A couple enjoying LED lighting

Comment: Demise of halogen lamps paves way for emergence of LEDs

LED light fittings are poised to fill the gap that will be left when popular types of bulb disappear from the market later this year. By Sam Woodward

The lighting industry is in the midst of a revolution. It’s just a few years since the question of whether light-emitting diodes could ever become a viable light source for architectural spaces was common. Now, thanks to the developments predicted by Haitz’s Law, which states that the amount of light of a given wavelength generated per package increases by a factor of 20 every decade while cost per lumen falls by a factor of 10, the debate has shifted to whether LEDs could ever become the dominant light source. Early challenges with colour temperatures have been overcome and we have spectrum akin to the warm tungsten sources of yesteryear, yet with significantly higher efficiency.

The days of cold or flickering LED lamps are over, and long life expectancies mean a light source no longer needs to be a user-replaceable item. Fixtures can be designed with new shapes made possible by integral LED light engines, as well as permutations that weren’t possible with replaceable lamp packages.

The LED era is arriving at just the right time. For example, legislation comes into effect in September this year that will remove directional halogen lamps such as the ubiquitous GU10 type from market in EU member countries, with the same fate in store for non-directional lamps in 2018.

The transition will not necessarily be straightforward. Simply replacing halogen light sources with new retrofit LED lamps can lead to problems with flicker, poor low-end dimming performance, audible noise and even interference with products such as audio-visual systems.

The part of the overall LED system that has most effect on performance is the driver, or electronic control gear, which converts the incoming mains voltage into the low-voltage direct current. There are many types, including constant current for LEDs in a series string with a fixed current to determine brightness, and constant voltage for LED tape where the exact length, and therefore the total number of LEDs to be powered, is unknown prior to installation.

One of the most important driver features is the quality and consistency of its DC output. The nearly instantaneous response of LED chips to a changing current makes them highly susceptible to flicker, especially compared with older incandescent sources where the white-hot filament meant that instantaneous darkness wasn’t possible, and the slower rate of cooling suppressed most fast-flicker issues.

There are two methods to dim an LED: chopping up the mains supply to the driver – also known as phase-cutting or mains-dimming – or using a driver that accepts a fixed mains supply and a low-voltage control signal to instruct the driver how much power to deliver to the load.

Mains-dimming is often used for retrofit LED lamps because the driver only has mains connections via the lamp cap. The challenge in Europe is that whilst there are many safety standards that apply to lamps and fixtures, there are none to ensure that drivers and dimmers are compatible. The only way a designer can be certain that a system will work as intended is to test each combination of lamp and dimmer, and in every quantity combination.

Verifying compatibility has to go far beyond a simple ‘look and see’ test; invisible issues such as inrush currents, power-factor problems, or electrical and audible noise can affect flicker, interference and system life. Thorough testing is time-?consuming, but in the absence of EU compatibility standards is the only route to achieving success in the real-world conditions of a building project.

When controlling LED fixtures rather than retrofit lamps, greater success can be achieved by using a driver that takes a fixed mains supply along with a low-voltage signal that indicates to the driver circuit what proportion of the output power should be supplied to the load. This technique is less prone to flicker because the driver’s input power supply is not chopped, so the driver circuit will always have the power that it needs to perform the voltage conversion.

Whatever type of LED is used, compatibility, longevity and proper performance are only reliably determined by thorough testing. The good news is that with good quality lamps, and robust controls, the LED era opens up exciting possibilities to enhance lighting projects.

Sam Woodward is customer education leader for Europe & Africa at Lutron and co-author of the IET Code of Practice for the Application of LED Lighting Systems.

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