I'm writing a raytracer and I'm using the assimp library to load objects. I have an object's material specified in a .mtl file:

newmtl rightSphere
Ka 0.01 0.01 0.01
Kd 0.01 0.01 0.01
Ks 0.30 0.30 0.30
Tf 0.10 0.10 0.10
Ns 200
Ni 2.5
illum 7


and I'm trying to load this material. I've been mostly successful so far, for example I know that to extract the diffuse component (Kd), I can use

aiMesh *mesh = scene->mMeshes[i];
aiMaterial *mat = scene->mMaterials[mesh->mMaterialIndex];
aiColor3D color;
mat->Get(AI_MATKEY_COLOR_DIFFUSE, color);


using the identifiers specified in here. However, I've been unable to extract the Tf component. I understand that this component defines the coefficient of refracted light per color channel, and I need this value along with the index of refraction (Ni) to get it to work.

My question is: how can I get the Tf value?