Flicker

Have you ever tried to take a photo of your Pixalux® panel, only to notice that it appears to be flickering on the smartphone or camera screen?

Unsurprisingly, this effect is referred to as 'flicker' and it occurs when dimming or controlling the colour of LED's using Pulse Width Modulation (PWM).

The colour of this panel is controlled by a PWM driver which appears to flicker when recorded.

The colour of this panel is controlled by a PWM driver which appears to flicker when recorded.

Why can't I see the flicker, yet my camera can?

The human eye has a threshold for perceiving flicker ranging from 60 Hz to 90 Hz - this means the light has to flash as infrequently as 60 times per second for the human eye to see it. Camera sensors will capture the flicker depending on the shutter speed or the frame rate of the video. The shutter speed on an Iphone can range anywhere from 1/2 a second to a 1000th of second. Any shutter speed faster than the rate of flicker means the camera can potentially capture it.

The cause of flicker

Standard LED's run off direct current (DC) which is a uninterrupted source of power so they don't normally flicker. Sometimes the use of a very low quality power supply can result in flicker due to the ripple from the AC input.

Flicker occurs when LED's are controlled using Pulse Width Modulation (PWM) which is the primary way of dimming or changing LED colour. As covered in the previous blog post, PWM controls the intensity of LEDs by changing the proportion of time the LED's are on per cycle, known as the duty cycle. If the frequency of the PWM is too low, such as close to the shutter speed of a camera or video frame rate, the effect will be visible.

When flicker matters

Flicker is only likely to be an issue if the lighting is to be used in a film or photography application. In this scenario, high quality PWM devices can be sourced which have frequencies as high as 300 Hz to prevent flicker. Appropriate use of camera settings can also do much to mitigate the effects of flicker.

See the articles below for further information on the subject.

  • http://www.davidsatz.com/aboutflicker_en.html
  • https://en.wikipedia.org/wiki/Flicker_fusion_threshold
  • http://www.xicato.com/technology/dimming-technology