Granted Patent
Utility
US 4,931,221 · App. 07/292,599
Photochromic spiropyran compounds
Inventor:
Harry G. Heller
Patented Case
View Patent ↗
Granted: Jun 5, 1990
Filed: Dec 30, 1988
Inventor
Harry G. Heller
Assignee (at issue)
PPG Industries, Inc.
This patent predates the USPTO's electronic file wrapper — prosecution documents from this era exist only in the Patent Office's paper records. A certified copy of the file history can be ordered from the USPTO's paper archives.