how to restore photos from icloud photo library?

Hi all,

I just reset my iphone (iOS 8) to the facroty setting. Before doing that, I backed up all the data to icloud and all photos to icloud photo library (I chose the "optimize Iphone storage" ).

I have successfully backed up all the data from icloud but could not get all the photos which previously saved on this iphone back from the icloud photo library. The photo album is completely empty on the phone. How could I restroe these photos from icloud photo library?