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Enable Caustics – Turns the rendering of caustics on and off.

New – Causes a new photon map to be generated. The new map overwrites any previous photon map left over from a previous rendering.
From File – V-Ray does not compute the photon map but loads it from a file.
Progressive – The Progressive method doesn't need most of the controls used for traditional Photon Mapping, so the controls are grayed out.

Multiplier – [Available only for New mode] Controls the strength of the caustics. It is global and applies to all light sources that generate caustics. If you want different multipliers for the different light sources, use local light settings. 

Note: This multiplier is cumulative with the multipliers in the local light settings.

Search Distance – [Available only for New mode] When Mode is set to Progressive, this parameter specifies the initial photon lookup radius in pixels. When Mode is set to New or From File, the Search Distance specifies the radius of search area circle. When V-Ray needs to render the caustics effect at a given surface point, it searches for a number photons on that surface in the area surrounding the shaded point (search area). The search area is a circle with its center at the original photon and a radius equal to the value. Smaller values produce sharper, but perhaps noisier caustics; larger values produce smoother but blurrier caustics. For more information, see the Search Distance Parameter (Progressive) and the Search Distance Parameter (Photon Map) examples below. 

Max Photons – [Available only for New mode] Specifies the maximum number of photons within the search area circle that are considered when rendering the caustics effect on a surface. Smaller values cause less photons to be used and the caustics are sharper, but perhaps noisier. Larger values produce smoother but blurrier caustics. The special value of 0 means that V-Ray uses all the photons that it can find inside the search area. For more information, see the Max Photons Parameter example below or the VRaySun Photon Radius example.

Max Density – [Available only for New mode] Limits the resolution (and thus the memory) of the caustics photon map. Whenever V-Ray needs to store a new photon in the caustics photon map, it first looks if there are any other photons within a distance specified by Max density. If there is already a suitable photon in the map, V-Ray just adds the energy of the new photon to the one in the map. Otherwise, V-Ray stores the new photon in the photon map. Using this options allows you to shoot many photons (and thus get smoother results) while keeping the size of the caustics photon map manageable.

File – [Available only for From File mode] File name with the caustics photon map to be loaded when Mode is set to From File.

Don't Delete from Memory – When enabled, V-Ray keeps the photon map in memory after the scene has finished rendering. Otherwise, the map is deleted, so that the memory it takes up is freed. This option can be especially useful if you want to compute the photon map for a particular scene only once and then reuse it for further rendering.

Auto Save – [Available only for New and Progressive modes] When enabled, V-Ray automatically saves the caustics photon map to the file specified in the Auto save file field when rendering is complete.

Auto Save File – [Available only for New and Progressive modes] Specifies the file to which the caustics photon map is saved when Auto save is enabled. Click the Browse button to the right to specify the file name.

Show Calculation Phase – [Available only for New mode] When enabled, displays the calculation of the caustics map.