A Guide for Studio Video Lighting for Professionals

23.11.2014 15:26

Flicker-FreeLights, and Why They are Important to You

 courtesy B&H PhotoVideo

If you only shoot outside, using the sun and reflectors for light sources, it is highly unlikely that you will encounter flicker problems. However, for the rest of us, flickering light sources can become a daily headache when shooting video. What is important to remember is that most electrically generated light sources flicker in some manner, but what matters is how perceivable that flicker is when we are shooting. I can hear you thinking: “Wait, that’s not true—tungsten lights don’t flicker and neither do LED lights, because they are DC powered.” Now, while that may be true for many applications, it is not always the case. Read on to see how to avoid having flicker ruin your day.

Under normal conditions, tungsten bulbs don’t flicker on video, but there are times they will and here is why. When plugged into an AC power source, the bulb’s filament is heating up 60 times a second, and cooling down 60 times a second (in the US). As the filament heats up, it starts to glow and emit light, and when the power is off, the filament glows even as it is cooling down—until the next cycle starts and the filament glows again. Now, all this happens too quickly for the human eye to see, and during normal shooting frame rates, the camera is capturing the same number of light pulses on each frame, so the flicker isn’t noticeable. As you increase the camera’s frame rates, or run off speed, you end up with frames that don’t have a consistent number of light pulses per frame, and these frames will exhibit flicker. This is most noticeable with smaller-wattage bulbs; once you get to a 5,000-watt bulb the filament gets so hot that it doesn’t cool enough in between cycles to flicker. You could use DC voltage to power your lights; this way, the bulb never goes through an on/off cycle, but finding 120 volts of DC isn’t as easy as it was around the turn of the 20th Century.

At this point you may be thinking, “I don’t shoot high speed (above 100/120 fps), and I’m using fluorescent lights anyway, and these don’t flicker.” If you are thinking this way, then you may be surprised to find out that flicker can be even worse with standard fluorescent lights than with tungsten lights, and that is even before you start dimming. For many years, in the US, the common setup for shooting under fluorescent lights was to use a 180-degree shutter and shoot at 24 or 30 fps. Nonetheless, with the increasing number of digital cameras offering shutter angle control and multiple frame-rate settings, shooting under fluorescent lights that are flicker free becomes very advantageous.

The light from a fluorescent bulb is generated by an arc, running through the tube, creating a plasma inside the tube that excites the phosphor coating on the inside of the bulb. The phosphors glow, releasing the visible light we see. A standard US household fluorescent light turns completely on and off 120 times a second—60 on, and 60 off, which is too fast for us to notice consciously, but this is not always the case with a film or video camera. In the US, for example, the household frequency is 60 Hz, and if you are shooting with a camera running at 30 fps, then that will give you two pulses of light per frame, no matter where in the cycle you start or stop the camera. This will remain consistent and not vary during the shot, so there is no noticeable flicker. This is, of course, all theoretical because in reality your camera is probably running at either 29.97 or 59.94 frames/fields per second (and not a true 30 or 60), which makes it easier to use your footage in a broadcast environment. Cameras that shoot at 24 fps can suffer from this, as well; unless you’re shooting a true 24p, your camera may be running at 23.976. How much of a problem is this under normal circumstances? It’s probably not a huge problem, although I have seen lots of color shifting going on when shooting under standard household fluorescent lights. This is something you definitely want to avoid.

What is ballast? 
The ballast is built into a fluorescent light fixture and takes the common household 120 volts, then generates the high voltage necessary to create the arc. After generating the arc, the ballast controls the flicker rate of the fixture.

However, if you are shooting at variable frame rates, or high speed, especially at frame rates that don’t easily divide into 60 (in the US) you are likely going to encounter flicker problems. To avoid this, you want to stay away from using household fluorescent fixtures, because they usually have very inexpensive ballasts, which have a very low flicker rate. You want to use “Flicker-Free” fixtures, which flicker at up to 250,000 times a second (250,000 Hz), such as Kino Flo or, if you are trying to light your scene using practical bulbs, you can try Lowel compact fluorescent bulbs, which fit into standard US household light sockets but have a flicker rate of 20,000 Hz, so they are significantly less likely to flicker than regular household fluorescents—but they are not as flicker free as Kino’s or other lights that use high-frequency ballasts.

This brings us to LED lighting, which can be powered by either Alternating Current (AC), or Direct Current (AC). LEDs that are powered by AC power supplies that step-down household voltage to levels appropriate for LED lights will exhibit similar frame-rate flicker issues as fluorescent lights. Most, if not all, LED light fixtures for video production are DC powered, either through a coax power input or batteries, which would seem to make these units flicker free. However, when powering an LED light from an AC outlet using an AC/DC power supply, you may run into a problem. A poorly made power supply can allow some AC voltage to pass into the LED light, creating what is referred to as “ripple.” This AC ripple can cause the LED fixture to pulse or flicker, affecting your shot. Using a battery can alleviate this problem, as the battery provides DC and DC only.

What is a Practical?
A practical is any light source such as a candle or flashlight; or a light fixture such as a desk or floor lamp that is in the frame and provides or appears to provide the light source for the shot.

Another consideration, when working with LED lights, is dimming. Dimming a light can cause problems across tungsten, fluorescent, and LED lighting. Dimming a tungsten bulb (household or professional) can lead to the bulb’s filament vibrating or “singing,” creating a problem for the sound recordist. With fluorescent bulbs, depending on how the bulb is being dimmed, you can end up destabilizing the arc, causing it to wander or pulse, creating a very obvious flicker. Some fluorescent lights have a limit (often 10%), as to how dim you can make them without flickering, so don’t assume a flicker-free light remains flicker free from maximum brightness to off (100 to 0%). LED lights can also suffer from flickering when dimmed, even if you power your LED light with batteries.

Dimming an LED light is often accomplished using a method known as Pulse Width Modulation, which works by cutting the power to the LED, essentially introducing off periods to the LED. This causes the light to be less bright over time by pulsing the LED. If the pulses happen rapidly enough, they are undetectable to the human eye and camera. Nonetheless, as with fluorescent lights, you now have a light source that is flickering. If the pulses don’t align with your frame rate, then your footage can show flickering, even if it is not noticeable to your eye. There is another form of dimming technology, referred to as Switch Mode Regulation, which operates at 500,000 Hz and is flicker free at virtually any frame rate.

So remember, when choosing your lighting, be it tungsten, fluorescent, or LED—most of the time, whether or not your lighting is truly flicker free or not probably won’t make a difference. But if you are shooting non-standard frame rates,  high-speed video, or dimming your lights, then using truly flicker-free lighting units may just save you from an unhappy surprise.