The Evolution of Automotive Image Sensors

It all started 18 years ago when I started working on one of the first automotive CMOS image sensors for rear view cameras (RVC). At the time, having RVC to help drivers see behind the car was a great innovation. Now, 20 years later, RVC is standard in modern vehicles and more cameras are laying the foundation for advanced driver assistance systems (ADAS). My career has evolved along with Aptina’s spin-off from what was then Micron’s image sensor division and later acquisition by ON Semiconductor , and ADAS systems have also undergone a series of major changes .

Today, more advanced automotive systems alert drivers when they detect close objects or cars in their blind spots, lane departures, and maintaining speed and distance in cruise control mode. Many of these familiar safety features are enabled by automotive image sensors. It has been a very rewarding experience for me personally to be a part of this transportation revolution. I have had the opportunity to work on the ON Semiconductor team and strive to bring many groundbreaking inventions to the industry that are now the norm. For example, we introduced dual-gain pixel technology and high dynamic range imaging (HDR), which are now used in many sensor designs. I am proud to say that most ADAS systems use image sensors developed by ON Semiconductor .

The development of image sensing technology has changed dramatically over the course of my career, and I have witnessed firsthand the remarkable advancements in image sensors used in vehicles.

Resolution

Image resolution is one of the most important parameters for measuring image quality. Especially for automotive imaging, higher resolution means images with sharper edges and finer details. Think about it, when video graphics array (VGA) sensors first came out, they could only produce 0.3-megapixel (0.3MP, 640 H x 480 V) images. Our AR0820AT is the first automotive-grade 8.3-megapixel (8.3MP, 3840 H x 2160 V) image sensor on the market. This high resolution allows a single camera to support multiple applications (such as vision and perception) and enable better object detection. As more and more automotive applications require larger amounts of imaging data to assist in making critical decisions related to safety, we can foresee that the market demand for higher resolution will continue to grow in the future.

Figure 1. ON Semiconductor Development trend of automotive image sensors

Pixel size

Pixel size is another factor to consider when selecting a sensor, and needs to be balanced with speed, sensitivity, and image quality. Larger pixels have more area to collect available light, but this does not always mean better image quality. A sensor with smaller pixels can outperform a sensor with larger pixels while covering the same optical area. Our Hyperlux series is an example of how a 2.1µm pixel sensor can outperform a 3µm pixel sensor in typical automotive conditions: excelling in low light, overall signal-to-noise ratios (SNRs), and HDR. As we develop more advanced image sensors, pixel size has shrunk from a larger 6µm to our current 2.1µm super exposed pixel, while improving overall performance.

Exposure HDR Technology

We were the first company to invent the big-small pixel technology to produce HDR images. With the big-small pixel approach, the sensor area dedicated to a single pixel is divided into two parts: a larger photodiode covers most of the area, and a smaller photodiode uses the remainder. However, we no longer use big-small pixel technology because it results in reduced image quality, increased dark noise, and reduced performance, especially at higher temperatures.

Figure 2. Comparison of exposure techniques

Our solution to these shortcomings is Super Exposure, also known as Overflow Multi-Exposure. This technology adds an area within the pixel to accommodate large signals or overflowing charge. This approach is like using a bucket to catch rainwater. If the rainwater overflows the bucket, we have a larger basin to hold the excess water. The signal in the "bucket" can be read very accurately, so we can achieve excellent low-light performance; while the overflow basin accommodates all the excess, thereby extending the dynamic range and the ability to capture the true colors of bright objects and scenes. Therefore, the entire pixel area can be used in low-light conditions, while it will not be saturated in bright light conditions. As a result, Super Exposure provides better image quality for HDR scenes in automotive applications, including capturing all the colors and details of flashing LED lights and logos.

Dynamic Range

Dynamic range is the ratio between the brightest and darkest parts of a scene. Our image sensors were the first to achieve 120dB and later 140dB High Dynamic Range (HDR). Our Hayabusa series was the first to market with 120dB HDR and LED Flicker Mitigation (LFM). Most recently, we introduced the Hyperlux image sensor series, which features industry-leading 150dB HDR LFM performance and enhanced image quality.

The 8.3-megapixel AR0823AT and 3-megapixel AR0341AT are the first products to feature Hyperlux technology. With its superior HDR performance, Hyperlux is able to deliver colorful, sharp, and detailed images, which lays the foundation for higher security designs due to its extremely stable nature and unaffected by changes in temperature or lighting conditions. As shown in Figure 3, Hyperlux sensors perform well in the most demanding environments and extreme conditions.

Figure 3. High dynamic range performance comparison between Hyperlux and competitor sensors

Automotive camera systems are a critical component in active safety, as they are the only sensor that can recognize the color, shape, and size of different objects on the road. Throughout its history, the ON Semiconductor team has continued to innovate and set new standards in low-light environments, high dynamic range (HDR), image quality, and clarity. This has helped automotive original equipment manufacturers (OEMs) successfully upgrade early rearview cameras (RVC) to Level 2 autonomous vehicle systems and actively implement Level 2+ and Level 3 driving automation. With industry-leading performance and features, Hyperlux sensors are expected to improve safety indicators and help achieve high-speed autonomous driving while reducing system development costs. Therefore, it is no surprise that many OEMs and Tier 1 suppliers choose to use ON Semiconductor image sensors in their ADAS camera designs.

The road to a safer, better future does not stop here. Together, let us continue to move towards greater road safety and driving automation. Learn more about the Hyperlux image sensor family.