Is this the end of the fluorescent lamp?

Almost anyone who has ever worked in an office will be familiar with the cold-white, shadowless light produced by most fluorescent strip lights.

Indeed, the fluorescent tube is today the most common choice of light source where the system designer requires very high efficiency and reasonably good colour rendering. Commonly available in the ‘T5’ format, fluorescent strip lights have an obvious appeal to business and home users, because they are cheap to buy and low on electricity cost.

But in fact, fluorescent tubes have significant drawbacks: they offer a limited lifetime (typically less than 20,000 hours); they contain mercury, a toxic chemical, and require special disposal arrangements; they are more difficult to drive than incandescent lamps; and they often require bulky optical fixtures to optimise light distribution and hide the unsightly tube.

Until recently, there was no alternative light source to rival fluorescent for efficiency and cost. But power LEDs are now starting to offer a competitive $/lumen performance: improvements in power LED design and manufacturing mean that the flux (quantity of light) per device is continually rising, while the cost per device stays fairly constant. The equation is turning in favour of the power LED.

In fact, there are a number of reasons why the market is interested in adopting power Research work by Polymer Optics Limited (POL) in the UK has shown how an LED-based luminaire can replace a standard 1.2m fluorescent strip light.

This demonstrates that a luminaire designed using LUXEON Rebel LEDs from Philips Lumileds can achieve a similar light output to that of a high-performance fluorescent strip light while consuming a similar amount of power.

Analysis of a typical fluorescent strip light
The benchmark for POL’s LED designs was a scaled model of a typical fluorescent tube. POL used a standard GE 4ft (1.2m) 28W T5 tube, with a colour temperature of 6,500K. This tube is nominally specified to provide 2,450 lumens under normal operating conditions.

To make it easier to perform a raytrace analysis of the light output from the LED system, the 1.2m tube was scaled down to 300mm. The 300mm scaled fluorescent tube model was rated to emit a luminous flux of 640 lumens while consuming 7.3W.

The fluorescent strip system showed a peak luminous intensity of 380lux at the centre of the photometric distribution pattern projected on to a 3.6x2.4m plane at 1m range. If the LED system was to rival the fluorescent luminaire, it would need to provide a comparable output and distribution of light, while consuming around the same amount of power.

Designing an LED-based alternative: key considerations
Among the important benefits of power LEDs are their long lifetime and their high efficiency. System design choices have a large impact on both these factors, and it is important for the designer to know how to balance the effects of temperature, drive current, number of devices and cost in order to match the performance of a fluorescent strip light.
For instance, in order to make an LED system competitive on cost with a fluorescent light, it must offer a very long lifetime: this balances the higher bill of materials cost (attributable to the LEDs) against the high repair and lamp replacement costs of the fluorescent system.

The GE tube used in the study has an average rated life of 20,000 hours.

Power LEDs are not specified with a similarly straightforward average rated life. LEDs do not fail completely in the way that an incandescent lamp does; rather, lumen output declines gradually over time. So LEDs have a ‘lumen maintenance’ rating, which shows how long it takes for lumen output to decline from its initial peak. Many LED vendors quote a 70% lumen maintenance figure in datasheets, but in truth, the system designer can choose whether to target a higher or lower lumen maintenance figure than this, depending on the desired lifetime/relamping interval for the luminaire and the expected usage.

The designer has a choice to make not only over the lumen maintenance percentage, but on how to achieve it. Put simply, the higher the junction temperature at the silicon die, the quicker lumen output will decline. In the same way, a higher drive current also reduces lumen maintenance more quickly, as well as lowering efficiency. 

LEDs replacing a strip light: a practical implementation
POL’s optical assembly for a fluorescent tube-like light distribution is of similar dimensions to a typical tube, so it can be retrofitted into existing strip light fittings. The optical assembly can also be made in a modular fashion, so that a number of different strip light lengths can be retrofitted from basic LED optical modules.

The Strip Light Replacement model is a 300mm assembly using six equi-spaced LUXEON Rebel LEDs (part number LXML-PWC1-0100) from Philips Lumileds. The manufacturer’s rating for the LEDs specifies a 100 lumen output (or ‘flux’) at a thermal pad temperature of 25°C. But the flux of an LED varies depending on the junction temperature of the device, which in turn is affected by the ambient temperature, enclosure design and so on, and these factors will be different in every application. Flux declines as temperature rises.

POL’s design is therefore modelled on an 80 lumen output, which is a conservative estimate of the output that most real-world designs will achieve. Thus the six LEDs produce a total input light flux for the system of 480 lumens. The choice of the LUXEON Rebel LED is important, because the device’s high efficacy – up to 100 lumens/Watt – helps the LED system compete with the very high efficiency of fluorescent technology.

The raytrace of the Strip Light Replacement design gives an illumination distribution as shown in Figure 3. It achieves a similar lighting effect to that of the fluorescent luminaire.

The efficiency of this LED design is around 65 per cent. The Strip Light Replacement optic diffuses light over a wide area; diffusion results in light loss. Nevertheless, the Strip Light Replacement achieves a peak luminous intensity of 320lux, which compares well with the 380lux peak illuminance achieved with the fluorescent strip light, especially considering that the input luminous flux of the LED system is 25 per cent lower than the input flux of the fluorescent strip light (fluorescent system input flux: 640lm; LED system input flux: 480lm).

A final consideration in comparing the results of the design analyses is that the 300mm-equivalent fluorescent tube has a scaled power consumption of 7.3W; the LED luminaires consume around 1.2W per LED, for a total of 7.2W.

This practical implementation of LED-based strip lighting by POL demonstrates that an LED system can achieve better illuminance than a fluorescent system for applications that require directed light, and comparable illuminance for applications that require even lighting of a wide area, while consuming the same amount of power.

Jaume Tarradellas is technical solutions manager at Future Electronics (Spain)