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© David Morgan Associates 2010. David Morgan runs David Morgan Associates, a London-based international design consultancy specialising in luminaire design and development.
Email:
david@dmadesign.co.uk Web:
www.dmadesign.co.uk Tel: +44 (0)20 8340 4009 The views expressed in this article are entirely those of David Morgan and not necessarily those of mondo*arc.
Comparative images of the Polestar LED 2010 Ambient Dim downlight against both tungsten halogen and a retro fit LED
used in the sample. The LED array pcb is mounted onto an appropriately sized stan- dard pin type heat sink and is held into the downlight, along with the seven element lens, with a single coil spring.
The only problem with this product is that, as far as I can tell, it is not very efficient. It needs two drivers to achieve the colour changing effect which doubles the typical driver losses of 20% and the dark light baffle in the downlight must cut out a significant amount of light.
According to the calculated figures we received from John Cullen the total light output from the LED light engine is around 400 lumens which is equivalent to a 20 watt halogen lamp. The power consumption was stated at 8 watts but we measured more than 10 watts going into the two drivers needed to control the two sets of LEDs so the efficacy figure of 53 lumens per watt mentioned in the specification sheet seems rather optimistic and a figure of 40 lumens per watt seems more likely. Once the light losses within the darklight baffles of the lu- minare are also accounted for it is unlikely that this downlight could comply with the Part L1 requirement of 45 lumens per watt for the complete system including drivers. We tried running all seven LEDs from one driver and the power consumption did fall to around the specified 8 watts but then the colour changing effect would not work. The acid test with any LED product is to make a direct comparison with the conven-
tional light source it is meant to be replac- ing. We compared the light output in terms of colour appearance, beam angle and lux level with various 12 volt MR16 halogen lamps run in the same downlight housing. The beam angle of the Polestar light engine sample we were given was fairly narrow and the closest halogen lamp we found to match the centre beam light levels from the Polestar LED downlight was a 10 watt 21 degree MR16. It could be that the large size of the LED lens array leads to greater losses in the dark light baffle. The 10W MR16 gave around 880 lux in the centre beam at 1 me- tre compared to 710 lux from the Polestar downlight.
On full power the Polestar LED 2010 Ambi- ent Dim downlight gave a pleasant qual- ity of light but it had a distinct pink tint compared to the halogen. When dimmed down to minimum level however the colour temperature of the two sources was very
similar so the dimmed colour changing ef- fect was definitely a success. The CRI of the Cree XPE used in the downlight is only 84 so the red level is low compared to halogen. However normal skin tones looked fairly natural in the light out- put. Since there are now a variety of LEDs with CRI’s over 90 and these do give better rendition of reds, hopefully John Cullen is investigating these for future use. Although the efficiency figures quoted for the Polestar LED 2010 Ambient Dim do seem to be rather misleading and the product would probably not comply with Part L, its miniature size, good glare control and neat appearance make it very usable on many residential and hospitality projects. It certainly provides much more attractive light quality than many LED downlights I have seen and the colour temperature shift feature works well.
www.johncullenlighting.co.uk
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