Light layers allow you to separate contributions from different lights onto different "layers". Each layer can then be manipulated separately, even after the render has completed.
You can change the brightness of each layer, or change the overall colour of each layer, or even turn each layer off completely.
Enabling Light Layers
By default, all lights in an Indigo scene are assigned to Layer 0. This means that the HDR image will have only one layer – Layer 0. However, in the exporter for your 3D modelling program, you can change the layer that a light is assigned to. All contributions from that light will then be rendered onto that layer.
Each layer has a number of controls that you can manipulate in the Indigo GUI; please see the corresponding section in the Indigo interface section.