Can you post a picture of these "rings" please?
As far as I know there are no caps behind the panel. There should be LED strips that provide the back light. These LEDs can fail, but when that happens, usually the entire strip or sometimes the entire back light will fail. Depending on how the strips are powered, the LEDs on the strip are connected in series which means that if one fails, the entire strip will be effected. Some sets power each strip while others have all strips in series. From your description, it sounds like one LED (or 5 if there are 5 "rings") is brighter than the rest. This may indicate an LED that is starting to fail.
A picture will help. In any case, either replacing the strip or the individual LED may resolve the problem.
Dan