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Understanding LED luminaire lifetime and reliability

LED luminaires are getting cheaper but they’re not cheap so a long life is required, hence the 5 year guarantee.

I last wrote about LM80 TM21 and the life of the LED and what it means in terms of the life of a luminaire.  Whilst the LED life is important there are other factors which need consideration.  The LED is probably the most reliable part.

A luminaire requires 2 parts, besides the LED; a housing and a power supply (driver).  Both of which will affect the overall expected life from the luminaire.

The driver, even more than the LED, doesn’t like heat.  The average for premature failure of a driver is around 5/1000, (0.005%), within 5 years, meaning you can reasonably expect several of your fittings to go wrong and need repair or replacement.  That’s assuming the driver is working at a 50 degree C max case temperature.  If the case temperature gets higher the life of the driver goes down.  Reduce the case temperature by 10 degrees C and you will double the life of the driver.

The housing, apart from providing the aesthetics, is required to remove the heat from both the driver and LED, which is why you often see lots of fins on many designs.  The larger the surface area; the greater the degree of cooling.

So, how does that knowledge help?

Allowing that in many corridors the ambient temperature may be 35 degrees C or more it should be appreciated that the temperature inside the luminaire will be considerably hotter, perhaps as much as 15 to 20 degrees hotter.  That makes the temperature inside the fitting – where the driver is – well over 50 degrees and above the maximum case temperature for the driver.  Care should be taken when a luminaire manufacturer quotes an ambient of 40degs – the internal temp could be well in excess of this and well in excess of the driver operating conditions thus reducing the life.  If the internal temperature is 60 degrees C the life of the driver/luminaire may be as little as 2.5 years.

Most lights now come with a 5 year guarantee so if they go wrong they should be replaced but it will still be very disruptive.

Contact us if you’d like to learn about how good design will help reduce premature failures.

LM80 TM21 – LED luminaire lifetime predictions

I mentioned previously that I’d expand on the LM80/TM21 LED theme and what it means in relation to a luminaire life.  The short answer is…absolutely nothing!  LM80 and TM21 refer, only, to the predicted lifetime of LEDs and COBs.  How the LEDs are mounted, cooled and driven are the key factors affecting luminaire life.

Having said it means nothing, that’s not totally true.  What the data is intended for is to provide a luminaire manufacturer with a way to directly compare LEDs/COBs allowing for the best choice of component to achieve a good luminaire life.

So, what does it mean, in simple terms?

IES LM80-80-2008 “Measuring Lumen Maintenance of LED Light Sources”  is the industry standard method for testing LEDs to determine lumen depreciation over time and is carried out over a 6000- 10000 hour period.  At 1000 hour intervals the luminous flux is taken.  As a typical example (CREE, because we use them) let’s say the depreciation is 3%. That means a 97% maintenance of light output. (and, perhaps, at 10000 hours, 6% depreciation or 94% maintenance) Historically that was it and LED manufacturers could draw their own curve through the test points and boldly quote a 50000 hour lumen maintenance of 70% (L70) output while others would quote 90% (L90).   – Before TM21 came along, there was no agreed standard as to how to predict the end of useful life. This was not helpful for us as luminaire manufacturers or you as customers.

IES TM-21-2011 “Projecting Long Term Lumen Maintenance of LED Light Sources” recommends a method to use the LM80 data to predict the lumen maintenance of an LED.  Basically, an exponential curve is drawn between the 1000 hour test points on a graph plotting lumen maintenance from 70% against a time line of up to 100,000 hours.

This could well give a calculated figure of L70 = 50000 hours

from CREE X-lamp lumen maintenance

However, it is also stipulated in TM21 that the maximum life that may be quoted is 6x the actual test duration. So, if the test duration is 6000 hours the maximum life that can be quote is 36000 hours and would be quoted

L70 (6K) >36000 hours if 6000 hours of testing have been carried out or L70 (10K )>60,000 hours if 10000 hours of testing has been done.

It’s obviously more complicated than this and the tests are actually carried out at 3 different LED case temperatures; 55deg C, 85deg C and a manufacturer selected temperature.  (The meaningful one is 85deg C as this represents practical conditions).

Don’t forget,this is just the LED in lab conditions with controlled cooling and drive current and doesn’t take account of ambient temperatures or power supply life time.

Beware, therefore, what is quoted as a luminaire life.  I’ve seen lifetimes of >100,000 hours to L90 at 25deg C quoted in big type on luminaire datasheets and, in small print, this is with a junction temp Tj  and is the LED manufacturers data. Rather misleading but looks good on the spec sheet.

Earlsmann uses the CREE COB CXA 2530 or 2540. TM21 predictions for these is L90 (6k) >36,300 hours with Tc 105deg and 808mA drive current.  We optimise for a lower case temp of 85deg C. which can only improve the life expectancy.

Correlated Colour Temperature (CCT) and Colour Rendering Index (CRI)

There are two parameters which define the colour of LED light. – colour temperature and colour rendering.

Colour temperature is the perceived colour of the light source.  It is compared with a glowing black body source.  When heated to around 3000K it will glow a warm yellow (think about a 40 watt filament lamp).  When heated to 4000K you get whiter (neutral white) and when heated to 5-7000K you get a bluey white (daylight or cool white).

Warm white is most useful in domestic environments and tends to provide a “homely” feel.  Neutral white is more suited to the office environment as it is slightly brighter than warm white and easier to see by.  Cool white provides the most light for a given amount of power but may appear harsher so but is ideal where good visibility is required such as a gym or car park.

