Nearly all AC-powered traditional light sources exhibit some degree of periodic modulation or flicker. Additionally, many traditional lighting sources produce a noticeable flicker as they near their end of life. Although desirable in some situations and not perceived equally by all people—both visible and non-visible flicker should be avoided, or at least minimized, in most lighting applications. Through its Cree Services Thermal, Electrical, Mechanical, Photometric and Optical (TEMPO) testing service, Cree has tested hundreds of SSL luminaires from streetlights to MR16 lamps to help characterize flicker and identify what levels of flicker are acceptable in certain situations, and how flicker can be minimized in LED lighting.
Metrics and Industry Standards
One of the greatest challenges with flicker is that an official industry standard does not exist to fully quantify the effects of flickering light sources. Visible flicker is usually noticed at frequencies below 100 Hz. The second volume of The Illuminating Engineer (1908) discusses the results of experiments to determine the “vanishing-flicker frequency” – the threshold where the effect is no longer observed. This is now known as the flicker fusion threshold or rate, and is influenced by six factors.
- Frequency of the light modulation
- Amplitude of the light modulation
- Average illumination intensity
- Position on the retina at which stimulation occurs
- Degree of light or dark adaptation
According to the Illuminating Engineering Society (IES) RP 16 10 standard, percent flicker is a relative measure of the cyclic variation in the amplitude of a light source, and flicker index is a measure of the cyclic variation taking into account the shape of the waveform. The drawback to this method is that it addresses only two of the six factors previously mentioned. In addition, it assumes that a light source will always flicker at a fixed frequency and amplitude, and does not address random, erratic events that cause flicker, such as a sudden decrease in electrical current or voltage.
The ENERGY STAR requirement for lamps, due to go into effect Sept. 30, 2014, specifies that the highest percent flicker and highest flicker index be reported, but does not specify a maximum allowable limit for either.
Moreover, the Alliance for Solid-State Illumination System and Technologies (ASSIST) defines flicker acceptability criteria based on their testing. Using the ASSIST criteria, at 100 Hz, percent flicker greater than 20 percent is unacceptable, and at 120 Hz, percent flicker greater than 30 percent is unacceptable.
Flicker in LED Lighting
Flicker is also nothing new with SSL. As a new technology, SSL is put under more scrutiny than the traditional light sources it is destined to replace, which is understandable after the many issues compact fluorescent lighting (CFL) had when it was first introduced to the market. Although our test results from a sample population of several SSL products show a wide range in flicker; a large majority of those products perform the same or better than other traditional light sources (to see the actual test results, read our white paper on flicker).
LED flicker characteristics are primarily a function of the LED driver. Most of the attention has focused on the ripple frequency that occurs on the output of the LED drivers, which is typically two times that of the input. For example, if the input voltage frequency is 60 Hz, the ripple frequency is 120 Hz. The light output of an LED correlates closely with the output waveform of its driver. Figure 1 shows a waveform of the ripple current from a driver. Figure 2 shows the resulting waveform of the light output of an LED connected to the driver. In this example, the driver ripple current fluctuates 46 percent and the resulting percent flicker of the LED is 36 percent.
Figure 1. Driver output ripple current
Figure 2. Measured Light Output
Flicker is also present with pulse width modulation (PWM), a technique commonly used to dim LEDs. Figure 3 shows the flicker index versus duty cycle for a square wave at three different modulation percentages. The worst case flicker index, with the value approaching 1.0, would be for a light that flashes in short, low frequency bursts.
Figure 3. Flicker index for square wave
Solutions to Flicker
A well-designed driver can reduce the perceived flicker produced by an SSL luminaire. If designing a custom driver for a luminaire, capacitance should be added to the output of the driver to filter out the AC ripple component; however, this comes with the trade-off of potentially decreasing system reliability, especially if low-quality capacitors are used. In many applications, such as replacement lamps, it may not be possible to add sufficient capacitance because of physical space constraints.
If a luminaire designer chooses to use a commercially available (i.e. off-the-shelf) driver, a driver that minimizes the amount of driver ripple current should be selected. If information on the percent ripple is not provided, it is important for a designer to get this data from the driver manufacturer before making a selection.
One cause of flickering is compatibility issues with dimming and control circuitry. It is important to specify and verify that the products are indeed compatible with the dimmers or other control circuits used in the lighting system. Problems can be caused by a faulty photosensor or timer.
Furthermore, random, intermittent flickering could be an indication of some other problem in the lighting system such as loose wiring and interconnections. Problems with the quality of the electrical supply can also result in power fluctuations. If those causes are suspected, it is important to investigate further to prevent any potential safety hazards.
Flicker index and percent flicker are typically not listed in product datasheets or labeling. Until they are, it is critical for the lighting designer to either obtain this information from the luminaire manufacturer or conduct luminaire testing to measure flicker directly.