updated 03:54 pm EST, Thu November 21, 2013
Document kept out of public eye until after 5s launch
The US Patent and Trademark Office has published an Apple patent application for the technology inside its Touch ID fingerprint sensor. The document is titled Capacitive Sensor Packaging, and was originally submitted on March 15th. Like a number of Apple filings, though, it appears to have been kept secret until after the product associated with the technology launched.
The application describes a sensor close to the final implementation of Touch ID, including even the use of a metal ring and sapphire. Some unusual aspects include a process for turning a sensor lens opaque, so users don't see any underlying parts, and alternate methods of proximity detection, like optical and infrared sensors. Apple suggests that asking a person to place their fingerprint within a specific space restricts "the design flexibility for the fingerprint recognition sensor."
Another cited possibility is embedding a sensor behind a display, something covered in a separate patent filing published this June. That may imply that Apple meant Touch ID to be built into the 5s' display, or simply that it was one of several options on the table. The concept could be revisited in future iOS devices.