In – Display Fingerprint Sensor

Did you ever rest your thumb (or finger) on the right part of your phone’s display, and unlock your phone?

• Almost all brands have introduced this technique in their cell phone models, to increase security from getting hacked.
• What this technology does is, It just adds an extra component in such devices which helps them sense your fingerprint or the fingerprint of the person who wants to use the device.

BIRTH of IN-DISPLAY FINGERPRINT SENSOR

• Looking into the history of fingerprint scanners in Mobile devices, we get to know that
  Pantech GI100 launched this technology back in 2004, Its fingerprint reader was the first of its kind.
• G900 and G500 of Toshiba in 2007.
• HTC, Acer, and Motorola joined later with their respective devices.
• Apple introduced this in 2013 with the iPhone 5s getting a fingerprint sensor.

TECHNOLOGIES

• Different technologies work in tracking fingerprints like optical scanning, capacitive scanning, or ultrasonic scanning.

Types of Fingerprint Scanner

• Optical Scanner (Used in In-display fingerprint sensors): Optical scanners are the oldest methods of fingerprint authentication. But, in-display optical sensors are comparatively new to smartphones.

• A charge-coupled device (CCD) is the heart of an optical sensor, it is used in digital cameras and camcorders.
• CCD is an array of light-sensitive diodes called photosites, which generates electrical signals in response to light photons.

• When you place your finger on the sensor, an array of light-emitting diodes (LEDs) light up to illuminate the ridges and gaps.
• The CCD camera captures its image and the CCD system generates an inverted image of the finger, i.e, darker areas(ridges) represent more reflected light and lighter(gaps or valleys) areas representing less reflected light.
• The image captured is then compared with the stored image.
• Ultrasonic Scanner (Used in In-display fingerprint scanners): these sensors make use of high-frequency ultrasonic sound to map fingerprints.
• These scanners lie beneath the display and waves from ultrasonic sensors travel through the display glass. It is advised not to add extra protection such as a screen protector.

The hardware

• Ultrasonic transmitter and receiver = Scanning process begin the moment the fingertip is placed on the sensor.
• The transmitter transmits an ultrasonic pulse which collides with the ridges and valleys on the fingertip, some of the applied pulse pressure is absorbed and some of it is bounced back to the sensor. Absorption and bounce back of the pulse vary with varying fingerprints.
• The sensor detects mechanical stress and calculates the intensity of the returning ultrasonic pulse at different points on the scanner.
• Ultrasonic sensors give a detailed replica of finger print, It is the costliest among other available technologies and not many devices use this.
• Samsung partnered with Qualcomm to bring the first device with an in-display ultrasonic fingerprint sensor the, Galaxy S10/S10+
• Capacitive Scanner are the most widely used sensors and can be found on every other device.
• The capacitors as the core component, for storing electrical energy.
• These sensors scan the ridges and valleys of fingerprints by using electric current instead of light to collect data. Fingerprint detail is collected by an array of capacitors that are placed below the scanning surface.

• Once a fingertip is placed on the scanning surface the charge stored on the capacitor changes and this difference in charge is tracked by an op-amp integrator circuit which is further recorded by an analog-to-digital converter.