“With the increasing use of LEDs in street lighting and headlamps, a phenomenon known as “LED flicker” is becoming a key issue for machine vision systems to address, resulting in LED traffic signals not being fully captured. As ADAS begins to gain popularity, LED Flicker Mitigation (LFM, LED Flicker Mitigation) is critical to keeping these systems safe.

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[ADAS (4)]Do you know that the red in the “eye” of a car is not red

Uncover how the car accurately perceives the “Electronic traffic police”-LED signal on the road.

introduction:

The combination of consumers, policies/regulations, and the market continues to promote the penetration rate of ADAS. In March 2022, the L2-level supporting volume of domestic new cars was close to 416,000, with a penetration rate of nearly 30%. ADAS becomes standard, how to upgrade CIS?

OmniVision Automotive CIS series special ADAS articles will be divided into 5 issues, each with a hard-core knowledge point, to help you understand the application, technology trends and automotive products in the field of ADAS in detail.

The last issue of popular science explained why ADAS front-view cameras especially need to reach 140dB HDR. This issue will focus on how the car accurately perceives the “electronic traffic police”-LED signal on the road.

With the increasing use of LEDs in street lighting and headlamps, a phenomenon known as “LED flicker” is becoming a key issue for machine vision systems to address, resulting in LED traffic signals not being fully captured. As ADAS begins to gain popularity, LED Flicker Mitigation (LFM, LED Flicker Mitigation) is critical to keeping these systems safe.

What is LED blinking

The use of LEDs can save power and reduce costs, and has been widely used in the field of lighting, but the LED light source is not continuous. LED light sources are not driven by direct current, but by pulsed alternating current. The LED light has a base frequency, and adjustable duty cycle to adjust the intensity of bright and dark. To the human eye, the LED light source appears to be continuous, but an HDR camera (High-Dynamic Range, or HDR for short) can capture the bright and dark properties of the LED and present it when watching a video “flashing” appears.

The HDR frame rate and the LED drive signal are asynchronous (11ms for an LED on/off cycle; HDR takes 16.6ms per frame in daylight), so HDR captures the resulting LED on/off cycle for each frame different brightness, resulting in “flickering”.

The above reasons cause the CMOS image sensor to be troubled by LED flickering, which causes the vehicle camera to “misread” the traffic signal indication issued by the LED, and it is easy to mistake the flickering of the LED light as an emergency light or a direction indicator.

How to deal with LED flickering?

If the camera can extend the exposure time and make it longer than the LED drive period, without overexposure, and maintaining a certain level of dynamic range, the problem is solved.

Theoretically speaking, the exposure time is the time that the camera’s shutter opens and closes. The longer the exposure time, the longer the shutter opening time, and the more light entering the camera, the brighter the image will be. Or completely white, called saturated or supersaturated. The shorter the exposure time, the darker the image. If the camera exposure time is longer than the drive period of the LED without going into saturation, and the dynamic range is maintained, then the HDR camera can capture the traffic signal.

To ensure that the HDR captures the full LED image, OmniVision has developed the LFM solution. LFM distinguishes the exposure time of SPD and LPD by small photodiode (SPD for short) and large photodiode (LPD for short) respectively, that is, small photodiode (SPD) can prolong the exposure time, make it long enough to ensure the capture of LED pulse ; Large Photodiode (LPD) ensures higher sensitivity and color compensation for scene exposures in low light conditions; while additional Very Short Exposure (VS) captures the brightest parts of the scene.

In response to the pain point of flickering LED lights, OmniVision launched LFM products using Split Pixel technology as early as May 2017, and is still the only image sensor manufacturer in China that can mass-produce LFM functions. For ADAS field, OX08B40, OX03F10, OX03C10, etc. all have LFM function.

In summary, the ADAS chapter analyzes the technical trends of automotive CIS such as HDR, LFM, and high resolution. What are the advantages of OmniVision in ADAS vehicle image technology?

Meng Shu, Marketing Director:

“The application of ADAS and autonomous driving has put forward higher requirements for the image quality of the camera.

For example, while pursuing higher dynamic range, ADAS needs to minimize the smear phenomenon caused by moving objects, which makes the traditional implementation of line overlap no longer applicable. OmniVision has developed two HDR technologies of large and small pixels and DCG very early, which can easily achieve 120/140dB dynamic range and avoid the smear of moving objects as much as possible.

LED flicker suppression (LFM) is another important technology for ADAS applications. OmniVision has its own unique HALE technology, which is installed in the full range of 2M-8M products. range of technologies.

Other cutting-edge technical functions involved in ADAS, such as functional safety, CyberSecurity engine, multi-color filter support, multi-ROI output, etc., OmniVision has been leading the technical direction of in-vehicle image sensing. 1A solution provider of a full range of automotive-grade image sensor products recognized by customers. “

With rich experience in mass production as the cornerstone and leading technology as the engine, what kind of vehicle regulation solution has OmniVision proposed for ADAS? What are the advantages? Please pay attention to the next issue of “Omnibus CIS Series Special ADAS Chapter (5) OmniVision Group Vehicle-level Solutions”.