The first step in creating a global illumination or caustic effect is to define a light as a source for photon rays. To generate photons using a particular light source, you must specify the number of photons to be generated by this light source and the energy being distributed into the scene.
To define a photon light source
Select the light you want to use as the global illumination or caustic source. Photons can be emitted from point lights, spotlights, or area lights.
Choose Modify 
Shader from the Render toolbar to display the selected light's property editor.
On the Photon tab, display all of the light's photon properties.
Deselect Light Falloff while setting up the light source. You can select it again later.
Define the light as a Global Illumination source or a Caustic source (or both) by selecting the appropriate option.
Define a custom Color for the global illumination or caustic light by using the color sliders or the Color Editor (click the color box beside the light color sliders).
For more information on defining colors, see Defining Color Properties [Interface and Tools].
Set the caustic energy Intensity for the light. Intensity is the option that creates the caustic effect. You can tune the brightness of caustics to make them either more or less visible by setting a value from 10 000 to 1 000 000.
Define the number of Emitted Photons for your caustic or global illumination effect using this slider. Keep in mind that the higher this value is, the longer it takes to render.
This value specifies the number of photons in the Photon Map (see Using Photon Maps), so it's a good indication of the quality of the generated caustics. It is also a direct indication of the memory usage, which is proportional to the number of photons in the Photon Map.
The photon effect is not be visible just yet. You must define receivers and casters and then enable photons in the render region. See the next section, Setting Photon Casting, Receiving and Visibility, for the next step.
