Evaluating High Power LEDs

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High power and high brightness LEDs (HBLEDs) have diverse applications. Prior to selecting these for the manufacture of lighting fixtures, it is important that they are subjected to close scrutiny. This article gives buyers a few key pointers.

By Deepshikha Shukla

High power and brightness are the two main characteristics of high power LEDs. An LED with a current rating higher than 20mA and power ratings of 0.5W, 1W, 3W, 5W, 8W, 10W, and so on, can be considered a high power LED. These are more costly than low power LEDs. High brightness LEDs, also known as HBLEDs or high power LEDs, provide much higher levels of light output than the traditional indicator LEDs. This places new challenges on the technology used to make them, although they can provide high levels of performance—with greater efficiency than other forms of lighting technology and a much longer lifetime. The brightness of high power LEDs is measured in lumens (lm), while for small power LEDs it is measured in candelas (cd). High power LEDs are widely used today in outdoor and industrial lighting, automobile lights, indoor lighting fixtures, flashlights, etc.

Notably, the terms high brightness HBLED and high power LED refer to slightly different aspects of operation of the same LED. Although high power LEDs and HBLEDs typically are the same, the two definitions refer to different characteristics or parameters. A HBLED is an LED that produces over 50 lumens while a high power LED consumes more than 1 watt in power.

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Improvements in high brightness LEDs
High brightness LEDs (HBLEDs) have several advantages over the standard LEDs as well as over other technologies like incandescent bulbs and CFLs. They offer higher brightness levels, longer life, and a high return on investment. Their RoHS manufacturing compatibility (lead-free) ensures they are far more environment-friendly than CFLs. The high power LEDs’ brightness is equivalent to the standard incandescent and halogen light bulbs. This makes the former perfect for automotive, industrial, homes and offices, and even hobby applications. Several enhancements have been made to basic LED technology to make HBLEDs.

Surface mount technology enabled the development of LEDs in such a way that the PCB (printed circuit board) could act as a heat-sink. With LEDs mounted onto the board, the heat could be removed with reasonable effectiveness, and this allowed light levels to be increased. With HBLEDs running at much higher power levels, dissipating the increased heat generated was a key issue. The next development was to add a thermal heat slug directly onto the bottom of the surface mount package. Being located directly under the LED junction, this allowed heat to be removed far more effectively. With this effective heat removal technique, the junction of the HBLED can remain within its safety limits while still producing the required light output. In addition, more effective manufacturing processes have enabled efficiency levels of 100 lumens per watt, and this is improving further. The high power LED incorporates an aluminium heat-sink to dissipate heat.

Types of high power LEDs
The high power LEDs are classified by package techniques —large dimension epoxy package, MCPCB package, simulated superflux epoxy package, TO package, power SMD package, etc. There can be low and non-low luminous decay in high power LEDs. These LEDs are available in both the star and ring shaped configurations, which come with a good colour temperature and colour rendering capability. These are particularly popular among architects and light designers because of their nominal correlated colour temperature of 3200K, which is close to a traditional indoor light source.

High power LEDs are available in assorted colours in 1-watt, 3-watt, and 5-watt models. They are also available in multi-colour LED form with three internal LEDs (a red, green and blue element) in both 1-watt and 3-watt options. Additional accessories such as lens assemblies are required in various models to focus the output light depending on the requirements or high-power current drivers.

How to measure brightness
Light is measured in lumens (lm) while watts measure power. Lumen measures the power of light. It is a photometric unit for the luminous flux from a light source. The term ‘photometric’ refers to the spectral response of the human eye and indicates how bright the light will be perceived. The perceived brightness of a 1W monochromatic light source will depend on the frequency of the emitted light.

Lamps are also classified in terms of their luminous performance or efficacy. The parameter ‘lumens per watt’(lm/W) measures the relationship between how much light is produced relative to how much electrical energy is consumed. This takes into account the power used by the complete lighting unit, including any losses in electronic ballasts, switching circuits, etc, that are necessary for some types of lighting. An inefficient electronic switching circuit will impact the efficiency of the entire lighting unit.

Luxeon high power LEDs (Courtesy: https://www.lumileds.com)

A candela is the measure of the luminous intensity from a small light source in a particular direction—a 1 cd light source emits 1 lumen per steradian in all directions. A steradian is defined as a solid angle that has its vertex at the centre of a sphere (at the light source) so that 1 steradian has a projected area of 1m² at a distance of 1m.

The parameter of luminous intensity applies to directional light sources like halogen lamps with built-in reflectors. Many LEDs are designed the same way. Along with this parameter, you will typically see the angle of the beam specified. The beam edges are defined as the regions where the luminous intensity falls to half of the peak value. Lasers, for example, have an extremely high luminous intensity because their beam angle is so small.

Some of the high power LEDs available in the market
A few high power LEDs available in India are listed below.

  • Luxeon high power LEDs are the flagship range from Lumileds. They are high on efficiency, helping customers keep their material costs in check, and offer a broad selection.
  • Cree’s new Extreme High Power (XHP) LEDs provide twice the lumen output and improved reliability, the company claims, enabling up to 40 per cent lower system costs. They are efficient, environment-friendly LEDs being used in backlighting, electronic signs and signals. XHP LEDs enable new lighting designs that require fewer optics, a smaller PCB, a smaller housing and less handling. They have a longer lifetime, even at high operating temperatures and currents, allowing lighting manufacturers to reduce heat-sink sizes and costs without impacting the rated lifetime.
  • Samsung’s high power LED range is the 3535. Its technology is optimised for advanced performance and compatibility. Its film coating achieves 20 per cent greater lens-ceramic adhesive strength, resulting in improved colour consistency, the company claims. These LEDs maintain maximised high flux and efficacy in a compact size, cutting down on the number of LEDs used, which leads to lower costs.
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