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John Cullen Polestar LED 2010 Ambient Dim

Issue 58 Dec / Jan 2010/11

David Morgan takes a look at the Polestar LED 2010 Ambient Dim™ by John Cullen, an LED downlighter that changes colour temperature to mimic the effect of a halogen or incandescent light source

John Cullen was founded in 1981 and provides a complete lighting design service as well as supplying a range of upmarket luminaires for residential and hospitality lighting applications.

Most of the luminaire range is designed in house by lighting design director Sally Storey with other members of the lighting design team and product designer Graham Lunn.

The Polestar downlight range was originally designed for use with MR16 halogen lamps and is small to match the scale required for residential projects. With a bezel diameter of only 69mm and made of thin steel the downlight is very discrete. The black anti glare baffles gives a well controlled darklight effect. With 30 degree aiming from the vertical it is designed for picture and feature lighting as well as general downlighting.
The Ambient Dim LED 2010 white light engine is a new development that intriguingly changes colour temperature as the LED light engine dims in an attempt to mimic the warming effect of a dimmed halogen or incandescent light source. This is certainly an effect that is very desirable for any residential low energy efficient lighting product as commercial LED downlights do not give a suitable ambience in these applications.

Although the colour temperature changing feature is not unique as there are now a number of retrofit lamps on the market which incorporate this effect the John Cullen system is certainly one of the first to be integrated into a residential scale downlight. Colour temperature changing retrofit LED lamps from Ledzworld from The Netherlands and Feelux from Korea are GLS or PAR sized and designed to fit into larger luminaires and table lamps.

The LED light engine used in the Polestar LED 2010 Ambient Dim combines a central warm white 1 watt LED with a surrounding ring of six cooler white 1 watt LEDs. The colour temperature shift effect works by keeping the warm white LED on full power while the cooler white LEDs are dimmed so as the amount of light from the cooler LEDs diminishes the overall effect becomes warmer until only the warm white LED is left on. A colour correction filter is also included in front of the standard collimating seven element lens and this enhances the warming effect still further.

The colour shift while dimming effect is not quite as smooth as dimming a halogen lamp as the driver for the cool white LEDs cuts out at around 5% instead of going down to zero so there is a step where the cooler LEDs suddenly cut off to leave the warm white LED on. However the overall lit effect is very pleasant and cosy when dimmed.

Thermally the downlight seems to operate within the limits of the Cree XPE LEDs used in the sample. The LED array pcb is mounted onto an appropriately sized standard 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 luminare 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 conventional light source it is meant to be replacing. 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 metre compared to 710 lux from the Polestar downlight.

On full power the Polestar LED 2010 Ambient Dim downlight gave a pleasant quality 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 effect 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 output. 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.


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