Colour rendering is the ability of the light source to allow colours to be seen accurately and is not directly related to colour temperature.

White light is made from a mixture of colours.  A white LED has a blue source with a yellow phosphor overlay which emits white light when illuminated by the blue source.  Needless to say, the colour rendering may not be great.  Reds will appear purple and oranges rather muddy.  

This is because there is no red within the light source to reflect from the red surface.

In simple terms, the solution is the addition of red phosphors to the yellow overlay.  This provides the red to reflect from the red surface and give a good colour rendering.  It would be desirable if this could be added to all LED coatings but, as always, the issue is cost.  Red phosphors are significantly more expensive than yellow so the cost of the LED goes up.

Does it matter?  In many cases no, but it may have safety implications.

For example, old low pressure sodium (LPS) street lights have a colour rendering index of 20 – pretty poor.  They may be bright but a red car will appear black and may simply be invisible at night.

A moderate to good colour rendering of 70-80 is almost certainly adequate.  The fact that a royal blue shirt may appear simply as dark blue probably isn’t important.  It’s also reasonable for security cameras as it will provide good contrast.

A high colour rendering of 90 or more is probably only required if comparing absolute colours, perhaps in paint or fabric manufacture or in retail.  With a really high colour rendering oranges will look really orange and may sell better than your competitors and you can match a shirt and tie exactly so they will look the same in daylight. (although it is still wise to ask the lady in your life).

So, it’s a choice between budget and application.  You could pay 15-20% more for a high colour rendering so it’s worth asking the question…is it necessary?

At Earlsmann we use CREE LEDs for most of our products.  These are available from 2700K to 6500K and with a CRI up to 95 so we can accommodate most requirements

Cree Announces Next-Generation XP LED Delivering 200 Lumens Per Watt

If you missed the recent announcement from CREE about their latest product release then it's worth a read now

DURHAM, NC -- Cree, Inc. (Nasdaq: CREE) introduces the XLamp® XP-L LED, the first commercially available single-die LED to achieve breakthrough efficacy of up to 200 lumens per watt (LPW) at 350 mA. Delivering up to 1226 lumens in a 3.45 mm x 3.45 mm package, the game-changing Cree® XLamp XP-L LED enables an immediate performance increase of 50 percent or more as a drop-in upgrade for lighting designs based on Cree’s market-leading XLamp XP-G LEDs. 

Read the whole story here

With a world leading manufacturer like CREE it won't surprise anyone to learn that Earlsmann uses their LEDs, almost exclusively, in our products.   In fact, we actually use their COB (chip on board) 2530 which achieves a very creditable 112lm per system Watt (350mA) in our Lincoln Street light with gull wing lens - this takes account of power supply and lens losses.  For the bare COB, at 800mA, the CREE published figure is around 24lm/W.

It is worth mentioning that CREE characterise their products with a junction temperature of 85 deg.C  Many of their competitors quote their products at a junction temp. of 25 deg C which is quite unachievable unless the ambient temperature is around freezing!   What's more, some lighting manufacturers quote the LED life in their data - a bit misleading, I think, as it can't be achieved. 

More on LED specifying next time...
LM80/TM21 

Indoor Tennis Court Lighting

I’ve recently come across a rather novel way of lighting indoor courts which I think worth sharing as it is very effective and very cost effective.

I was asked to quote to replace the lighting on an indoor tennis court.  Simple enough, but the current arrangement was rather unusual.  It was lit with 4x6ft fluorescent fittings the entire length of the court on both sides located more or less at 6m directly above the doubles side lines. (off the playing area) They were angled at about 30deg from vertical.

One solution is obviously to simply replace the tubes with LED but this would mean rewiring each fitting and cleaning the diffusers.

I considered a different approach and modelled the court with our Turin 600x600 suspended ceiling panel and was amazed at the result.

Good overall illumination and uniformity, minimal glare and, since these panels are solid, unlike the fluorescent fittings, they are robust and less prone to damage from mis-hit balls.

 Of course it's not just indoor tennis you can light this way.  Bowling greens, squash, badminton - anything where glare would be an issue.

If you have something you need lit - give us the challenge to come up with something different

Daylight Harvesting with Earlsmann LED Lighting

Daylight harvesting is a technique that is becoming popular as a very good way to keep energy usage to a minimum.  Why pay to run your lighting when the sun can provide all the light you need?  How often do you go into an office and discover the lights are on and the sun is blazing outside.  In fact, you may not actually notice the lights are on the sun is so bright!

All you need to do is turn the lights off to make immediate savings but that (almost) never happens.

A good starting point is a motion sensor as they will turn the lights off for you when nobody is about but this has its own problems as it can quite often be much gloomier at desks farthest from the window.  The solution, of course, is daylight harvesting sensors to maintain a constant lux.  These continuously monitor the amount of light falling on the work surface and adjust the light output from the luminaire accordingly to keep it to the required level.  These can be incorporated in the luminaire so those sitting closest to the window where daylight could easily be 700 or 800 lux would have minimal artificial light whilst those sitting with, perhaps, only 100 lux from daylight can have it boosted to 300 lux or more, as desired, by the luminaire.

With modern controls and LED luminaires this is very easy to achieve.  The initial investment in controls is slightly higher than for fittings without but the payback time is similar and the ongoing savings far greater.

Constant Lux with Brighton LED Bay light

In the picture above we used our 120 Watt Brighton LED bay light to achieve around 700 lux

If you’d like to know more, or to discuss your own requirements, please get in touch.