Overview of Materials in Cycles - Blender 2.6 Cycles: Materials and Textures - page 34

4. Switch the Emission node output from the first Shader input socket
of the Mix Shader node to the second one, as shown in the following
screenshot:
5. Save the file as 1301OS_01_meshlight_final.blend .
The plane is now emitting light only in one direction.
How it works...
We have already seen that in a Mix Shader node the first (upper) green Shader in-
put socket is considered equal to a 0 value, while the second one to a 1 value. So,
the Backfacing output of the Geometry node is stating Cycles to make the plane
emit light only in the face-normal direction, and to keep black and non-emitting (just
like a blank shader) the opposite "backfacing" side of the plane.
By switching the Emission node connection to the first Mix Shader input sockets,
it's obviously possible to invert the direction of the light emission.