How to make light ray with Luxcore I'm trying to make a realistic prism with Luxcore since yesterday but it's not really working, there's no ray! But I know that the prisme is working because if I put a plane in front of the prism I can see the light on it 
How could I do that ?
In the World tab you can add a volume to the entire scene. This allows you to create fog which scatters light from your laser and thus makes it visible. You will likely need to adjust the Scattering Scale to the size of your scene, larger values result in stronger scattering.