Addressing Flicker Issues with LEDs and Photography|
Article - July 3, 2017 By LarsonElectronics.com
Addressing Flicker Issues with LEDs and Photography
The widespread adoption of LEDs can be seen in professional film-making and photography. Both industries benefit from the modern luminaries’ sturdy, compact and energy efficient features. With focus on photography, the demanding requirements of shooting under artificial lighting systems have caused some issues to surface.
One of these concerns is flicker; defined as constantly fluctuating light output – from an on to an off state. This phenomenon can cause some photos to appear dark, while others appear bright, resulting in inconsistent images. It is important to highlight that flicker is usually only an issue during indoor photography, as natural sunlight does not flicker.
Riding the Wave
In AC-powered lighting systems, from incandescent to HID lamps, flicker-related issues (more often than not) can be traced to the way power is fed to the luminary. Like a constant wave, the current peaks and falls as it is fed through the light. At every peak, the light glows or heats up and at every fall the light cools down. This occurrence happens too fast for the human eye to detect without special equipment. However, cameras, with emphasis on units with adjusted frame rates and run off speeds, can pick up such manifestations fairly easily.
Flicker is more noticeable with low wattage lighting systems, compared to high wattage variants, because the latter doesn’t experience a significant cool down period in between cycles. Furthermore, DC-powered luminaries are very effective in addressing flicker, since the current doesn’t follow the traditional on/off cycle that is associated with AC power.
AC/DC Power Conversion
The type of LED lights used in photography are mostly DC powered. As mentioned earlier, flicker isn’t normally associated with DC power. However, flicker is sometimes observed when an LED light uses AC power, which typically requires a conversion from AC to DC power. This process can be a leading cause of flicker, as a “ripple” is created from the driver to the luminary.
To rectify this issue, non-oscillating current is needed. A high frequency driver could be used to eliminate or reduce flicker. Moreover, individuals could also use batteries to power their LED lights (that run on DC power), as this would forego the need to convert AC to DC power. This workaround is not always effective, especially for long photo shoots. One would be forced to carry around a set of portable power cells to get through the session.
What About Dimming?
Another cause of flicker in photography is light dimming. Some dimming processes, depending on the shape of the waveform, prolongs the peak and cool down periods of the cycle. For example, in a triangular waveform, the rise and fall points are abrupt and sharp, resulting in minimal flicker. As the peaks and falls of the waveforms are stretched, which can be seen in sine and square waveform shapes, flicker becomes more of an issue.
A solution to flicker originating from a faulty dimmer is to use one that leverages high frequency currents. For a PWM dimmer, Wanda Lau from LEED AP recommends using a unit that is capable of high frequencies surpassing several thousand hertz.