Another real world found prop, the Monitor 4 is another radiation detector, though in Ghostbusters 2 is used in measuring emotion. These regularly come up on ebay though are pricey, though on occasion you can snag a bargain with one. The Monitor 4 is used by Egon in his laboratory at the start of GB2 and can be seen on Ray’s belt during the montage sequence where they capture the jogger ghost.