I recently had a desire/need to limit the light from my Cycles light panels (lights created from mesh objects with emission materials). I searched high and low and eventually found some indications that this might be possible in the nodes setup of the emission material. The default action in Blender Cycles is that mesh materials are visible from both sides, whether they are emissive or not. This particular behavior is a bit different from what you see in some 3D viewports, such as with game engines like Unity, where the backside of a face renders nothing and is, in effect, invisible.
Blender can operate in this mode as well by enabling Backface Culling on the Shading tab of the Properties panel (hotkey 'N' in the 3D viewport). However, this doesn't affect the render in Cycles -- the backside of the face will still render (as black). Nonetheless, this type of display may still be useful (in texture or material mode) for some types of modeling, for example, when working with interiors with walls facing inward.
So, back to our desire to create a single-sided light plane for Cycles rendering. As it turns out, this is extremely easy to do. It requires a simple node setup for the emission material you've applied to your light panel. And make a note here, while this doesn't make the backside of a face invisible in your render, it will allow you to put two different 'looks' on either side of a material's face. For example, you could put a red diffuse on the frontface and a blue diffuse on the backface. Or, in this instance, emit light from a single-side and nothing from the backside.
Check out our quick-tip video tutorial on how to set this up (it's 5 minutes long). Or if you just want to see the node setup, check out the example images below!
Watch the video!