What is LED Flicker Mitigation (LFM)?

To understand what LFD is we first have to look at what is meant by flicker and how it affects image quality.

Light flicker is defined as a periodic change to the brightness or intensity of a light source.

Like with a flashing light from an emergency vehicle, this change can be intentional whereas an example of an unintentional flicker, are the light fluctuations from digital road signs. As a result of its changing nature, flicker can be challenging to quantify in a test setting. Nevertheless, with the continuing advancement of cameras and sensors in specific industries, such as advanced driver assistance systems (ADAS), other automotive systems and security, it is imperative to recreate and test flicker in a lab setting.

In traditional camera testing, camera systems are generally not tested for flickering light performance. Other cameras, such as automotive systems, must perform at the highest level in any illumination scenario to ensure safety standards are met. Unfortunately, there is currently no internationally recognized standard for flicker performance testing.

Flicker often occurs from LED-based light sources that use Pulse Width Modulation (PWM) technology to control light intensity. Flicker is a Temporal Light Modulation (TLM) of the light that hits a camera sensor.

As that sensor has a discrete sampling of light intensity (exposure time), the combination of the light turning on and off quickly and the capturing process can lead to unwanted digital artefacts.

In general, the frequency and duty cycle of the PWM used in the light source together with the phase shift between the light source and the capture rate of the camera, determines the number of observable artefacts in an image or video due to flickering light sources.

In the automotive industry, flickering light is broken down into two categories, reflectance flicker and illuminant flicker.

A flickering light source that illuminates the scene but is not within the camera’s field of view (e.g., illumination in a parking garage) is referred to as Reflectance Flicker.

A flickering light source that illuminates the scene within the field of view of the camera (e.g., headlights of an approaching vehicle) is referred to as Illuminate Flicker.

Light flicker can affect almost all industries that rely on cameras and sensors, but it’s most evident in the automotive and security sectors. Flicker is common in these industries because the surrounding environments constantly change and produce a broad range of lighting situations. The camera and sensor systems in these industries need to mitigate the modulation of the light source (flicker) to adapt to the changing environment to ensure high performance and safety.

For example, an ADAS system must always react correctly even with an inconsistent light flicker to avoid a dangerous driving situation.

By using LED flicker mitigation (LFM) technology for automotive use, it is possible to acquire high-quality information by suppressing the flicker that occurs when imaging traffic lights and signs that use an LED light source.

The LED light sources used in traffic lights and signs, etc, as explained above, are repeatedly turning ON and OFF at a frequency that cannot be detected by human eyes.

When imaged with an image sensor, if the exposure time is shorter than this ON/OFF timing, then the image may be taken during the time when it is OFF, so it will not be possible to capture the LED light emission.

There is also a risk that this leads to the lit state being mistakenly recognized as a flashing state.

Using an exposure time longer than the emission period makes it possible to capture an LED light source, but there is the problem that a pixel can become saturated in some situations and blowout may occur.

Steps have been taken by manufacturers of automotive image sensors to ensure that pixel saturation will not occur even if the exposure time is longer than the LED emission period.

This makes it possible to capture the LED light source each period to acquire a high-quality image signal with no flicker, thus this solution is termed as LED Flicker Mitigation (LFD).