It is feasible to obtain good quality information by reducing the flicker that happens while imaging traffic signals and signs that employ an LED light source by using LED flicker mitigation (LFM) technology for automobile applications.
Understanding LED Flicker and Mitigation Techniques
LED flicker is a major concern in digital imaging, as it can cause discomfort to viewers and can also make an image appear blurry or inconsistent with the varying exposure times of digital cameras. This phenomenon has been attributed to non-linearity and quantization errors in the imaging system, along with temporal aliasing caused by factors such as the integration time of the camera, the frame rate of a video sequence, and digital shutter effects.
It happens whenever a modulated signal is used to power a light source. To alter their perceived brightness, LED lights typically flicker several hundred times per second with a variable duty cycle. While LED flicker is more noticeable in some images than others, it can have a significant impact on the perceived quality of an image, making it difficult to use in various applications such as medical imaging or security surveillance. In contrast, a camera imaging the light source could need a relatively brief exposure period to properly record an image, especially in intense brightness.
To address the issue of LED flicker, researchers have developed various techniques to reduce its impact. Let’s discuss them further in the following section.
Reasons for LED Flicker in Camera Systems
The flickering of LED light is incredibly fast and far too minuscule for the human eye to catch. This rapid movement of light is referred to as ‘flicker’ and is defined as a change in luminance or brightness that happens over a short time. Let’s examine the underlying cause of the LED lights flickering in the camera system.
Power Supply: Flicker is more prominent when LEDs are connected to an AC supply because the voltage from an AC source has a varying current and frequency. On the other hand, flicker is not usually an issue with LEDs connected to a DC supply, such as an LED lamp.
As an AC passes through an LED, it releases energy in the form of light, heat, and sound. As the number of Hertz increases, LEDs produce more light, as they are designed to emit more photons when there is a greater concentration of electrons passing through them in a given period. Accordingly, LEDs can operate at frequencies between 50Hz and 60Hz, meaning that a complete cycle of electrons can occur from 50 to 60 times in one second. Depending on the country where it is used, the frequency may vary.
If your LED lights are flickering on a DC power supply, they may have been hooked up to a pulse width modulation (PWM) system for dimming. This could cause the LED to flicker on and off, so it can be important to adjust the PWM frequency and duty cycle to ensure that the LED doesn’t flicker. The frequency, duty cycle, and phase shift between the light source and the camera’s capture rate all play a role in how many observable artifacts from flickering light sources can be detected in an image or video.
Shutter Speed: The LED light flickering in the video is caused by the video itself being a collection of still images captured per second at various points in time.
The shutter speed is important in taking pictures, as it affects how much light enters the camera and is ultimately captured on film. This phenomenon is referred to as the “rolling shutter effect” and occurs when the light levels of a scene change faster than the camera can record them. The image was captured by the camera as the light was cycling out when the LED light on the screen abruptly went black. This enables the camera to accurately capture a moment in time, freezing the motion and providing viewers with an accurate representation of what took place.
You will see that the lights have a strong tendency to flicker in the slow-motion footage. This flickering effect is a result of the slower frame rate capturing more rapid fluctuations in the lights. This is because the lighting frequencies are exaggerated and become much more visible to the naked eye.
To avoid this, the LED frequency must be adjusted to match the exact frame rate of the camera.
How can LED lights that are being used in cameras avoid flickering?
To avoid this issue, engineers have developed an algorithm that determines the ideal balance between the camera’s FPS and LED frequency.
LED Flicker Mitigation
Reducing LED flicker is a big challenge. The automotive imaging industry is not currently in agreement on the required level of mitigation. In general, most applications require that a light source never seem to be “OFF” when it is being recorded. To accomplish this, make sure the length of the camera exposure is greater than or equal to the period of the PWM light. However, unless the camera’s exposure time is an exact integer multiple of the PWM light source’s frequency, the brightness of the light will deteriorate over time.
New pixel architectures have been created by many sensor manufacturers to enable longer exposure times without saturation. These architectures allow the pixel to hold more electrons before they overflow and saturate, making it possible to increase exposure times without creating too much noise or distortions. Increasing exposure time to reduce LED flicker has the effect of exacerbating motion blur, making it difficult for an observer to maintain focus on a moving subject. To address this issue, a combination of exposure time optimization and anti-blur algorithms is necessary to reduce flicker without creating image distortion or motion blur.
Evaluation of LED Flicker
Two distinct metrics were used to determine LED flicker: reflectance flicker and illuminant flicker.
The term “reflection flicker” describes a flickering light source that brightens the scene however it is not visible to the camera. This type of flicker has significant implications for computer vision applications, as the changes in light can create confusion for algorithms trying to track objects in the scene. The evaluation of reflectance flicker is used by the flicker modulation index,
Illuminant flicker is the term used to describe a light source that flickers while illuminating the scene. This can result in image artifacts like temporal aliasing, which can lower the quality of the images that are captured. To counteract the illuminant flicker, it is necessary to have a higher and balanced FMI value. The Flicker Detection Index (FDI) takes into account the environment and camera parameters, such as the position of the camera to the LED light source, ambient light level, and exposure time. Therefore,
Although the FDI equation is an effective flicker detection indication, it loses accuracy if the system must replicate the light source signal level, which is often necessary for automotive-grade cameras. To accurately reproduce the light source signal level, a more sophisticated and accurate flicker detection algorithm must be used with the help of a modulation mitigation probability equation,
By using these equations to balance the ambient light, light source, and exposure time in a camera, it is possible to reduce or even eliminate flicker in varying conditions.
Automotive Image Sensor from Sony, Onsemi, Omnivision
The AR0233 is a new 2.3-megapixel CMOS image sensor from On Semiconductor that establishes a new standard for LED flicker mitigation technology in automotive applications. By removing high-frequency LED flicker from traffic signs and vehicle LED lights, this image camera sensor enables Traffic Sign Reading algorithms to function in any lighting circumstances.
The OAX4010 automotive ISP from OmniVision Technologies has a revolutionary HDR and LFM Engine hybrid algorithm. The OAX4010 provides simultaneous LFM and HDR functionality while working over the automotive temperature range, enabling more intelligent decision-making in conjunction with the company’s OX01A10 and OX02A10 LFM image sensors.
The simultaneous exposure feature of Sony’s IMX490 is advantageous for reducing LED flicker. For the sensor to balance out the flicker, the particular sub-pixel that is more sensitive to light regions can be adjusted to the same longer exposure as the other pixels.
Final Words
Nearly every industry that uses cameras and sensors can be impacted by light flicker, but the automotive and security industries are where it is most noticeable. These sectors experience a lot of flickers because the surrounding environments are continually changing and produce a wide range of varying illumination. To maintain good performance and safety, the camera and sensor systems in these industries are showing interest in reducing the modulation of the light flicker.
Correspondingly, we (Vadzo) produce embedded cameras based on automotive camera sensors with LED flicker mitigation, which enables your project to eliminate LED flicker. This technology provides accurate image processing and stabilization while eliminating LED flicker, making it ideal for automotive cameras. With our cameras incorporating LED flicker mitigation technology, you can be sure that no pixel saturation will occur, and your cameras will capture the image without any distortion. As a result, it can capture the LED light source every time and acquire high-quality, flicker-free image data.
Hence, if you have any queries, feel free to Contact Us
