LuxRender Light groups
From LuxRender Wiki
Light Groups is a feature that can greatly help you composite and evaluate the lighting in your scene. It allows you to bundle one or more light sources in groups and change their intensity and colour temperature during the rendering process.
LuxRender actually renders the scene using each light group and produces different images. LuxRender then provides controls for how much these images are mixed together.
Light groups are intended to help setting up the right mix of lights, but for any final renderings it is advisable to adjust the light intensity values correctly before starting the rendering. The reason for this is that LuxRender bases the importance of the various light sources on their initial brightness. If a light source is boosted a lot, this will increase the noise level in the image. Also adjustments to the colour temperature are an approximation and setting the colour temperature right from the beginning will result in a more accurate rendering.
The normal procedure for creating light groups is to select each light object (or mesh emitter) and assign it to a group. In the example shown below, we have a scene with lights from the sun, TV and some incandescent lamps, assigned to the groups "sun", "tv" and "spots". When exported and rendered, the light groups interface can be accessed by clicking the "LightGroups" tab.
When viewing the "LightGroups" tab in LuxGUI, you should see a panel for each light group in the scene.
Group: Each panel has a title bar. The Group field shows the name of the light group that you're working with.
On/Off Button: On the right of the title bar is the On/Off button, which turns the group on or off.
Minimize: This button minimizes the controls of the light groups to make the interface tidier.
Gain: The Gain slider controls the intensity of the lights in the current light group. Note of course that the image will be tone-mapped to accommodate the new Gain value. Also note that the Gain value does not control the actual light intensities in the scene file; it simply controls how much of the image containing the light group is mixed with the other images.
RGB: This check box allows you to change the color of the overall lights in the light group; click the white square box to choose a different color.
Black Body Temperature: This check box and slider also allow you to modify the color of the light group, this time with a temperature setting. Note that this slider is not the same as the one in the exporter; it adds to the effect. Think of it as a fine tuner for the lights' temperature.