I found this article
https://forums.lenovo.com/t5/Lenovo-Yoga-Series-Notebooks/Yoga-2-Pro-Auto-brightness-feature-and-how-to-disable-in-Windows/ta-p/1294763
I don't know how much applies as I don't know which version of windows your using (this guide is for 8.1), but essentially I think your looking for the "Adjust my screen brightness automatically", or something similar.
Let me know if this helps.