TND6449/D
Rev. 1, July − 2024

Super−Exposure Pixels Mitigate LED
Flicker in the Most Demanding
Automotive Environments© Semiconductor Components Industries, LLC, 2024

July, 2024 − Rev. 1

1Publication Order Number:
TND6449/D

Super−Exposure Pixels Mitigate LED Flicker in the Most
Demanding Automotive Environments
Prepared by: Sergey Velichko, Sr. Manager, onsemi
Automotive cameras with new generation of image sensors enable the latest advancements
in autonomous and assisted driving systems. Advanced driver assistance systems (ADAS) use
the images captured by these cameras to identify objects around the vehicle and calculatedistance (with stereo cameras), which can be used in features like automatic braking, highway
cruise control, lane departure, or parking assist. However, the light−emitting diodes (LEDs)
used in most modern automotive lights and traffic signs can cause problems for some image
sensors. onsemi utilizes image sensors with pixel overflow approach, also known as super
exposure, that uses very large in−pixel overflow memory to extend dynamic range many times
and achieve 120 dB LED flicker free operation. With the advanced super exposure pixels, they
can address the challenges of ADAS requirements and flickering LEDs.
LED lights are generated using pulse−width modulation (PWM), a digital technique commonlyused to reduce power consumption in headlights, taillights, traffic signs, and vehicle messaging
systems. Their on−off pulse frequency is imperceptible to the human eye, but it causes
problems when trying to capture images. To prevent oversaturation from occurring in the brightarea within a scene, it is often necessary to use short exposure times − thus missing LED pulses.
As a result, sometimes it can appear as if a light is off, as shown on the left image in Figure 1.
In a video scene, LED lights seem to be continuously turning on and off − a phenomenon referred
to as LED flicker. As LEDs are commonly used in modern vehicle lighting and digital signage,
the video captured may show different striped or segmented artifacts which may be very
distracting for drivers and passengers.

Figure 1. LFM Samplings at 30 fps 10% Duty Cycle 10 ms Period (100 Hz)

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2This is a critical problem as it can distract a driver who looks at a display only to see theflickering headlights behind them and mistakes it for a first responder vehicle. It can also
potentially create a problem for algorithms being used to determine the number being displayedon a digital speed sign or color of a traffic light or detect brake lights (in Figure 2), to makesafety−critical decisions accurately. Automotive image sensors must address challenging
scenarios, including flickering LEDs, while providing a high dynamic range (HDR) of over140 dB and operating at high temperatures (up to +125°C junction). In this article, we briefly
discuss the operation and limitations of some optical techniques commonly used for HDR andLED flicker mitigation (LFM) in automotive image sensors before presenting a new type ofsensor that overcomes these limitations to provide multiple types of LFM with even better HDR.

