Create/import a mesh. Set it's properties to not cast shadows, and tick the light emitter box.

Apply a material that boosts the render engine's response to ambient strength. I have a partial MT5 (it's only a group of nodes - add it to your material, don't replace your material) in a freebie Superfly material sampler.

Set Superfly render settings with about 15 mesh light samples and at least 4 diffuse bounces. Use branched path tracing (unless) GPU rendering.

If you want to look at pre-made Superfly lightcaster materials, try the Atlantis gate room, or some of the Superfly stuff I have on ShareCG - seachnasaigh's ShareCG

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5