February 18, 2014 7:58 am
The LED Revolution – Flickering issue
LED lighting is rapidly replacing traditional incandescent and fluorescent lighting.
So large are the savings that analysts forecast that up to 90% of the lighting market will be LED by 2020. In an effort to capitalize on this market surge, new suppliers emerge every day; unfortunately, a growing proportion of these suppliers cut corners that result in lower lighting quality.
The LED driver is responsible for converting line current to the well-regulated current needed by LED lights. As the key component for assuring lighting quality, their design determines whether LEDs turn on and off smoothly, retain their color under all load conditions and when dimmed, and—perhaps most crucially—whether they will suffer from flicker.
Any artificial light “flickers”.When a current flows through the filament of an incandescent bulb, light waves emanate from the tungsten filament based on the frequency of electricity supplied—in the US and North America at 50 cycles per second. The reason why we do not perceive the resulting flicker is related to both the biggest advantage, and the biggest disadvantage of incandescent lighting. When electricity flows through the incandescent filament, it releases over 80% of its energy in the form of heat—which means that incandescent lighting is actually better classified as a heating element than a light source! It also means that the heated filament continues to emit visible light in-between AC cycles. Although the largest pulses of light occur at input line frequency, a significant amount of light emerges along the long thermal constant of the filament. Therefore, the flicker associated with standard incandescent bulbs is quite minimal and does not appear to be visible to the naked eye.
Fluorescent lamps glow when the electrons traveling inside the tube collide with phosphor atoms coating the surface of the tube.The energized state of these phosphor atoms declines steadily but, like incandescent filaments, continues to throw off light photons for a short period of time—enough to reduce the sensation of flicker.
Since the human eye and brain process information based on electro-chemical transfers that are associated with time-delays, most people do not notice the flicker of artificial lighting. This is especially the case when the light is exceedingly bright, which makes the pupil opening very small, further “filtering” the flicker effect. Our eyes and brain are more sensitive to low ambient light because the pupil opens very wide to capture all the available light. However, studies consistently prove that most people suffer deleterious effects from flicker despite being unaware of it.
Here is why flicker is more visible in LED’s:
- LEDs response-time to current changes is faster than incandescent or halogen lamps. LED response time is measured in nano seconds. In incandecent or halogen lamps the response time will be in milliseconds or even hundreds of milliseconds.
- Devices that are connected to the electricity grid may generate RFI or EMI (radio frequency or electromagnetic interferences) that can be visible on LED luminaries especially when dimming is at the lower end of the range.
- Incompatible dimmer such as trailing Electronic Low Voltage (ELV) or trailing edge dimmers add their own artifacts to lighting
- Over load. Remember: the “D” in LED stands for “diodes”. Diodes do not act as a constant resistive load–regardless of the driver output. At t=zero the LEDs are acting at high resistive load therefore the driver will start to apply output power. Once the LEDs start to conduct the load (thereby “illuminating”) they become highly conductive and drop the resistance dramatically.A miss match of the correct load will thus cause overload of the driver and will cause the driver to cut the power to the load. This creates a reactive “on-off” flicker cycle based on a built-in feedback loop. The following table summarizes percent flicker and flicker index for light sources. Higher flicker index is more noticeable to the human eye.
|Max||Min||Ave||% Flicker||Flicker Index|
|100 W MH||9.1472||3.2066||6.5147||48.088||0.1398|
|LED at DC||43.4||41.0||42.2||2.84||0.0037|
|LED w/ Flicker||15.996||0.0555||6.3026||99.309||0.4498|
Magnitude Drivers technology drivers include a specially designed transformer optimized for use with LED load. The drivers where developed in conjunction with all major dimmers currently in use to insure a wide dimming range as well as flicker free operation. The drivers integrate full wave rectification and Synchronous rectifiers insure a high efficiency rating and therefore smooth operation when loading the drivers.