LED or Luminous Efficacy Demystified

Efficacy, power and costs of high power LEDs have now reached levels that make them attractive in general lighting applications.

The luminous efficacy of a light source is the ratio between the emitted luminous flux and the amount of the absorbed energy to transmit it. It is expressed in lumen/watt (lm/W), where the lumen is the measure unit of the luminous flux. However, the lumen is based on the subjective perception of light by the average human vision, corresponding to a particular curve inside the visible spectrum. To put it simply, a standard incandescent light emits radiation both inside and outside the visible spectrum. The radiations emitted in the infrared and in the ultraviolet do not contribute to our perception of brightness. A light source will have a higher luminous efficacy as much as it will be able to emit in a spectrum suitable for the human vision.

The LED technology has made significant progress on the emitted power front. Today standard high power LED are available in 1, 3 and 5 watt, and multi-chip emitters are becoming available that push LED power towards the 15 watt. But above all high power LED have ten times more efficacy than the incandescent sources. Every serious manufacturer offers high brightness LEDs with minimum luminous efficacies well above 80 lm/W. Nowadays 100 lm/W minimum luminous efficacies are quickly becoming the norm for white light.

To quickly illustrate the point, a 60 watt standard light bulb, with a source efficacy of 15 lm/W, produces a luminous flux of 60 x 15 = 900 lumens. A light source built with nine 1 watt high power LEDs, with a luminous efficacy of 100 lm/W will produce the same luminous flux, but will use only a power of 9 watt instead of the 60 watt of the standard light bulb.

For a more accurate comparison between different lighting systems we must take into account the entire system that produces the luminous flux:

• Source luminous efficacy (lm/W): it is the primary luminous characteristic of the light source, and varies according to the given technology.
• Electrical efficiency (%): it defines the incurred losses to adapt the standard electrical source to the need of the considered technology. For example, incandescent lamps are directly connected to the power line without any adjustment. This is not the case for other technologies such as the fluorescent lamps, which require ballasts with 60-70% efficiency. Similarly, an inverter for compact fluorescent lamps has an efficiency of 80-90%, while an AC/DC driver suitable for LEDs can have an efficiency higher than 90%.
• Fixture efficiency (%): standard incandescent and fluorescent light sources radiate in almost all directions and require reflectors and diffusers to shape the light beam for the required application. These systems have an efficiency which is usually estimated between 30 and 50%. By comparison, light emissions from LEDs are inherently directional, and 95% efficiency can be assumed.

Taking into account the entire chain, the efficacy of incandescent lamps will be as low as 7 lm/W, whereas fluorescent will achieve 38 lm/W and high power LEDs reach 76 lm/W. A better evaluation of the electrical power necessary to produce the 900 lumens would give 128 watts in the case of incandescent lamps, 23.8 watt for fluorescent fixtures and 11.8 watts for LEDs. A significant energy saving!

Evolving view points for an evolving technology

To me a great majority of blog posts talking about LEDs are frankly disappointing. Although some try to take an exhaustive approach at presenting the technology in terms that can be understood by anyone, their content often remains static and academic, often copied from the same source. As these blogs are likely to come on top of the search engines responses, the casual reader may end up drawing the wrong conclusions with regards to the usability of the LEDs technology. Obviously at this point in time it would be biased to present LEDs as “the” perfect, “do-it-all” technology for general lighting. But this is a fast evolving technology, and, as for computer hardware, Moore’s law is applicable: today’s “truth” may become obsolete within a year or two. As a result, one has to remain cautious when presenting LEDs technology strengths and weaknesses.

In general lighting applications LEDs have advantages and disadvantages when compared with other light sources such as incandescent or fluorescent lamps. When looking at the positive side, the most significant advantages are fast turn-on, lower heat generation, lower power consumption, higher operating life, and high resistance to shock or vibration.

On the negative side, many blog have not been updated and retain obsolete information which may induce the casual reader into drawing hasty conclusions. Amongst the recurring limitations these blogs describe are the narrow viewing angle, and the need for electronic driver circuits to operate.

Starting by the later, LEDs need to be driven properly to ensure optimal performance and long life. An effective driver is key in obtaining all the benefits of LEDs. If early driver's implementations made of discrete components were not cost effective, this is not the case anymore. Pushed by the fast adoption of LEDs in the automotive and aeronautical industries, today almost every integrated circuit manufacturer proposes a vast array of LED drivers to suit almost every aspect of general lighting requirements. Furthermore, the leading manufacturers’ constant current sources, which until recently were only able to drive a limited number (usually 3 to 6) of power LEDs have recently been superseded by new affordable sources with 3 to 4 times more capacity.

Let’s now look at the viewing angle. First of all, light emissions from LEDs are inherently directional, thus reducing the need for reflectors and diffusers that can trap light. As a result, general lighting LEDs fixtures can potentially deliver light more efficiently to an intended location, leading to potentially higher application efficiency than other light sources in certain lighting applications. By comparison, fluorescent and incandescent lamps emit light in all directions. In their case, much of the light produced by the lamp is lost within the lighting fixture, reabsorbed by the lamp, or escapes from the fixture in a direction that is not useful for the application. For many fixture types, it is not uncommon for 40-50% of the total light to be lost before it exits the fixture.
Now, if the early generations of power LEDs were exhibiting narrow light emission angles (from 30º to 50º), the latest generations emit light at much larger angles, between 120º and 160º. This cannot be qualified as “narrow” anymore, and opens up new general lighting applications to the use of power LEDs.

Once again, as in too many cases of emerging technologies, the information for the public tends to remain far behind the actual technical advance...

