The Dimensional Frequency Scanner is a two piece hand held device used by Egon Spengler to pinpoint the location through a being's unique frequency.
Some days after he saw his long thought dead college friend Eugene Visitor, Egon returned to the crosswalk near Delgado's Gym. Seeing Eugene during an ongoing surge in the ambient P.K.E. field drew his suspicion. Egon took out the Dimensional Frequency Scanner and started taking readings. For 20 minutes, he only got several inconclusive scans. He finally got a ping. After poking an elderly man in the shoulder, Egon was shocked by what the scanner picked up. He returned to the Firehouse and drove Ecto-1a to Belleville, New Jersey. He verified that Eugene was alive and learned it was due to capturing Death in his backpack.
Egon used the scanner on Abby Yates, to her annoyance, as she declared her team was the Ghostbusters and were in operation for well over a year. She stopped and asked what he was doing. Egon revealed her dimensional frequency was in a pronounced state of flux.  She denied it. Ray Stantz clarified it was part and parcel of how they dealt with travelers from alternate dimensions.
- A component of the scanner is based on Doc Brown's stopwatch in "Back to the Future"