Guest Post by Kevin Willmorth
Flicker presents itself in several forms, each with its own cause. For lighting decision makers, avoiding flicker requires understanding what causes it and what can be done to avoid it.
For a good understanding of what flicker is and how we see it, NEMA’s position paper TLAs-2015 Temporal Light Artifacts describes the basics of flicker, while NEMA 77-2017 outlines guidance criteria and test methods. The DOE also offers a great deal of information on flicker in its Flicker Fact Sheet. Further, California has published JA 10, describing its required procedure for measuring flicker.
Ideally, solid-state LED products should not present visible flicker. However, electronic devices do exhibit behaviors when used in combinations, that may produce undesirable flicker when dimmed. The best practice is to use only dimmers manufacturers have tested and recommend. Unfortunately, the variety of dimming products precludes every possible combination from being pre-tested.
Steady State Flicker – Consistent and Constant Frequency
Steady state flicker is when a light source is modulating at a constant frequency low enough to be seen. The most common example of this is the 120Hz flicker of uncorrected AC powered light sources. When LED sources are connected to power supplies that allows incoming AC sine waves to reach the LEDs, flicker of 120Hz will be present and noticeable. Dimming will increase this effect.
As a guide, the IEE 1789 Recommended Practice sets a limit of no more than 10% flicker at 120Hz, while modulation above 1,250Hz is not visible to the human visual system. Achieving 10% at 120Hz with phase cut dimmers in not attainable without special luminaire electronics to mitigate flicker.
Steady state flicker may also manifest itself with line voltage dimming controls that cut incoming sine wave signals into truncated segments to reduce light output. Leading edge dimmers, also known as TRIAC dimmers, cut the leading edge of the sine wave off. Trailing Edge dimmers, also known as MOSFET or ELV dimmers, cut the back side of the sine wave off. Most luminaires are designed to operate on one or the other type. Be certain the dimmer used is of the appropriate type. Further, look for products that comply with NEMA SSL 7A, which sets a standard for behavior of LED Drivers and phase cut dimming products. Should dimming increase apparent flicker as luminaires are dimmed, there is likely an issue with dimmer compatibility.
Another issue with line-voltage dimming is mixing products of different driver types or specification on a single dimmed circuit. Interference or interaction between incompatible products sharing a common dimmer control can produce unpredictable results, including flicker, truncated dim function, flashing, and buzzing. This is particularly troubling when products designed for trailing-edge dimming are connected to the same dimmer as fixtures designed to be dimmed from leading-edge controls. These issues can be avoided by only connecting like-products, and products proven (through testing) to be compatible on individual dimmer controls.
Flickering in 0-10V or digital low-voltage controls is frequently caused by mixing product that use drivers that are conflicting with one another, incompatibility between the dimmer and the driver’s dimming circuitry, noise on the low-voltage wiring (frequently picking up a 60Hz signal from a nearby high voltage wire), or issues of driver input voltage vs. load voltage within a luminaire that causes the driver to flicker as it struggles to maintain current flow. For this reason, it is prudent to keep dimmed circuits clear of high-voltage wiring, use shielded control cables, limit luminaire types on any given control circuit, and utilize luminaires and dimmers that have been verified compatible with one another.
Flickering that occurs as the dimmer control is moved is a common dimmer problem caused by either poor dimmer circuit design or too few digital “steps” being used over the dimmer control’s range of movement. Higher-quality dimmers include features and firmware to create smooth dimming function, or use a minimum of 256 steps to create the impression of smooth dim response.
Intermittent Flicker – Flashing, Strobing, or Low Frequency Pulsing
There are several types of intermittent flicker. To start, driver products frequently produce pulsed light output as an indicator of a connection error, miswiring, or reversed polarity or another fault. If luminaires strobe only when dimmed, the issue may be caused by dimmer incompatibility, or mixing drivers from different manufacturers, or manufacturing batches on the same control, creating interference between drivers. Strobing is also caused by a driver being loaded with too little forward voltage to maintain current regulation. This is more common at 277V than it is at 120V. Diagnostics of these issues requires bench testing to isolate the issue and allow the manufacturer to determine a proper corrective action.
Resolution of flicker in dimmed products requires careful selection of products that are proven compatible with luminaires to be controlled. This may require pre-testing products during the selection phase. Should flicker be experienced after installation, manufacturers will need detailed information describing the behavior of the product(s), operating voltage, connected load, dimmers used, and luminaires sharing the dimmed circuit. Both control and luminaire manufacturers need quality information to produce satisfactory solutions.