Makers of complementary metal oxide semiconductor (CMOS) image sensors for the automotive market face growing challenges with respect to achieving high-dynamic-range (HDR) imaging that delivers improved flicker mitigation and enhanced performance in low-light conditions. Implementing lateral overflow integration capacitor (LOFIC) technology in single-exposure LED flicker mitigation (LFM) sensors can help optimize dynamic range and reduce image flicker.

CMOS image sensors convert photons to electrons to create digital images in still and video cameras. As light enters the camera and is focused onto the image sensor, each pixel accumulates an electrical charge that’s proportional to the local incident light. The image sensor’s analog-to-digital converters processes the electrical signals to yield a digital image. This established approach has continued to evolve with emerging technology requirements—mostly notably, the need for HDR capabilities that enable the sensor to achieve crisp image capture, even in areas of the scene that are extremely bright or dark.

Existing HDR imaging techniques often struggle to provide consistent performance in terms of dynamic range, flicker mitigation and low-light performance. Pulse-width modulation (PWM) in LED light sources can cause image detection problems when short exposure times are used. If the exposure window falls in between the LED pulses, the sensor misses detecting the LED light source. Video stream flicker is created by frames missing the “on” pulse when the sensor tries to capture bright LED light with a short exposure time; however, increasing exposure time to capture the LED pulse can cause pixel saturation.

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In the automotive market, flicker from LED traffic lights or signs pose a serious challenge for HDR solutions, preventing driver-assistance and autonomous driving systems from being able to correctly detect lighted traffic signs. Mitigating this challenge requires an approach that can be easily integrated into a CMOS image sensor. Some solutions that have achieved good LFM capabilities have resulted in reduced image quality, especially for low-light, high temperature operation and other conditions.

The goal is not only to achieve HDR performance, but also to maintain a high signal-to-noise ratio (SNR) over the dynamic range. This improves sensing accuracy over a wide range of illumination conditions.

For single-exposure HDR, several technologies have been explored for LFM applications, including dual conversion gain (DCG), split-diode (large/small photodiode) architecture with light attenuation in the small photodiode, or a combination of these. Together, these approaches can achieve a dynamic range of around 100dB, making them useful for automotive applications like in-vehicle cameras. Introducing another technology—LOFIC—enables the creation of 2.1µm pixel LFM sensors with higher total dynamic range, enabling them to achieve optimal object detection in vehicle driver assistance and autonomous driving systems.

What Is LOFIC?

LOFIC technology is key to realizing the combination of low noise readout, high sensitivity operation, large full well capacity and a linear response using a single exposure. A sensor that utilizes LOFIC stores more signal electrons by collecting charge that overflows the photodiode in a large capacitor within each pixel. Collecting all of the charges during each frame enables both higher dynamic range and signal to noise ratio at the same time. By harnessing LOFIC’s unique capabilities, HDR imaging can be effectively enhanced, allowing for superior image quality across a wide range of lighting conditions.

In DCG pixels, a photodiode’s photoelectrons are read out twice with different gains: low-conversion gain (LCG) and high-conversion gain (HCG). The higher gain enables lower readout noise, and the lower gain enables larger effective full well capacity.

Combining DCG with LOFIC or DCG and split-diode with LOFIC significantly improves LFM range for single-exposure HDR. A split-diode LOFIC sensor achieves the ultimate LFM range, while a single-photodiode LOFIC sensor enables a good balance between low-light SNR and dynamic range in a small-pixel product. The key to automotive image sensor success is to achieve the best performance in the areas of greatest importance to the market, including:

* Low-light image quality

* High device temperature

* LFM/HDR range

OmniVision’s new 2.1µm single-pixel TheiaCel LFM sensor technology is the first automotive solution to deliver high LFM dynamic range without sacrificing image quality. TheiaCel combines next-generation LOFIC capabilities with the strengths of OmniVision’s proprietary CMOS image sensor technologies. This enables CMOS image sensors with TheiaCel technology to capture extremely high-contrast scenes for optimum content and image quality.

By: DocMemory
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