If your 3d artist and hungry its a must to check this out: Understanding Light Select Render Element In Vray For 3ds Max.
The Light Select Render Element represents the lighting contribution from one or more selected lights in the scene. Each Light Select channel can output selected lights’ raw, diffuse, or specular contributions to the illumination, or overall (normal) contribution. When multiple lights are selected, all the contributions from the selected lights are combined into a single render element. Multiple VRayLightSelect elements can be rendered for a single scene, and lights may be included in more than one VRayLightSelect element.
This element is similar to the Lighting Render Element. However, the Lighting element combines the effect of all lights in the scene while the Light Select element allows for a user-selection light or set of lights to be broken out, showing their own individual effect(s) on the scene’s illumination. By using these render elements, specific lights in the resulting render can be adjusted (color, temperature, brightness, etc.) in a composite without the need for re-rendering.
For example, by generating a Light Select element for all of the backlights in a scene, an artist may adjust the backlighting of the rendered scene easily in the composite without affecting the rest of the scene’s illumination.
The following diagram shows the compositing formula to recreate all the light in a scene in its most basic form, but only when each light in the scene is accounted for in exactly one VRayLightSelect element for a particular mode. If a particular light is used in more than one VRayLightSelect element, this equation will result in brighter lighting than intended because that light will contribute lighting more than once.
- When using VRayLightSelect, a good practice is to have one VRayLightSelect element for each light source in your scene. This way the Lighting element can be recreated from them, and adjusted as needed while compositing without re-rendering.
- If the VRayLightSelect element mode is set to Direct illumination, the specular contribution is added to the render element. While this is perfect for simple compositing, a better workflow is for each light to be selected for two VRayLightSelect Render Elements, one with its mode set to Direct diffuse and another with its mode set to Direct specular. In this way, the specular and diffuse lighting can both be controlled independently at a composite level.
Source Credits to: Chaosgroup