High Power LED Technology

Presented by ELEKTOR

LED Look around and you'll discover that everything from flashlights to tail lamps are going diode... light emitting diode that is. Improvements in LED technology have led to more and more design situations where LEDs are replacing traditional filament lamps.

LEDs are distinguished by excellent reliability and recently
high-powered LEDs have become commercially available opening up new avenues. In this article we shed light on the basics of optoelectronics and the role of LED.

Lumens or watts?

CIE Eye sensitivity curve Light is measured in lumens (lm). While watts measure power lumens measure the power of light. It is the photometric unit for the luminous flux from a light source. The term 'photometric' indicates that the spectral response of the human eye and gives an indication of 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 (If it is in either the IR or UV part of the spectrum then nothing at all will be visible). The relative sensitivity of the eye (daylight adapted) is shown in Figure 1.

Let's do some light math. As demonstrated, the eye's sensitivity is at its maximum at a wavelength of 555 nm. If we are using 1 W monochromatic light source emitting at this single frequency would result in a luminous flux of 683 lumens. This figure is the sum of all the light emitted irrespective of direction. If, however, it were possible to maintain the same output power but change the color of the emitted light to red (with a wavelength of 650 nm), then the luminous flux will fall to 73 lumens (683×0.107 = 73 lumen).

Lamps are also classified in terms of their luminous performance or efficacy. The parameter lumens per watt (lm/W) gives 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. Look at the table below. It is interesting to see the spread of figures for each of the different types of lamp technology. Low pressure sodium lamps boast the highest value of efficacy and their unflattering yellow light is a familiar sight in a street lamp. White light sources, on the other hand, have a lower efficacy because its energy is spread throughout the visible spectrum.

Light source Electrical Energy
(Watts)
Luminous Flux
(Lumen)
Luminous Intensity
(Candela)
Efficacy
(lm/W)
Filament lamp 75 900   12
Fluorescent lamp 58 5400   90
Low Pressure Sodium 130 26000   200
Hg High Pressure 1000 58000   58
12 Volt Halogen 65 1700   26
Halogen Reflector 10° Beam 50   12500  
Halogen Reflector 60° Beam 50   1100  
Luxeon LED 1 18   18
Nichia LED 20° 0.08   6.4  

Characteristics of common light sources

Candela

Why don't you see luminous flux when you look at the light output on an LED data sheet? Instead, you will see the term "cd" which stands for candela (and not a music disc). 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 lm per steradian in all directions where a steradian is defined as a solid angle that has its vertex at the center of a sphere (at the light source) so that 1 steradian has a projected area of 1 m2 at a distance of 1 m:

Luminous intensity = Luminous flux per steradian

The luminous intensity parameter is usually given for 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. Be careful trying to compare luminous flux and luminous intensity. It's not always possible to compare the two. Plus, it's also important to take into account the spectral content of the light emitted.