The general all-purpose answer to your question is: The Emissive Shader.
The cheap-and-quick way to get wall-to-wall interior lighting is:
1. create a primitive plane - Create --> New Primitive. The default should be Plane (a flat 2d surface). Drop that into the scene. Expand it to roughly the size of your room, and raise it to near the ceiling.
2. The basic plane does not have the attributes we have so we have to add them.
a. Click on the Surface Selection Tool
b. Select the plane
c. Go to Surfaces - Editor and you can see all the attributes. We're going to change those.
d. Go to Presets - Shaders. Expand Shaders. Go down to iRay
e. Select the iRay shaders. Apply 'iRay Uber Base' to the plane.
3. Once we go back to the editor, we see a huge new set of attributes. Go down to 'Emission Color'. It will be black. Changing it to anything else causes that surface to emit light. Change it to White. This also adds a further set of new attributes.
a. Specifically, you're going to be concerned with: Emission Temperature, Two Sided Light, Luminance, and Luminance Units.
4. Keep the default temperature for now. The basics: the higher the number, the more blue it adds. The lower, the more red.
5. Turn Two Sided Light off. We only want one side of the primitive to emit light.
6. Luminance is the strength of the light emitted by the plane. It's going to default to 1500 or so.
This is where things get tricky and finicky. The light entirely depends on how much area is being lit. Having a two-hundred foot ceiling is very different from having an 8-foot ceiling.
For now, keep it at the default. This is where you're going to be doing a lot of back-and-forth as you test things.
7. Luminance Units is also an attribute you might fool with a LOT. For now, though, leave it alone.
Render the scene. Light should now flood the area because you have this massive plane emitting light like a giant ceiling light. It's probably too much light, in fact, washing out some details.
But that's where the back-and-forth comes in.
Change Luminance up or down. Change the color - make it more grey to 'blunt' the effect a bit.
I highly suggest Sickleyield's tutorials.
https://www.youtube.com/..._yhVozEcxG8O&index=2 https://www.deviantart.com/sickleyieldhttps://www.youtube.com/watch?v=L5FZ5gS9v50