Figure 2. Flicker of LED Taillights
Traditional Multiple Exposure HDR
Capturing scenes with HDR typically requires taking multiple images, each with different
exposure time settings, which are then processed and combined into a single image. Using
various exposure settings allows more usable image data to be gathered from the brighter anddarker areas of the scene. For example, longer exposure times will capture more image
information in darker areas, while shorter exposure times capture more usable data in brighter
areas. The final processed image takes the best data from each exposure and combines it into
an HDR image (see Figure 3 top). While traditional multiple exposure HDR imaging is excellent
for static scenes (with little to no motion), real−life dynamic scenes such as automotive
applications with the camera moving at a high speed, can produce motion artifacts due toobjects changing position between different exposures. This happens because each exposure
takes place at different times, sequentially, and this delay causes the HDR post−processing togenerate a final image with objects that appear to be distorted and with color artifacts.
Simultaneous Split−Pixel Exposure
Split−Pixel sensor technology has been used to capture images with HDR in automotive
applications because it captures multiple exposures simultaneously rather than sequentially,
thus reducing blurring and color artifacts caused by motion. To produce an image with HDR, raw
images are captured using both large and small sub−pixels, and these are then combined in thewww.onsemi.com
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HDR output (see Figure 3 middle). To extend the dynamic range, a large sub−pixel is used for
darker parts of image and a small sub−pixel is used for mid to high light parts of image captures.
The big sub−pixel collects more light, and therefore saturates in brighter conditions. Conversely,
the small sub−pixel can be exposed for a longer time but will not saturate easily because lesslight is collected. The overall effect of having the large sub−pixel working in low−light conditions
and the small sub−pixel in bright conditions is extended dynamic range. The relative size of thephotosensitive area of the pixel (fill factor) plays a very significant role in the image quality. A
disadvantage of split−pixel is that less pixel area is available for image capture in low−light
conditions as significant part of the pixel is occupied by small sub−pixel. Even less area isavailable for small sub−pixel thus generating order of magnitude smaller signal andconsequently lower signal−to−noise ratios. When viewing an identical scene, the output image
will be different, depending on whether large or small sub−pixels are used. Also, significant color
artifacts and shifts can occur because of the incident angle of light rays from the lens interacting
differently with the two−sized sub−pixels in the array. Removing these artifacts requires special
sensor calibration and elaborate on−sensor corrections. This is the reason of generally higher
power consumption of split−pixel sensors. Furthermore, the performance of image sensors
based on this technique can degrade at ambient temperatures exceeding +100°C and often
even +80°C, with SNR transition dropping much below 25 dB.
Super−Exposure Pixel
onsemi has developed image sensors with a unique set of features that cater to both sensing
and viewing camera applications. From one side, they provide a mode of operation to capture
120 dB HDR images with LED flicker mitigation (good for surround view and e−mirrorapplications). On the other side, a classical multiple exposure operation extends the total
dynamic range to 140 dB and beyond to address even the most challenging scenes for front andsurround sensing. They also feature a third pulsing LFM mode which covers the extreme caseof the brightest flickering headlights. In all cases, these sensors keep all signal−to−noise ratio(SNR) transitions at 30 dB and above, even for high temperatures (up to +125°C junction).
These sensors deliver the performance needed for ADAS safety and autonomous driving
system designs that may rely on the image sensor, especially on hot sunny days when sensors
within tightly packed front cameras heat up to scorching temperatures.

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4Figure 3. Different Exposure Techniques

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5onsemi’s latest automotive HDR image sensors are built with super exposure technology.
These sensors implement a set of operations for effective, across entire automotivetemperature range:• Super−exposure with 150 dB HDR LFM• Pulsed LED flicker mitigation (pLFM)• Multiple exposure 140 dB and beyond HDR image capture
These modes cover the full range of typical automotive scenarios and enable effective
mitigation of flickering LEDs and other lights. The HDR performance of image sensors
combined with high conversion gain (HCG) signal readout provides a cinematographic image
quality with sharp, high color fidelity captures as a result of excellent pixel linearity from very low
to very bright light levels. Some sensors provide an additional mode of operation in conjunction
with two photodiodes (PDs) covered by a single mlens enabled distance extraction and a color
image using in−pixel phase−detect. Characterizing 4−exposure HDR mode at room
temperature shows a 140 dB dynamic range and all total SNR transitions at or above 30 dB withthe first T1 exposure readout using HCG.

Figure 4. 4−exposure Room Temperature 140 dB Dynamic Range Total SNR
These super−exposure HDR LFM image sensors better captured bright flickering LEDs andeliminated motion artifacts compared to conventional multi−exposure image sensors, as shownin Figure 5.

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6Figure 5. Multi−exposure HDR (top) vs. LFM HDR (bottom) Image Capturesof Flickering LED Lights
Superior Performance
The super−exposure pixel HDR image sensors provide greater HDR and superior LFM acrossthe entire automotive temperature range and are used for safety and other applications in themost demanding lighting and temperature environments. Our second−generation super
exposure technology is based on the same architecture principals to enable even smaller andbetter performing HDR LFM pixels.

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