In-Display Fingerprint Sensor Explained: Optical vs Ultrasonic and Which Is Actually Better
How in-display fingerprint sensors read a fingerprint through glass, the real difference between optical and ultrasonic sensors, and why some unlock faster or more securely than others.
The in-display fingerprint sensor quietly became one of the more impressive pieces of everyday phone engineering, letting a phone read a fingerprint straight through a solid sheet of glass with no visible sensor or button. Not all in-display sensors work the same way, though, and the technology behind one phone’s sensor can genuinely explain why it unlocks faster, more reliably, or more securely than another.
Optical Fingerprint Sensors: The More Common Approach
Optical in-display sensors work by using a tiny camera positioned beneath the display to photograph the fingerprint’s ridges and valleys, illuminated briefly by light from the screen itself (which is why the unlock area glows when a finger touches it). This method is cheaper to manufacture and has become the standard on most midrange and many flagship Android phones. Optical sensors have improved substantially in speed and accuracy over their early implementations, but they generally perform less reliably when a finger is wet, dirty, or pressed too lightly, since the sensor relies on capturing a clear optical image.
Ultrasonic Fingerprint Sensors: More Secure, More Expensive
Ultrasonic in-display sensors, most notably used across Samsung’s Galaxy S and Note flagship lines, work fundamentally differently. Instead of capturing an image, they send out ultrasonic sound waves that bounce back to create a three-dimensional map of the fingerprint’s ridge pattern, including some depth information that optical sensors can’t capture. This 3D mapping makes ultrasonic sensors meaningfully more resistant to being fooled by a photograph or a simple mold of a fingerprint, and it generally handles wet or slightly dirty fingers better than optical sensors, since sound waves aren’t blocked by moisture the way light can be.
The trade-off is cost and sensor size. Ultrasonic sensors are more expensive to manufacture and historically required a larger sensing area than optical sensors, which is part of why they’ve mostly stayed limited to flagship phones from a small number of manufacturers rather than becoming the industry default.
Why Some In-Display Sensors Feel Faster Than Others
Beyond the underlying sensor technology, unlock speed depends heavily on the software processing the fingerprint data and how much of the display area the sensor covers. A larger sensing area makes it easier to register a proper fingerprint placement on the first try, reducing failed unlock attempts, while more efficient processing shortens the time between a finger touching the screen and the phone actually unlocking. This is why two phones using the same underlying sensor type can still feel noticeably different in daily use.
Security Considerations Worth Knowing
No consumer fingerprint sensor, in-display or otherwise, is marketed as a substitute for strong device security on its own, and biometric authentication is generally recommended as one layer alongside a strong PIN or password rather than a complete replacement for one. In several jurisdictions, legal protections around compelled unlocking also differ between biometric authentication and a memorized passcode, which is a practical consideration some security-conscious users factor into how they configure their phone’s lock screen.
Bottom Line
In-display fingerprint sensors have become reliable enough that most users barely think about the technology working underneath their thumb, but the underlying method still matters. Ultrasonic sensors generally offer stronger security and better performance with wet or dirty fingers, while optical sensors remain more common thanks to lower manufacturing cost and have narrowed much of the early performance gap. Either technology, when well implemented, offers a meaningfully more convenient unlock experience than the physical fingerprint buttons phones used before.
Sources
- Manufacturer fingerprint sensor technical briefings (Qualcomm, Samsung)
- Independent biometric security testing from major tech outlets
- FIDO Alliance, biometric authentication standards documentation
- Legal analysis of biometric vs passcode device-unlock protections
