However, it is not that simple, and it is difficult for TFT-LCD to identify off-screen fingerprints. This is because LCD emits light passively, and it emits light through TFT through LED backlight at the bottom. This layer of TFT itself is not so light-transmitting. If you put your finger on it without making any changes, it is difficult for the sensor under the screen to identify fingerprints. Therefore, if TFT-LCD is used for off-screen optical fingerprint identification, it is necessary to improve the TFT layer in technology, such as adding some gaps or opening up an area for LED backlight. But even so, LED backlight will cause great interference to the light reflected by fingerprints. Therefore, it is difficult to realize optical fingerprint recognition under TFT-LCD screen.
On the other hand, organic light-emitting diodes (OLEDs) emit light actively, which can control each sub-pixel accurately in theory, so the screen made of OLEDs is a more ideal light source. In addition, the organic light-emitting diode display module is thinner, which can also alleviate the problem of overall body thickening caused by placing the fingerprint sensor under the screen.
At present, there are three development directions of using OLED screens in the industrial chain: 1, a CMOS sensor is arranged directly below the screen, and the light passes through the gap between OLED subpixels to identify fingerprints; 2. Shrink the sensor and insert it between the pixels of the organic light emitting diode; 3. Make the CMOS sensor transparent and stick it directly above the AMOLED screen, and make the optical fingerprint recognition as the recognition layer.
In the aspect of fingerprint identification on screen, many companies have begun to try and have achieved initial results. Ding Hui Science and Technology has demonstrated the case of using AMOLED screen to realize off-screen fingerprint identification. The demonstration models are Samsung Galaxy S7 Edge and vivo Xplay6. Ding Hui technology is to arrange a CMOS sensor at the bottom of the screen. According to the patents registered by Ding Hui Science and Technology in the United States, the answer lies in these three pictures.
At present, there are not many large-scale ultrasonic fingerprint identification mobile phones, mainly LeEco's LeMax2 and Xiaomi 5s. LeMax2 put the fingerprint on the back and Xiaomi 5s on the front. At that time, the ultrasonic wave could not penetrate the thick glass, and the maximum thickness was about 0.4mm, while the thickness of the mobile phone cover glass was about 0.6mm~0.9mm, so the effective thickness of 0.4mm was not enough to penetrate the thickness of glass+display screen (0.6mm+0.3mm). Xiaomi had to discuss with Lance Technology to dig an extra piece of glass for the front panel to ensure that ultrasonic waves could penetrate, so the glass in the fingerprint recognition area was thinned. According to the information released by official website, Qualcomm, the new generation Sense ID can penetrate 1.2mm OLED screen or 0.65mm aluminum alloy or 0.8mm glass. This penetration ability is enough for current glass or organic light emitting diode screens.
But why isn't vivo used on mass-produced X20 and other mobile phones? This is because the optimization algorithm takes time. New technology needs a debugging process from release to formal application. Fingerprint identification is a biometric technology that requires high security, and it needs time optimization algorithm to improve security, recognition speed and recognition rate.
However, the plan just announced by FPC is even crazier. FPC says that whether your hand is dry or wet, whether your screen is AMOLED or LCD, or even whether your surface material is glass, we can recognize it. How thick can it penetrate? 20mm！ 20mm！ 20mm！ Say the important things three times. In contrast, the Sense ID of the first generation in Qualcomm is 0.4mm, and the second generation can penetrate1.2 mm. The thickness of FPC is more than 16 times that of Qualcomm.
Advantages of the new FPC technology include:
1. Support the clean front design of smart phone (any other device), which can be used for display, and also include fingerprint recognition function to optimize the ratio of screen to mobile phone;
2. Full screen can be used for fingerprint identification. There is no need to visually or physically highlight a specific area of the smart phone as fingerprint recognition;
This technology can capture fingerprints on different surface materials, such as thick glass and metal. It can also work when fingers are wet or immersed, and this technology can work well in all different glass thicknesses, even the thickest glass on the market;
4. This unique technology can also work well in LCD panels and OLED panels.
Fig. 2 1A and fig. 2 1B show the location of the fingerprint identification sensor from the top view and the side view.
Fig. 24 shows how light penetrates the OLED screen from a microscopic point of view, with fingers on it; The gray area above is the screen part of the mobile phone. The small hole passing through the screen is called the "collimating hole" at the top of the meeting, and the light reflected by the finger is collected and processed by the optical sensor.
How to ensure that the collected light is the reflection of fingerprints? This requires that the light be collimated. As shown in fig. 27, Ding Hui has customized a special microlens array and an optical spatial filter array, and the microlens array needs to be subjected to MEMS technology or chemical treatment. These two arrays can ensure that the light entering the sensor is basically reflected by fingerprints, not the screen or sunlight.
Another direction of off-screen fingerprint recognition is to use ultrasonic fingerprint recognition.
Qualcomm's scheme is called Sense ID, and FPC, the leading enterprise of fingerprint identification, has just released their scheme. Ultrasonic wave needs neither photosensitive element nor capacitive induction, and is more suitable for fingerprint identification under screen. The full-screen fingerprint identification used by Vivo presenter adopts Qualcomm's scheme and Oufei's module.