How Under-Display Fingerprint Sensors Actually Work — Optical vs. Ultrasonic Explained

Tapping your thumb on a blank stretch of screen and watching your phone unlock in under a second feels almost magical. But there is real, clever engineering happening beneath that glass, and understanding it changes how you think about your device every time you pick it up.

Under-display fingerprint sensors — often called UDFS or in-display fingerprint sensors — come in two distinct flavors: optical and ultrasonic. They share the same goal but take fundamentally different paths to reach it.

Optical sensors work with light. When you press your finger to the screen, the display itself — almost always an OLED panel, which can illuminate individual pixels — flashes a burst of light directly beneath your fingertip. That light bounces back off the ridges and valleys of your fingerprint. A photodetector array sitting below the panel captures the reflected pattern, and the processor compares that captured image against the encrypted template stored on the device. Ridges reflect light more strongly than valleys, giving the sensor enough contrast to build a reliable map. The process happens in milliseconds.

The limitation of optical sensors is straightforward: they are essentially cameras reading a lit image. A high-quality photograph of a fingerprint can, under certain conditions, fool them, which is why optical sensors tend to appear in mid-range devices where the security trade-off is considered acceptable for the price point.

Ultrasonic sensors take a more sophisticated approach. Instead of light, they fire high-frequency sound pulses — inaudible to humans — into your fingertip. Those pulses penetrate the outer layer of skin and bounce back at different speeds depending on whether they hit a ridge or a valley. Because sound behaves differently through living tissue than through a flat surface, ultrasonic sensors can construct a three-dimensional depth map of your fingerprint rather than a two-dimensional image. This makes them significantly harder to spoof with a photograph and means they can read through water, light sweat, or minor screen grime where optical sensors sometimes struggle.

The trade-off is cost and complexity. Ultrasonic modules require piezoelectric transducers — materials that convert electrical signals into sound and back again — and the processing overhead to interpret returning wave data is higher. That is why ultrasonic UDFS technology has historically appeared in flagship-tier devices.

Both types store your fingerprint template in a secure enclave, a protected partition of the main processor that keeps biometric data isolated from the operating system and apps. Neither type transmits raw fingerprint data externally — your template never leaves the device, which matters for privacy.

As display technology and sensor miniaturization continue to improve, the gap between optical and ultrasonic performance is narrowing. Keeping an eye on which sensor type a phone uses before you buy is still worth doing — and now you have the knowledge to read between the lines of any spec sheet.