Light flicker during filming is a serious concern that can affect the final outcome of the clip. Addressing such issues is no easy task, because there are several factors that can contribute to flickering, such as a fixture’s electronic ballast, dimming techniques and camera recording frame rates (just to name a few). Almost all artificial luminaries do not emit continuous light beams. In most cases, humans are unable to detect such occurrences, because they happen very quickly.
What Causes Light Flicker?
One of the main factors that contributes to light flicker is supply voltage. During operation, a mains voltage switches its polarity in a rapid manner. This causes the fixture’s intensity to fluctuate, resulting in flicker. People on the set are typically unable to pick up on light flicker; but recording devices, such as cameras and some audio equipment, can detect the issue due to interference. Because of this, some individuals on the set use monitors to check for flicker problems in real-time during recording. It is important to consider that when using digital cameras, one must playback raw data by downloading the recording on a computer before checking. In some cases, meta data recordings do not reveal light flickering issues on the set.
Light Flicker and Lamps
The type of lamps used on the set may also influence light flickering concerns. For example, incandescent fixtures are known for displaying minimal flickering, because they do not react instantly to voltage changes. By comparison, discharge-based lamps are more sensitive to shifts in voltages and are prone to flickering. However, the application of electronic ballasts helps mitigate such issues by maintaining current levels being used by the unit. Ideally, to avoid flickering problems, individuals should consider investing in “flicker-free” luminaries with high flickering rates up to 250,000 times per second or 250,000 Hz.
Below are common fixtures found on movie sets and their light flickering behaviors:
• HMI: HMI lamps use discharge technology to generate light. They rely on an electronic ballast during operation, which can be “flicker-free.” One should note that artificial lights are never 100 percent flicker-free; but they can appear to be flicker-free, when such occurrences become undetectable by the human eye. Continuous use of an HMI’s ballast may degrade its features, often leading to light flicker on the set. This is primarily due to the increase of space between the electrodes over time and delays in arc extinguishing during zero-crossing. HMIs are capable of reaching flicker factor values as low as 1-3 percent (low flicker factor rates equate to minimal flickering and better performance).
• LED: LEDs are very sensitive to voltage changes, making them highly prone to flickering. As pointed out earlier, power supply is a major factor in a luminary’s flicker rate. Hence, stabilized voltages could help reduce flickering in LEDs. This becomes a problem when the lights need to be dimmed. When addressing LED flickering issues on the set, individuals must check the fixture’s flicker factor values and frequency. LEDs dimmed at 50 percent supports flicker factor rates up to 99 percent. While LEDs on transformers display 40-70 percent flicker factor values, and high quality LEDs support 0-3 percent flicker factor rates.
• Tungsten: Tungsten lamps on movie sets generally incorporate large, high-watt bulbs, resulting in minimal flicker. Smaller bulbs, around 100 watts or smaller- usually found in residential spaces, are more prone to flickering issues. Like LEDs, tungsten lights are prone to flickering problems when dimmed. Furthermore, when using thyristor dimmers, the lamps are more susceptible to displaying light flicker, especially when dimmed at 50 percent. Industrial tungsten units have a flicker factor rate of 0-10 percent, while household variants range between 10-15 percent. This figure increases dramatically when the fixtures are dimmed below general thresholds.
In some cases, adjusting the camera’s settings can decrease the effects of light flicker during recording. Cameras that shoot at normal frame rates do not pick up light flicker, because the device captures the same number of light pulses for each frame. Increasing the device’s frame rate or run off speed may cause a camera to “see” light flicker. This is due to the misalignment or inconsistent number of light pulses per frame.
Lastly, for lights that need to be dimmed, it might be worth applying Switch Mode Regulation (SMR) technology. Such methods operate at 500,000 Hz and are “flicker-free” at almost any frame rate setting on the camera. On the other hand, relying on Pulse Width Modulation (PWM) dimming techniques may increase light flicker risks, due to interference with other equipment on the set.