Stroboscopic, flickering with frequency, the rapid and repetitive change of light with time is called stroboscopic, and the direct feeling is that the light source is adjusted and unstable. The reason for the stroboscopic flicker is that the lamp uses AC power supply, and its brightness will change with the periodic changes of the AC power.
LED lights are a common light source with stroboscopic hazards and are widely used in traffic lights, car lights and other fields. It seems that there is nothing wrong with ordinary people. However, when the camera began to be used as a source of car information collection, LED lights The hazards of stroboscopic light began to appear.
LED lights stroboscopically interfere with the collection of information by car camerasToday's automotive camera systems use CMOS image sensors, which are high-definition sensors that can reach 1 to 2 million pixels. The fusion of single/binocular cameras and radar systems can provide drivers with information such as the speed, distance and appearance of obstacles or moving objects. Autonomous driving is nothing more than using deep learning algorithms to analyze the information collected by cameras, etc., and then substitute human drivers to make driving behaviors.
Liu Qi, Director of Automotive Electronics Business Department of Howe Technology China, said: “LED lights use a basic frequency and an adjustable duty cycle to adjust the intensity of bright and dark. For the human eye, the LED light source is continuous, but HDR The camera captured the attributes of light and darkness, and it flickered when watching the video."
Combining the two, when a car with an HDR camera captures the LED street signs and the LED light source of other vehicles, there will be missing information or information errors in the picture returned by the camera. Under such circumstances, can humans still dare to believe that smart rearview mirrors are even autonomous driving?
Analyzing the reasons for this, Liu Qi pointed out: "The current frequency of LED lights is mostly between 90-270 Hz, and usually a bright and dark period is less than or equal to 11 milliseconds. The HDR camera on the rearview mirror of a car streaming media is usually 16.6 milliseconds, but the exposure time is mostly between 3-4 milliseconds. Therefore, the camera cannot capture the LED pulse at any exposure time (long, short or very short), and the LED cannot be observed in the video."
We saw a demo demonstration of this situation at Howe Technology Shanghai. The camera on the left is an ordinary HDR car camera without technical support. In the eyes of human eyes, the LED light is always bright, but the camera on the left is formed. The light is a dark picture.
Schematic diagram of Haowei Technology demo
How to solve LED "flashing"?Judging from the current situation, in order to achieve high-level autonomous driving, cars cannot do without the assistance of cameras. If the problem of LED lights "flickering" is not solved, safety must always be a question mark. Suppressing and eliminating the "flicker" of LED lights is a hurdle that cannot be bypassed on the road to research on autonomous driving.
Liu Qi introduced that there are currently three methods to solve the problem of LED lights "flicker", namely LOFIC technology, chopping technology and separate pixel technology. The principle is to ensure that it takes at least 11ms of exposure time to capture a 90Hz LED pulse. The LFM (reduce or eliminate LED flicker) technology requires the camera to extend the exposure time and make it longer than the LED drive cycle without overexposure. And maintain a certain level of dynamic range, then the problem is solved.
Among the three technologies to suppress LED "flicker", Howe Technology has chosen separate pixel technology. Liu Qi mentioned in the process of analysis: "LOFIC has a linear diode with additional storage capacitance in the pixel, which provides an extended full well capacity. The higher the full well capacity, the easier it is to capture the LED pulse and will not exceed the saturation of the pixel. Used in SPD (small photodiode) or LPD (large photodiode), but there will be noise problems at high temperatures. Chopping is the transfer of part of the charge from the photodiode to the external storage capacitor in the time domain (integrated distribution of the frame period), It can ensure that the LED pulse is captured and can be applied to SPD or LPD. However, there will be noise problems at high temperatures."
Regarding the separated pixel technology, Liu Qi said: “Using a separated pixel structure and reducing the sensitivity of small photodiodes is the most ideal solution at present. First of all, small photodiodes can extend the exposure time and make it long enough to ensure that the LED Pulse capture; secondly, the large photodiode can ensure high sensitivity in low-light conditions; and the extra short exposure (VS) can capture the brightest part of the scene."
Based on the separated pixel technology, Howe Technology introduced two HDR image sensors OX01A10 and OXO2A10. These two sensors will be used in side-view and rear-view camera monitoring systems (CMS) respectively. The 0X01A10 and 0X02A10 sensors are based on OmniVision's 4.2-micron OmniBSI pixel architecture, and use pixel splitting technology to provide excellent high dynamic range (HDR) and first-class low-light performance. They are representative of the industry's leading LED flicker reduction solutions.
"The short exposure time of the ordinary HDR camera causes the image Sensor to miss the LED's'on' pulse, which creates a phenomenon of'flicker' in the video stream on the display. Simply increasing the exposure time of ordinary pixel technology to capture the LED pulse does not To solve the problem, it will lead to saturation and loss of dynamic range." Omnivision product marketing manager Marius Evensen said. "The 0X01A10 and 0XO2A10 image sensors we designed specifically solve this problem by adopting the technology to reduce LED flicker, so that E-view mirrors can be widely used in the automotive market. These two sensors have been incorporated into our expanding machine and vision displays. System in the automotive dedicated digital imaging solution group."
The dynamic range of OXO1A10 and 0X02A10 can reach 110dB, while ensuring the capture of LED pulses, so that the camera can play an excellent imaging function in all lighting conditions. This enables the on-board camera to capture both light and dark scenes at the same time, ensuring the best quality image effect even under the most demanding lighting conditions. OXO1A10 supports a resolution of 1280×1080 with an aspect ratio of 1:1.2, which is very suitable for side-view cameras. The OXO2A10 sensor dedicated to the rear view camera supports a resolution of 1840X940 with a 2:1 aspect ratio. The chipset algorithm on the sensor can reduce the data output rate, facilitating data transmission and back-end processing.
Two products, 0XO1A10 and 0XO2A10, are currently in mass production.
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