Doctor Clendenning was a Human scientist and resident of the Omicron Theta colony before its destruction. His research involved new methods of detecting gamma radiation. He was one of many colonists whose experiences and memories were partially implanted into Data.
Clendenning was working on new detectors in 2338, when the Crystalline Entity attacked the colony and killed its inhabitants including Clendenning. By 2368, Kila Marr was familiar with his records, but had never read about the gamma scans. The knowledge of his work made it possible for the USS Enterprise-D to detect the antiprotons the Crystalline Entity left behind when travelling through space. (TNG: "Silicon Avatar")