This article mainly introduces the most important part of the fingerprint recognition hardware ~ the sensor. In fact, sensors and chips also need conform to the basic principles of physics. We can understand the classification and implementation of mainstream fingerprint recognition sensors from the aspects of optics, electricity, sound, heat and force. JUKA Optical can customize and manufacture optical prism, optical lens or other optical components for optical fingerprint sensor.

1.Optical Sensor

The history of the development of optical sensors is probably the longest, and it has been 30 years. The principle of implementation is also the simplest.

It mainly uses the principle of light refraction and reflection. Place your finger on the optical lens. The finger is illuminated by the built-in light source, and the light is emitted from the bottom to the optical prism and emitted through the prism. The angle of light refracted on the uneven lines of the fingerprint on the finger surface and the brightness of the light reflected back will be different. A prism is used to project it onto the CMOS or CCD on the charge-coupled device, thereby forming ridge lines and valley lines. The ridge line is black and the valley line is white. Then form a multi-gray fingerprint image that can be processed by the fingerprint device algorithm.

The advantages of the optical fingerprint sensor are as follows:
1. Strong antistatic ability, good system stability and long service life;
2. High sensitivity;
3. It can provide high-resolution fingerprint images (can reach 500 dpi).

The disadvantages of optical fingerprint sensor are also obvious:
1. Potential fingerprints (multiple presses) will reduce the quality of the fingerprint image;
2. The platen coating and CCD array will cause loss over time, which may cause the quality of the fingerprint image collected to decline;
3. Larger size and poor power consumption control.

Application of optical fingerprint sensor:
In view of the relatively large volume of optical sensors, its application areas are mainly focused on fingerprint door locks, safes, and car fingerprint anti-theft.

Development of Optical Fingerprint Sensor:
Optical sensors still occupy a large market share in industries where traditional requirements for volume and power consumption are not critical. But it is lost in the big market of mobile phones and other mobile terminals.

Previously, Synaptics released the new optical sensor FS9100, mainly because of the penetrability of the optical sensor (it can penetrate about 1mm of glass). And with the general trend of fingerprint recognition in In Display screens, optical sensors will inevitably make a comeback with new technologies.

2.Capacitance Sensor
Capacitive sensors should be the most popular sensors in mobile phones and portable terminal equipment applications.

The principle is to integrate the capacitive sensor in a chip. When the fingerprint is pressed against the surface of the chip, the internal capacitance sensor will generate a fingerprint image according to the difference in charge generated by the peak and valley of the fingerprint.

Advantage:
1. Chips and modules can be made very thin
2. Power consumption is easier to control

Disadvantages:
1. The penetration rate is relatively low
2. The price is relatively high
3. Waterproof and antifouling are not strong

3.RF / Ultrasonic sensor
The principle of a radio frequency sensor is similar to a sonar that detects seabed material. It relies on the signal reflection of a specific frequency to detect the specific form of the fingerprint. The biggest advantage of this type of fingerprint module is that the finger does not need to be in contact with the fingerprint module, so it will not affect the appearance of the mobile phone too much. Based on this, the RF fingerprint module has also become one of the main development directions of fingerprint identification in the future.

The principle of ultrasonic testing is the same. It is to detect the difference between the ridge and the valley by the time difference of the reflected wave. According to this distance difference, a fingerprint image is drawn.

4.Mechanical / Thermal Sensor
Force transmission requires a harder conductive carrier, but if the carrier is harder, the difference in fingerprint distance is so small that the image formed will be blurred; the thermal sensor has also been developed, but the thermal sensor is subject to external temperature. The time interval between two presses is too large to meet the requirements of consumer electronics. Therefore, these two sensors are hidden in the long river of time.

If you need to customize optical prism, optical lens or other optical components for optical fingerprint sensor, please contact JUKA Optical.