Kobo glo defect
I had this same problem on my Kobo glo. I emailed Kobo and convinced them to send me a replacement which had the same problem only worse. They probably sent me a refurbished one someone else had returned. I keep thinking how can the people at Kobo not know about this defect. It is impossible to see until you are in the exact lighting situation though. It only shows up under two different lights in my house so I just do not read in those places anymore. I would not buy another one even though I love my Kobo unless they acknowledge there was an issue and it is now corrected.