Lights Overrides

UI Path: ||Render Settings|| > V-Ray > Overrides tab > Lights rollout

Use Lights – Enables lights globally. Note: Disabling this causes V-Ray to use the default lights. If you do not want any direct lighting in your scene, you must disable both this and the Use Default Lights override parameter. 

Use Hidden Lights – Determines whether or not lights marked as hidden (i.e. not Visible) are exported to V-Ray. 

Use Default Lights – Enables the usage of the default lights when there are no light objects in the scene or when you have disabled lighting globally.

Shadows – Enables shadows globally. 

Disable Self-illumination – Disables the rendering of self-illuminated objects.

Light Evaluation Mode – Determines how lights are sampled in scenes with many lights.

Full Evaluation – V-Ray goes through each scene light and evaluates it at each shading point. In scenes with many lights and lots of GI bounces, this leads to a lot of shadow rays being traced and rendering can become extremely slow. When GPU rendering, this introduces probabilistic light calculations but does not allow the Number of lights parameter to be set. 
Light Tree – V-Ray selects a variable number of suitable finite light sources based on proximity and evaluates only those. This mode may speed up the Light Cache and Brute Force calculations for scenes with lots of lights. It can also reduce flickering due to missed light sources. 
Adaptive – Uses information from the Light cache to determine which lights to sample. If Light Cache is not used, uniform sampling is used. Depending on the scene, it can be faster than the Full Evaluation mode.

Number of Lights – Number of lights from the scene that are evaluated by V-Ray when the Light Evaluation parameter is Adaptive. To achieve a positive effect from probabilistic light sampling, this value must be lower than the actual number of lights in the scene. Lower values make the rendering faster, but the result is more noisy. Higher values cause more lights to be computed at each hit point, thus producing less noise but increasing render times.