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Intro

This part is all about interior lighting workflow, where and which lamps to place, choosing colors, balancing etc. I will also try to show how to use composite nodes to get the desired effect (simply because You cant get it otherwise...).
I'm going to show You the basic workflow of preparing a day time lighting in Blender Internal and, as a bonus, same in Yafaray. Here we go!

Setup

As I said in the previous part You always have to answer one simple question: where is the scene illuminated from? For most bright day renders there are 2 most important light sources, sun and sky. For the direct sun light illumination we're going to use the standard sun lamp and for sun light bounces I use AO as it's a very simple and reliable method. Lastly for the sky we'll use area lights in all sorts of windows, they will act like portals for the sky dome light rays.

Sun light

That's probably the easiest to set up, but giving it the right direction can make the scene more interesting. I always try to make it illuminate some furniture to give more depth to the image. Another thing to remember is that there are no point lights in real life so You should give the sun some size, the lower the sun the softer shadows should be. (note: to get soft shadows a light has to have samples above 1)
For a finishing touch i also add an extra area light from the sun's direction to get that extra sun light scathered in the atmosphere.

Skydome

The standard, most popular way to get skydome illumintation is to put sky colored area lights in windows. This way we create portals for the sky illumination to our scene. This technique is especially important while using photon map based renderers (such as vray, mentalray, yafaray), as these lights become the primary source of photons bouncing in our scene (but more on that later...).
For blender internal I tend to make these area lights slightly bigger than the window frame and also offset the light a bit outward, this way the window frame itself is illuminated evenly (if You put the light right behind the window frame it will look as if the frame was lit by a point light).
Important note: Do not use scale while You fit the light in the window frame. Area light's energy is multiplied by the light's scale, so I advise to use the size property instead.

Indirect Illumination

Obviously, we're going to use Ambient Occlussion for (faking) indirect illumination, as it's the the only option in Blender Internal (for now). I decided to go with quite a generic AO setup with falloff distance of 1 meter (the scene was modelled so that 1 blender unit = 1 meter) and strength of 1. It's really a matter of trial and error. We want to get fairly long AO shadows, otherwise the scene will just get a constant light added and the sharp shadows will make the scene look unnatural. We also want AO to compute fast so the distance has to be limited, with added falloff to make the transition more realistic.
As for the AO color I always use Add method, because it's just another light source, plus Sky Color set to desaturated sun color. The scene will get a nice blue fill from the sky (area lights in the windows) so what we really need is the sun light bounce.

Light Balance

For now I haven't written anything about the energy/power of specific lights and that's for a good reason: Most likely, after You added all the lights to the scene, You would have to redo it anyways ;). We need to adjust all the lights, more or less, at the same time to:

• avoid overexposure
• get the right color balance
• control the shadow's brightness

Now at this point You have to keep in mind that we will brighten up the Blender Internal render with a gamma node (value of about 0.5). This is important for a couple of reasons:

• higher gamma means less contrast - which is a good thing, we don't have to worry that the shadows are too dark now
• higher gamma means less saturation - we will need to account for that with oversaturating light colors
• most likely we will have to tweak the light balance while working with composition nodes

All of that means that You shouldn't try to make the balance perfect at this moment, just get to something that looks allright.
As for the light energy, I start with area lights at 0.5, sun at 1.0 and AO at about 0.75. A couple of tips how to get the right balance:

• the point's where sun light meets the floor should be close to white, slightly yellow
• walls should get a bit of sky blue color, they shouldn't be gray, control this with the saturation and power of window area lights.
• the overall color should be slight yellow from the sun light, control it with both AO saturation and power.
• if the shadows are too dark, brighten up AO
• if the shadows are too sharp, adjust AO's falloff distance and strength

Environment

As I've already used the Sky Color for AO and I don't want to have a yellow sky outside the window, I put a simple texture to the environment to make the sky blue. You could also create a gradient that would look more like a real sky, but it's very likely that You will replace it with an image background in post production anyway.

Composition

This is the part where we can finally fix some big issues, most importantly fix the gamma. Now the scene is too dark, because the gamma is very low (1.0, the standard's are 1.8, 2.0, 2.2, so much higher). Add the gamma node at about 0.5 and everything should look a lot better, not perfect though. As said before increasing the gamma will make the scene loose contrast and saturation, so I add an RGB Curves node to increase the contrast a bit and a HSV node to fix the saturation. For a final touch I added another RGB Curve for color correction and make the render a bit warmer by adjusting the red color curve.
This looks like a very short part, but I actually spent more time on this than on the initial setup. It's because now You should also focus on adjusting the light balance to fit the compositing and the final render. As with most of the process it's trial and error, but then it wouldn't be so much fun otherwise ;)
And here it is!

The final render

And now for something completely different...

... not really. The workflow for Yafaray is almost the same. There's just that tiny, little difference: we can drop Ambient Occlussion and use Photon Mapping instead (actually Photon Mapping and Final Gathering, but more on that later...). This makes a big difference, because:

• Photon Mapping is much more physically accurate = realistic
• we get much softer lighting and shadows
• we don't have to manually set up the bouncing light (energy and color)

And thanks to other yafaray features:

• we don't have to apply as much post production as we did with BI
• getting the right color balance is easier (no AO to adjust, less post pro)
• better area light handling (eg. scale doesn't affect energy)
• faster render times (this might change, BI raytracer is being optimized as I'm writing this tutorial)
• ... and more, which i can't recall just now ;)

The only problem is that yafaray, like any other renderer, isn't fully integrated with blender, so:

• there's lack of support for material/texture nodes
• there's poor integration with the compositor
• You need to use the yafaray's exporter to set up materials, lights and so on.
• You need to use the exporter's render button instead of the standard blender Render button
• yafaray uses a separate render window, which freezes blender, till the window is closed.

To sum up, I encourage everyone to try out Yafaray. It's a very good renderer for all kinds of visualisation purposes.
Ok, now back to the scene...

Setup

As I said, it doesn't change too much. If Your area lights were off set from the window frame, it's time to get rid of the gap and resize the lights so they fit the window frame. This is because lights will emit photons from their area, so any overlapping places will produce photons which will never reach our scene.
Another thing to fix is the light saturation, You could probably cut it by half.
And that's all, standard setup, area lights in the windows + sun light.

Indirect Illumination

Now the boring part...

... to be continued ...

The final Yafaray render