Han van der Steen Ph.D. Innolumis Public Lighting

han.vandersteen@innolumis.nl

52 TRAFIK & VEJE • 2012 OKTOBER

Concept for lightingVisual perception at public lighting is about the perception of brightness, sharpness and colours during illumination levels up to about 15 lux or 1 cd/m2. These levels are low for the human eye. Compare daylight conditions having an illuminance of 10.000 to 100.000 lux. It should be noted that our visual perception at lower levels is slightly different from the illuminance levels at day-light. Using a tuned spectrum of the light source, only little energy is needed to pro-vide excellent visibility.

Applying 40 pure red, green, and blue LEDs per luminaire gives full control over the spectrum. This renders excellent visi-bility. Furthermore, esthetical atmospheres can be emphasized, such as emphasizing green nature areas by using a spectrum with a green hue, or applying a pure white spec-trum in more residential areas. Besides an optimal spectrum, it is important not to hinder the perceived field of view as a result of blinding light sources and also to pro-vide uniformly distributed light on the road surface. Applying flat-glass luminaires with mirror optics can achieve this. The amount of direct light from the LEDs is reduced to a minimum. As a result, the perception of details, objects, motion and colours is opti-mised to its best.

Dimming the light levelDimming conserves more energy and there-fore reduces costs. The great advantage of dimming an LED over dimming conven-tional lamps such as PL or sodium lamps is

that the volume of light is proportional to the energy consumed. Reducing the electric current in the LED dims lEDs. An LED that burns at half its capacity also consumes half the energy. Dimming LEDs always yields energy reduction and is therefore usually paired with cost reduction. PL and sodium lamps, on the other hand, cannot be dimmed as efficiently, as a result of which the energy reduction is less. Dimming may then even be more expensive.

Best of LED techniquesIn the area of LED concepts there are basi-cally two techniques to produce a spectrum suitable for public lighting. There is the technique that applies LEDs with a phos-phorous coating and the technique using only pure LEDs of different colours. Coated LEDs suffer efficiency losses over time due to heat problems from the activity in the phosphorous layer (Stokes shift) and due to degradation issues of the phosphorous layer itself. Luminaires having these phos-phorous LEDs need a change of the light sources after some years. Pure LEDs on the other hand, do not have a coating and consequently do not suffer these efficiency issues. The item that remains, and which holds for both LED techniques, is to cool the LEDs appropriately in order to keep the temperature low, at least below a critical temperature.

Heat managementLEDs have a long lifespan. When the elec-tric current running through them is not too high and the LEDs are maintained at the right temperature, they will last for more than 100.000 hours. The strength of the electrical current is easy to regulate and

limit with proper electronics. Cooling it is more complicated. Whereas compact fluo-rescent lamps need heat to operate, this is not the case for LEDs. When in operation, LEDs produce heat and this heat needs to be disposed. It is a great challenge to dispose the heat from an LED as quickly as possi-ble so that the LED does not get too warm. The temperature of the LED must always remain beneath a critical temperature, the so-called junction temperature. To accom-plish this, the LED must transfer its heat to the material on which it is mounted: the fixture.

Absorbing and radiating heatThe design philosophy is that LED light sources and the streetlight fixture need to be designed together, in coherence. Only in this way, the crucial cooling process by the fixture can be tuned to the light source: from the tiny LED surface to the fixture. The fixture material, to which the LED dis-perses its heat, needs to be able to absorb the heat and radiate it. In order to absorb, the fixture needs a certain amount of mass. On the other hand, the luminaire may not get too heavy. Their heat radiation coefficient is 0,9. As a comparison: a perfectly radiating black body has a radiation coefficient of 1. A polished piece of aluminium or a piece of shining chrome has an emissivity of less than 0,2; such materials are relatively poor at emitting heat. The design is optimised for the transfer of heat and we are able to make the outside smooth, without fins or other heat sink solutions. This saves materials and results in a great advantage for maintenance. A smooth design does not get as dirty and is easy to clean. The smooth design also

LED luminaires with 20 years lifetime

An innovative concept for lighting public spaces makes light using sustainable LEDs. The light spectrum corresponds optimally to the human eye sensitivity at dusk and night. As a consequence, the lighting needs less energy than conventional lamps to provide better visibility. In September 2012 Innolumis produced their first Danish street lighting project in the city of Esbjerg. The luminaires have 80.000 hours / 20 years warranty. The secret of this long lifetime is in the concept and technology.

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TRAFIK & VEJE • 2012 OKTOBER 53

provides another advantage. The maximum wind load surface is small, namely 0,12 m². This makes the luminaire relatively insen-sitive to wind and does not put too much pressure on the mast.

Selective in componentsLED technology is mature since only a few years while clients frequently ask for life-times of 20 years. That is why it is important to know the electronics and its components in technical detail and use certified and

RoHS clean products from reliable manu-facturers. Furthermore, choosing products on the basis of technical details such as the dimensioning, mechanical robustness, heat management, and redundancy. Although these capacitors are low cost, they have lim-ited lifetime and are not suitable as a com-ponent in a sustainable electronic circuit. There are sustainable alternatives.

Long lifetimeLED lighting is used in residential areas,

along roads, in parks and parking areas. Also in areas where light pollution should be lim-ited to a minimum, such as nature areas, our lighting proves its added value. With the in-novative concept and technology, products are assured of the quality offered and can live up to a long life expectancy of 20 years / 80.000 hours. Sustainable through inno-vation.

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Figure 1. Green nature areas by using a spectrum with a green hue.