Hello, I’m sorry for posting again. I decided to add a bit of fog with Volume scatter, and the result in the 3D view in rendered mode is really satisfying to me :
However, the final render on Cycles is completely different 
You can barely see the lamp at all, and I increased the strength of the lamp to 10 000, and it still wasn’t enough. I’m pretty sure Volume Scatter is the origin of this issue, but I don’t know how I could fix it.
Your help would be greatly appreciated. (I actually have to finish this project in 2 days…) Thank you in advance for your answers !