A “persistent sexual offender” was sentenced Thursday to more than four years in prison.

Robert C. Peterson, 64, pleaded no contest in April to attempted sexual exploitation of a child.

The case stemmed from an incident in August 2006, when police were called to a Lawrence business. Co-workers reported they saw Peterson viewing child pornography on his work computer and the images from his monitor were reflecting onto a glass surface near his desk.

Judge Paula Martin sentenced Peterson on Thursday to 52 months in prison.

Douglas County District Attorney Charles Branson said because Peterson had a prior offense, by statute he’s classified as a persistent sexual offender and will be subject to lifetime supervision and registration.