What the flicker
Why car headlights seem to flicker out of the corner of your eye
You’re stopped at a red light when something suddenly catches your eye out of the corner of your vision. It looks like the light reflected in your side mirror is flashing. You look directly at it, but nothing seems to be flickering; the car behind you doesn’t have flashing headlights.
TL;DR: No, you’re not crazy; many modern cars use rapid light modulation to control their lights. Human perception of flicker is complex, and while you might not see it when looking directly at a light source, several other factors can make flicker visible under the right conditions.
Why do car lights flicker?
First, we need to understand the three lighting functions found on most cars:
- Daytime running lights (DRL): Make the car more visible to other road users.
- Low-beam headlights: Illuminate the road without causing excessive glare for other drivers.
- High-beam headlights: Provide greater illumination in dark conditions when other road users won’t be dazzled.
On older cars, DRLs and low-beam headlights often used the same halogen bulbs, operating at different intensities for day and night. With the widespread adoption of LEDs, their compact size and relatively low cost have made it easier to separate these lighting functions. Their small size also enables sleek lighting patterns that make cars look like they came straight out of the Tron movie.
During the day, DRLs often operate at high intensity to remain visible in bright sunlight, much like turning up your phone’s screen brightness on a sunny day. At night, these lights need to be dimmed to minimize glare for other drivers. One common way to achieve this is through pulse-width modulation (PWM). Instead of supplying a constant current to the LED, the current is rapidly switched on and off, reducing the average light output.
What is flicker
In simple terms, flicker is a rapid variation in a light source’s intensity that can make it appear to flash. Common examples include old film projectors, strobe lights, or a light bulb flickering just before it burns out.
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We often associate flicker with lights flashing very rapidly. This raises an interesting question: How fast must a light flash before it becomes imperceptible?
The answer depends on several factors. The size of the flickering object, its peak light intensity, and its location in our field of view all influence how we perceive flicker. Humans are particularly sensitive to large, high-intensity flickering objects in their peripheral vision [1]. This might explain why a bright LED from the car behind us, seen in our peripheral vision, can suddenly catch our attention. However, we generally find it difficult to perceive flicker above about 120 Hz, even under favourable viewing conditions.
What flickering frequency do cars use?
Well, I didn’t know the answer, so I decided to experiment myself. I limited my investigation to the car I had available: a 2026 Subaru Uncharted. Unfortunately, my camera wasn’t fast enough to capture the flicker frequency, but the video still shows the DRL flickering at night.
Even if I wasn’t able to find the flicker frequency from the video, I was able to infer the flicker frequency to be 240hz (see explication in phantom arrow effect)
What the flicker is going on
This is the title of a recent report on flickering lights [2]. Many standard uses of flicker date from the recent past, when flicker beyond 120 Hz wasn’t much of an issue. However, the reality is much more complex. Research over the past two decades has shown that the visual effects of flickering light can remain perceptible at frequencies well above the threshold for direct flicker perception.
Two particularly interesting phenomena are the stroboscopic effect and the phantom array effect.
Stroboscopic effect
This is how you perceive a moving object under flickering illumination. The best way to show it is to move a chopstick very rapidly in front of a flickering light. The movement becomes choppy instead of blurry. Flickering affects how we see a moving object. Studies have shown that the stroboscopic effect can be seen up to 1280hz.
Phantom array effect
This is how you perceive the flickering object when either your eye or the object is moving. The effect is similar to the stroboscopic effect, except that the flickering light source itself may appear as a series of distinct images rather than a continuous streak. A good example is looking at a car with DRLs; the moving light will show some duplication along its axis of motion instead of a blurry trail. Studies have shown that, on average, the phantom array effect can be seen up to 10,000Hz for the average population and up to 15,000Hz for a sensitive person [4].
In the picture, you can see six duplications of the light. We acquired the picture with an exposure time of 25 ms. With six duplications, the light turns on and off six times during the 25 ms period. A little math gives us 240 Hz.
Enough science; why is the car flickering only from my side mirror
First, flicker in your peripheral vision is always more noticeable because humans are more sensitive to it there. Then, the road is a dynamic environment where objects around you are constantly moving, and your eyes are continuously scanning your surroundings. These movements can make the stroboscopic and phantom array effects more noticeable. However, when you look directly at the headlights, relative motion decreases, making these effects less apparent. Several studies have examined these flicker phenomena over the last two decades, and the automotive industry may need to review past standards using new metrics and knowledge.
Reference
[1] Brooke Krajancich, Petr Kellnhofer, and Gordon Wetzstein. 2021. A perceptual model for eccentricity-dependent spatio-temporal flicker fusion and its applications to foveated graphics. ACM Trans. Graph. 40, 4, Article 47 (August 2021), 11 pages. https://doi.org/10.1145/3450626.3459784
[2] Miller, N. and Irvin, L., “What the Flicker Is Going on Here? Temporal Light Modulation in Automotive Lighting,” WCX SAE World Congress Experience, Detroit, Michigan, United States, April 16, 2024, https://doi.org/10.4271/2024-01-2462.
[3] Małgorzata Perz, Dragan Sekulovski, Ingrid Vogels, and Ingrid Heynderickx, “Stroboscopic effect: contrast threshold function and dependence on illumination level,” J. Opt. Soc. Am. A 35, 309–319 (2018)
[4] Kang H, Lee C-S, Lee J, Lee K-M. Phantom array effect can be observed above 15 kHz in high speed eye movement group for high luminance warm white LED. Lighting Research & Technology. 2024;56(7):669–675. doi:10.1177/14771535221147312
