevent it from losing definition. It only works on models.
Note:Since 
Source 2007, half-lambertian lighting is forcibly enabled on VertexLitGeneric regardless of $halflambert setting when $phong is enabled, unless $phongdisablehalflambert(in all games since 
) is enabled.Note:This parameter is disabled since 
due to conflicts with cascade shadow mapping.
Important:Does not work on static props which have lighting baked per-vertex or into lightmaps in stock VRAD; this could be achieved with a custom VRAD.
Syntax
$halflambert 1
