I'm working on a project for a client, where they have a panther model with a tarp or cloth over his head.. they want the eyes to glow red from underneath the tarp, but I haven't been able to figure out a way to do it? I currently have the faces of the model's eyes set as an emission material, but haven't been able to get the cloth to allow light through it.. any help would be super awesome!
What you're asking for is called translucency. There's a Translucent BSDF available in Cycles (not to be confused with Transparent BSDF) which you can use in conjunction with other nodes. Here's a basic-as-it-gets setup where I'm simply mixing it with the default Principled BSDF:
You can find tons of other tutorials and guides about translucent cloth materials on Youtube as well as here.
Here's a comparison between using a Mix Shader (which is subtractive) vs using an Add Shader node with an HDRI environment, as per Allen Simpson's suggestion. It does look like using Add Shader results in a more natural look.
Mix Shader and Add Shader results, respectively.