Direct light is the light we see in our scene without Global Illumination turned on. GI will trace the light and make it bounce between surfaces to give a more real-life light behavior.
The thing that makes GI work is the use of light photons that is emitted from the light source and travel through the scene, illuminating the surfaces by Gathering information about color and luminance on the surfaces they come in contact with. This information is picked up and then the photons bounce to the next surface repeating the work.
Choosing the light type to use is important, as it will affect the way photons work within it. Generally you can think that the photon emission will follow the light emission for each light type.
There are two types of photons in Mental Ray, the first being Global Illumination and the second is Caustics.
The photon intensity setting of the light refers to the light-energy used for the photons when they leave the light source. The default value in Maya is set to 8000, which is pretty intense and often leads to blown out renders when used with small scenes.
The Exponent setting determines how fast the photons will lose their energy as they travel through space.
High values give shorter life so they lose energy faster.
Keep in mind that all these settings are calculated on values that exists in the sizes of objects and the size of your scene in the Units setting in Maya.
So if you build a scene that is many units in size this would need totally different settings in Mental Ray than if your scene is only a few units in size.
When using GI in your lights it is easy to forget that you also need to consider the direct light property, so that the direct light intensity is adjusted up or down depending on
the light you are trying to mimic. These are independent settings and can drastically alter your overall lighting!
There are many settings to control in the Indirect Lighting tab of Mental Ray. I will continue to post tips and tricks on lighting and rendering with Mental Ray in the time to come.