Understanding LED Flicker in Dashcams and Camera Footage: The Role of PWM and Incandescent Bulbs

Have you ever noticed that lights seem to flicker in the rear or side view camera of vehicles? This phenomenon is often attributed to pulse width modulation (PWM) and the unique characteristics of both LED and incandescent bulbs. In this article, we will delve into the reasons behind this flicker, its impact on dashcams and other cameras, and how it can be mitigated.

What is Pulse Width Modulation (PWM)?

PWM is a technique used to control the brightness or intensity of an LED by rapidly turning the LED on and off at a high frequency, typically in the range of 1,000 to 50,000 times per second. During each cycle, the LED is either fully on or fully off, with the duty cycle (the proportion of the cycle during which the LED is on) determining the perceived brightness.

How Flicker Happens in LEDs

LEDs are particularly prone to flicker because they can turn on and off almost instantaneously, leading to visible oscillations in brightness when the camera captures the light. To understand this, consider the typical use case of an LED daytime running light. These lights are often dimmed using PWM to ensure they do not consume excessive power while still providing adequate visibility.

The flicker is particularly noticeable in dashcams and security cameras, which capture footage at a high frame rate. Unlike human vision, which has a limited persistence of vision, cameras capture each frame continuously, making the rapid on/off cycles of the LED visible as a flicker.

The Impact of Incandescent Bulbs

In contrast to LEDs, incandescent bulbs have a much slower response time. They take time to heat up and cool down, which smooths out the flicker effect. However, some incandescent bulbs use a low-wattage filament, which can still result in some level of flicker, especially when dimmed.

Technological Factors

Camera Sensitivity and Rolling Shutter

The flicker effect is also exacerbated by the rolling shutter of cameras. Cameras with rolling shutters capture the image line by line, capturing one pixel at a time and then moving vertically to the next line. This creates a sequential capture that can cause an illusion of flicker, especially when the light source is rapidly blinking. The flicker can appear to scroll vertically across the image.

LED Signage and Multiplexing

LED signage often employs a technique called multiplexing, where only one row of LEDs is lit at a time, with the next row lighting up and the previous one turning off. This reduces the number of control signals needed to drive a large LED display, but it can also result in visible flicker when viewed from a distance.

Rectification and Filtering

Directly powering an LED from rectified AC power results in a high-frequency blinking due to the alternating current's half-cycles. To avoid this, it is essential to filter the voltage to smooth out the current and eliminate the rapid on/off cycles that create flicker.

Intentional Design of Traffic Lights

Interestingly, some traffic lights are intentionally designed to flicker quickly. This may not be visible to the naked eye, but it can trigger peripheral vision, making traffic lights more noticeable to drivers.

Conclusion

In summary, the flicker effect in LEDs is a result of the rapid on/off cycles induced by PWM. Incandescent bulbs, while less prone to flicker, can still exhibit some level of flicker, especially when dimmed. The combination of high-speed cameras, rolling shutters, and multiplexing techniques can exacerbate this flicker, leading to visible artifacts in dashcam videos and other camera footage. By understanding these technical factors, you can better appreciate the nuances of light behavior and take steps to mitigate this effect if necessary.