Lighting & Rendering: Linear Workflow

What is 'Linear Workflow'?

Linear workflow ensures that all of the elements interacting in a scene (light colours, textures and shader colours) correspond linearly with actual amounts of light so that the renderer does proper light computations in a physically accurate way. This is needed since light works linearly in the physical world (if you double the amount of photons, the colour values are also doubled). 

Lecture Notes