Overcome technical problems, LCD screen can also be used for fingerprint identification

Fingerprint recognition and unlocking has become the default solution for most smartphones. However, the fingerprint collection window required for traditional fingerprint unlocking affects the screen ratio of mobile phones to a certain extent. As full-screen smartphones have become mainstream, more "invisible" off-screen fingerprint recognition has flooded the market.

A few days ago, according to data from CINNO Research research institutions, 2020 is expected to be the first year of mass production of fingerprint recognition mobile phones under LCD screens. Analysts predict that the annual shipment of smart phones equipped with this function may exceed 6 million units, and market acceptance It is expected to reach 190 million units in 2024.

What is under-screen fingerprint recognition, and why is this feature equipped with a common LCD screen becoming a hot topic this year? With these questions, a reporter from Science and Technology Daily interviewed relevant experts.

In the past, fingerprint recognition under an optical screen required an OLED screen

"Mobile phones with off-screen fingerprint recognition capabilities can complete the process of fingerprint recognition by directly pressing the designated area of the mobile phone screen." Sun Yanbiao, the dean of the First Mobile Phone Research Institute pointed out that this design can avoid the extra occupation of the screen panel by physical buttons Furthermore, the screen ratio of the mobile phone is increased, and the display performance of the mobile phone is improved.

It is understood that there are three main mainstream fingerprint recognition technologies, namely capacitive fingerprint recognition, optical fingerprint recognition and ultrasonic fingerprint recognition. So, which kind of fingerprint recognition technology is more "collocation" with the fingerprint unlocking under the screen?

"Ultrasonic fingerprint recognition has strong penetrability and can penetrate common mobile phone materials such as metal and glass. However, its current market share is low and research and development is difficult; capacitive fingerprint recognition technology is mature, but it is recognized when there are fingers wet. In case of malfunction, the recognition rate is low, and it cannot penetrate too thick things. "Li Junyi, senior director of R & D of Tianma Microelectronics, said that optical fingerprint recognition technology is more mature and faster. From the current technology and market perspective, optical fingerprint recognition is more suitable for solving the problem of fingerprint recognition under the screen of mobile phones.

The principle of optical fingerprint recognition technology is to use the refraction and reflection of light. When the finger presses the screen, the screen emits light to illuminate the finger area, and the reflected light that illuminates the fingerprint returns to the sensor close to the screen through the gap of the screen pixels. The fingerprint image obtained is the same as the image first entered by the mobile phone. Compare, and finally identify and judge. Fingerprint recognition under the optical screen can effectively avoid the interference of ambient light, and its stability is better in complex environments.

For a long time, the fingerprint recognition under the optical screen applied to the smart phone has required the cooperation of the OLED screen. Why can't the LCD screen be completed?

It turns out that optical fingerprint recognition has certain requirements on the distance between the sensor and the lens. "The traditional LCD screen has a complicated structure, and it requires a backlight module to provide backlight. It is this layer of backlight module that prevents light from penetrating the LCD screen, resulting in failure of fingerprint recognition under the screen. The OLED screen can emit light without backlight. The source is thinner and has better light transmittance. "Li Junyi said that this allows the fingerprint recognition sensor under the OLED screen to accurately capture the light emitted by the OLED screen after illuminating the user's fingerprint and then reflecting the light back, so as to realize the fingerprint smoothly Identify.

Infrared imaging helps low-cost LCD screens regain favor

"Although the OLED screen has excellent performance in fingerprint recognition under the optical screen, from the cost point of view, the LCD screen has lower production costs and a longer service life." Yanyan Biao said.

The functions and performance of smartphones are constantly being upgraded. LCD screens are one of the key components of smartphones. The introduction of new processes such as smaller apertures and high refresh rates are constantly increasing the added value of LCD screens, which has also promoted many The manufacturer began to develop the off-screen fingerprint recognition technology applicable to the LCD screen.

At the beginning of January this year, Tianma Microelectronics showed off-screen fingerprint technology display products based on LTPS LCD technology, with display, touch and fingerprint recognition functions at the US Consumer Electronics Show. A few days ago, according to the news released by Lu Weibing, general manager of the Redmi brand on Weibo, Redmi's R & D team has also implemented an off-screen fingerprint unlock on the LCD screen, and said it has mass production.

"Because of the existence of backlight, if LCD screen uses visible light to realize the under-screen optical fingerprint recognition, it will face the problems of poor penetration and backlight interference that are difficult to overcome." Sun Yanbiao said.

According to a solution previously exposed by the industry, the use of under-screen infrared imaging technology can solve the above problems. Specifically, the infrared transmitter, infrared fingerprint receiver and infrared optical material system are used to greatly improve the infrared transmittance and the sensitivity of fingerprint imaging, and ultimately achieve a high level of recognition.

Li Junyi explained that on the basis of ensuring the display effect, in order to achieve a better infrared transmittance and fingerprint recognition effect, it is necessary to optimize the optical film and glass of the LCD screen, and even change the film structure to improve infrared Penetration rate. Secondly, in order to improve the fingerprint recognition performance, the sensor originally located at a specific position under the screen and the infrared signal transmitter also need to be specially modified.

The breakthrough in the laboratory is only the first step. "Under-screen fingerprint LCD screens have a slightly higher overall cost than ordinary LCD screens, and they still have a greater cost advantage than under-screen fingerprint OLED screens. It should be noted that because of the customization of LCD screens using under-screen fingerprints, they are more traditional LCD screens, in order to truly achieve mass production in the future, require close cooperation between upstream and downstream manufacturers, which puts higher requirements on the supply chain management and control capabilities of terminal brands. "Li Junyi said.