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Contributed By Electronic Products
2011-11-15
Figure 1: Luminous efficacy improvements of light sources. Note the rapid increase for white LEDs.¹
When high power white LEDs were first commercialized in 1996, LED luminous efficacy was about 5 lm/W. Today, for example, commercial chips such as Everlight Electronics’ Shuen 1W Series and Lite-On’s LTPL Series boast efficacies of 90 and 95 lm/W, respectively (at a current of 350 mA). And, as we shall see below, things are about to get much brighter.
A multifaceted approach
Overall efficacy of LEDs is influenced by the efficiency of four processes.²
ηoverall = ηinj x ηint x ηelec x ηextraction
where ηinj is the carrier injection efficiency, ηint is the internal quantum efficiency, ηelec is the electrical efficiency, and ηextraction is the photon (light) extraction efficiency.
The rapid increase in LED efficacy has come about because scientists and engineers have been able to improve the performance of the devices in each of these areas simultaneously. For example, light extraction efficiency has improved by changing the shape of the die, roughening the surface, and encapsulating with a material of a refractive index close to that of the semiconductor (see TechZone article).
Knowledge of the physics behind these processes allows scientists to make reasonable estimates as to the natural limits to LED efficiency. For example, it is known that internal quantum efficiency is influenced by a phenomenon known as efficiency droop, which causes efficiency to drop as drive current is increased (see TechZone article).
It turns out that the theoretical limit for white LEDs (using a blue LED and yellow phosphor, see TechZone article) is around 263 lm/W.³ (Incidentally, the highest possible efficacy of any light source is pure green light at a wavelength of 555 nm which can reach 683 lm/W.4)
Approaching the theoretical limit
As we’ve seen above, LEDs with an efficacy of around 100 lm/W (at 350 mA) are commonplace, and there are even commercially available devices that offer more, such as OSRAM’s Golden Dragon Plus (110 lm/W at 350 mA) and Cree’s XP-G Series that promise 120 lm/W (at 350 mA).
But still more efficient LEDs are lighting up R&D labs across the world.
In March 2011, for example, OSRAM set a new laboratory record of 142 lm/W (at 350 mA) for the efficacy of a warm white LED light source. Warm white LEDs are designed to mimic the ‘warmth’ of the light emitted by an incandescent bulb. The company’s test chip has a correlated color temperature (CCT) of 2755 K and a color rendering index (CRI) of 81.
“If we explore this technical approach further and allow deviations from the Planckian curve we should even now be able to achieve higher efficiency values of up to 160 lm/W for a CCT of 3000 K,” explained Dr. Norwin von Malm, Predevelopment Manager at OSRAM (Figure 2). “If we apply this approach to a 2-mm² chip we can improve efficiency by a further 10 to 15 percent for the same operating current. We would then expect 180 lm/W for a pure warm white LED and good color rendering.”
Figure 2: OSRAM’s warm white LED set a new record for this type of device at 142 lm/W. By deviating from the Planckian curve, the company expects to push the record to 160 lm/W.
For its part, Cree broke the 200-lm/W barrier (Figure 3) for a white power LED with a CCT of 4579 K and then in May 2011, the company announced it had produced a device with a remarkable efficacy of 231 lm/W (at a drive current of 350 mA) using a single-die component.
Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.
Electronic Products
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