Let's begin this simple tutorial I've written not so long time ago about Outdoor Illumination in mental Ray.What you see below is the final render of what we need to achieve with this tutorial.

Step1 - a simple scene

Modelling the pottery of the scene

It is very important to have some simple stuff on the scene so you can look on how Global Illumination and Fg work on your surfaces and their shaders .For this little tutorial I created some potteries,which are very easy to make but also smooth enough to let you all see
how the lights and shadows work.I assume you to have some modelling skills so I won't get into pottery explanation(just create simple Nurbs Geometry with the Revolving Tool...then convert it to poly and tweak it a little bit more....but as long as we are talking about Illumination we shouldn't really care about how objects look like...)
You need to arrange the pottery with 3 nurbs planes like in the picture.that's Easy!Isn't it?

some sample of the pottery

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arrange the scene in this way

Step 2 - The lights approach
One of the most popular way to illuminate 3d models is the three-point-lighting method,wich gives your scene a fully-thridimensional form.The three point lighting method is a very known scheme also in Hollywood movies.In this tutorial i'd like to explain you how to achieve the same kind of effect with Golbal illumination and Final gather point in mental ray.
The three-point-lighting is basically a way to 'reveal' your models...not only to lit them brightly;But as long as we are talking about Gi and Fg we don't need to care about having three lights.We can work just with one,(in this tutorial two are better).Global Illumination and Final Gather point are some very usefull and powerfull features of mental ray,they can provide very realistic rendering based on specific phisical photon emissions.
In Maya only spotlights and points light can emitt photons...so in this tut we will work with them,discovering some usefull things about how to save photons maps so that our rendering time will be shorter.