Display of power

Yesterday as I was reviewing various power LEDs’ technical datasheets, trying to extract useful parameters and feed them to my lighting design software, it struck me how manufacturers use such diverse ways to express the same characteristics of their products.

Luminous efficacy, expressed in lumen/W, is one of the most important parameters for lighting design. But when it comes to high power LEDs, it can be wrongly interpreted. It is probably the result of using a simplified description for these LEDs calling them 1W LEDs.

In effect, the actual usable power in the LED is equal to the driving current multiplied by the forward voltage. The latter changes significantly between LED families of the same or different manufacturers, and may vary according to application usage between 3V and 3.6V.

For LEDs driven at 350mA, the usable power will vary from 1.05W to 1.26W, inducing a 20% variation of the emitted light flux, which is certainly not to be ignored. By the way, this is the power value to be used when comparing luminous efficacies, as it includes the small amount of power wasted as heat in the LED.

Did you say white LED?

The LEDs (Light Emitting Diode) are semi-conductor diodes that emit light when traversed by a current flow. A photo emission is taking place at the diode PN junction region, and the total quantity of emitted photons, therefore the light intensity, is proportional to the current intensity that passes through it.

The spectrum of the emitted light is primarily defined by the type of materials used to build the diode's PN junction, although it also depends on the current’s intensity and on the junction temperature.

LED manufacturing uses diverse technological process variations that lead to the production of different families of light color, such as:

• Gallium arsenide (GaAs) for light from infrared to red (650 nm);
• Gallium arsenide and phosphate (GaAsP) for light from red to yellow (630-590 nm);
• Gallium phosphate (GaP) for wavelength from blue to green (565 nm);
• Gallium nitride (GaN) for blue light (430nm);
• Indium and Gallium nitride (InGaN) for light from deep blue to ultraviolet (390 – 360nm);

The white power LEDs are usually based on blue LED chips of the more recently introduced InGaN family. The white color is produced using a blue light chip covered by one or more semi-transparent layers of phosphors. Using the light’s complementary color combination, appropriately chosen phosphors layers combined with the base blue light allow the creation of various white lights.

As a result, today’s white power LEDs generally behave like the InGaN products' family, with some variations, due to the presence of the phosphors.

Is the general lighting industry ready for LEDs?

Reading this post I was wondering if today’s lighting industry is ready to design and produce power LED lighting addressing general users’ needs.

LED technology is something we have been hearing more and more about from television, magazines and news reports. However, very few people understand why this light source is such a powerful alternative. The advantages of LED light sources over traditional lighting sources are numerous but can be simplified into energy efficiency, longevity, power and versatility. Despite these advantages, creating power LEDs based lighting fixtures is challenging because the technology and the associated constraints require a different knowledge from the traditional general lighting know how. To highlight just a few differences, a power LED based lighting fixture generally involves:

• a number of electronic components which are not usually found in today’s standard lighting fixtures,
• a thermal management system to ensure the proper functioning temperature of the semi conductor which produces the light,
• an optical system to propagate and adapt the light for the specific purpose of the lighting fixture.

Beyond these technical differences, I see two major human reasons behind the slow adoption of LED technology in general lighting:

• the rather feeble interest shown by designers of general lighting fixture for the technology
• the heavy investments made by the industry in trying to make existing technologies more efficient

First of all, the components of power LED fixtures come in different form factors and present different usages patterns from what many lighting professionals, including designers, are using in their day to day work. In summary, using power LEDs requires a different type of knowledge, which in turn implies a certain learning curve.

Besides, I am under the impression that many interior lighting designers behave as stylists, “kooky kids” who like to do fun, pointless things, being more preoccupied by the actual “look” than by the “function” of the lighting fixture. When a given style becomes “hot”, legions of designers imitate it until it is everywhere, and as a result, most of today’s interior lighting designs boil down to flurries of boring light shades and diffusers stuck around inadequate incandescent, halogen or fluorescent sources.

As to the manufacturers, their weak interest make me believe that they re-iterate a well known incumbent behavior: they dismiss a disruptive technology to “protect” investments made in the previous technology. Furthermore, LED manufacturers only address part of the overall market, forgetting to properly educate other sectors of interest. This is particularly obvious of power LED manufacturing, where a vast majority of today’s production is destined to the automotive industry. Obviously, with the ever increasing amount of electronic in our cars, automotive engineers do not seem afraid of an additional “pile of semiconductor chips” and certainly know how to put them to good use.
Unfortunately, the same does not apply the general lighting industry! There, in view of the substantial investments made over time in studying, designing and using the current range of halogen and fluorescent lighting, everything looks like the adoption of power LED technology will take another century.

Finally, there is a split within the general lighting industry regarding the form LED luminaires should take. Some manufacturers are focusing on producing LED light sources that will accommodate existing lighting infrastructure and resemble existing decorative and task lighting fixtures. In my view doing so is rather short sighted, and wastes the potential of a promising technology. A better approach is, in my opinion, to design new light source formats that will deliver lighting in unexpected and innovative ways that could not be accomplished with previous lighting technologies. The fact that LEDs can be incorporated into just about any material, including fabrics, means that lighting could be made to emanate from the very surfaces and shapes of objects, from architectural elements to furniture.

At the risk of sounding overbearing, I believe some of these attitudes by the industry's actors delays a particularly attractive and efficient technology and limit the opportunity of participating to higher level discussions. In particular, although it is unlikely that incandescent lamps will be banned outright, the lighting efficacy standards under consideration all over the world would set minimums so high that most incandescent lamps would become ineligible for use by 2012 at the earliest. In this context, the lack of traditional sources efficient enough to meet the proposed efficacy standards will leave a huge gap in the market, that could be partially filled by LED lighting fixtures and luminaires.