I've been thinking a lot about the under-display fingerprint scanning technology lately. I know it's divided into two types: optical and ultrasonic, but the details are a bit fuzzy. Specifically, I'm curious about how optical sensors, which are placed behind the screen, use light to detect fingerprints. Does the ultrasonic version work with sound waves? If you have any detailed explanations or source recommendations, I'd really appreciate it!
How does in-display fingerprint technology work?
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The foundation of in-display fingerprint technology is actually based on a kind of "image capture" principle, especially for optical sensors this is more evident. In optical sensors, LEDs (often using red or infrared light) illuminate the finger. The ridges and valleys (depth differences) on the finger's surface cause light to reflect in different ways. The thin-layered light sensors located beneath the sensor capture these reflections and convert them into a digital image. The critical point here is the number of layers of the light sensors. If the sensor beneath the screen is single-layered, depth perception is weak, which can lead to poor fingerprint capture. Therefore, understanding how advanced the sensor is and how light transmittance is optimized is important.
As for ultrasonic sensors, the principle here is the use of sound waves, but this isn't just simple "reflection." Active ultrasonic sensors (such as Qualcomm's 3D Sonic Sensors) send high-frequency sound waves that penetrate deep into the finger. Different textures of the finger's skin reflect sound waves differently, and the sensor collects these echoes to create a 3D map. The advantage is that sound waves can distinguish between different materials (scratches, screen, fingerprint oils, etc.). The disadvantage, however, is that the sensor includes both components that send and receive sound waves, making it more costly and larger in size compared to optical sensors. So, there's a balance in the market in terms of both depth and cost.
However, it's worth criticizing that these technologies cannot completely solve the problem of "fake fingerprints." Optical sensors often capture a simple photo-like image, making them easily fooled by high-quality photos. While ultrasonic sensors are somewhat more resistant, they can still be bypassed with fake fingerprints made from materials like latex or molds. So, while the security level increases, a "cost-effective" solution needs to be found. So, do you think the future of biometric security in mobile devices lies in other technologies?