In this case, we have a torus as a mesh light.

The mesh light color is facing-ratio ramp, so that in the Beauty AOV, the polygons facing the incoming camera ray are red, and polygons facing away are green.

The plane below the light has a [slightly non-realistic] combination of diffuse, specular, and reflection (Kd=0.7, Ks=0.5, Kr=0.2).

The question here is: why do we see red in the reflection AOV, but green only in the diffuse and specular AOVs? Why does the facing-ratio ramp return red for reflection rays, but green for diffuse and glossy rays?

The answer is that there are no diffuse or glossy rays in this situation. A mesh light gets a meshLightMaterial shader, which MtoA assigns to the mesh light color (and to the torus shape) when MtoA translates the scene to Arnold.

The meshLightMaterial shader doesn’t handle diffuse, glossy, or shadow rays. The first few lines of the meshLightMaterial shader look like this:

if (sg->Rt & (AI_RAY_DIFFUSE | AI_RAY_GLOSSY | AI_RAY_SHADOW))
 {
 sg->out.RGBA = AI_RGBA_BLACK; 
 return;
}

Let’s take a look at what’s happening in the different AOVs.

Reflection

The reflection rays hit the bottom of the torus, go through the facing-ratio ramp, and return red because most of the polygons are facing down towards the plane. In this case, we get the facing ratio of the polygon normals and the reflection ray.

Specular

We get the green from the light sampling, where there’s no rays.

Since there’s no rays, we get the dot product of the surface normal and  0 0 0, which is 0. And that returns green from the ramp:

Diffuse

The diffuse rays don’t make a contribution (because the meshLightMaterial doesn’t handle them) so we get green from the light sampling (just like for specular).

Indirect Specular

This would be black, since the meshLightMaterial doesn’t handle glossy rays, so the mesh light makes no contribution to the glossy reflections.