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What should I know before selecting LED lighting
for my aircraft?
When LED lighting is suggested, the irresistible inference is that your cabin interior will look
great and that the system will be exceptionally reliable.
LED lighting can deliver these benefits and much more, but your choice of system is important
because some manufacturers are building more into their designs than others. In this paper
we outline what you should know about LED lighting before selecting a system for your
aircraft.
The benefits of LED cabin lighting
Well-designed LED cabin lighting products can be installed
rapidly and with no changes to aircraft wiring and procedures.
Detailed below are the benefits which can be delivered by the
very best LED lighting systems.
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Passenger experience benefits: these include bright illumination
which is consistent throughout the cabin and a choice of colour
accents which can be matched to brand livery. Full mood lighting
systems also enable the use of different colour accents during the
flight which may be helpful in influencing passenger behaviour.
Maintenance benefits: a well-designed LED cabin lighting system
can deliver exceptional reliability and significant reductions to
maintenance costs.
Environmental benefits: these include reductions to weight and
electrical load compared to OEM equipment, both of which result
in fuel savings, and replaceable lighting units containing no
hazardous waste e.g. mercury.
© Cobalt Aerospace Ltd. 2015
It is also worth noting that reduced maintenance costs and fuel savings
provide direct financial benefits and enhanced passenger experience may
also have a positive impact on profitability.
So there are clear advantages to replacing fluorescent tubes with a good
LED lighting system. However, upgrading to LED lighting can be
considerable investment and some manufacturers are building more into
their designs than others.
In this paper, we outline what you should know about LED cabin lighting
before you select a system for your aircraft, by answering the following
questions:
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Is an MTBF of 100,000 hours as good as it sounds?
Should the system I choose include built in age correction?
Is LED binning enough to ensure brightness and colour consistency
throughout the cabin?
Is blue light the right choice?
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MTFB deals with the period of product life during which it exhibits a
constant failure rate, meaning it hasn’t ‘worn out’ and failures are random
events that occur at a constant rate in time, for example: ‘0.01 light units
fail per hour’ or to put it another way, if we had 100 light units we would
expect to see 1 fail per hour.
MTBF is the inverse of failure rate - the failure rate in the example above is
0.01 per hour, giving an MTBF of 1/0.01=100 hours.
25 year old humans have a failure rate of about 0.12% per year which gives
the published MTBF (strictly Mean Time To Failure, MTTF) of about 839
years. They do however wear out after about 70-80 years rendering the
MTBF pretty useless in determining how often you need to replace human
beings.
Cobalt Aerospace LED Lighting System Installed on an Airbus A319
Is an MTBF of 100,000 hours as good as it sounds?
Mean Time Between Failures (MTBF) is probably the most
misunderstood and deliberately misused term in product
marketing.
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What does this mean in terms of choosing an LED lighting product to
replace your aircraft fluorescent tube system?
Although the typically quoted MTBF of 100,000 hours sounds impressive,
there will be in the region on 100 units fitted to a typical aircraft which
gives a system wide MTBF of 1,000 hours. This means it should be
expected that one unit in the system will fail every 1,000 hours and, given
typical usage, that is equivalent one or two units failing per year. This is
good but not very different from a fluorescent lighting system.
Statistically it is true the MTBF of a 25 year old human is about 830
years.
Statistically it is true that only 33% of units will reach their MTBF
time (and not because the stated MTBF is incorrect).
However, LED lighting units actually are more reliable than fluorescent
tubes but the important measure of reliability to consider is the wear out
point.
MTBFs are much quoted by those selling LED lighting systems but these
two statistics clearly show that we need to think a little harder about what
MTBF figures mean before we accept them at face value.
A fluorescent tube will be considered to have worn out due to complete
failure, colour shift or dimming long before an LED lighting unit
incorporating age-correction measures shows any sign of age.
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© Cobalt Aerospace Ltd. 2015
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Should the system I choose include built in agecorrection?
An LED emits light over a very narrow spectrum making it
virtually monochromatic red, green or blue. White light consists
of wavelengths from the entire visible spectrum.
To provide apparent white light using LEDs there are two possibilities:
1. Use red, green and blue LEDs and mix the light they emit together;
or
2. Coat a blue LED with phosphor which absorbs the blue light and reemits it across a range of wavelengths to provide an approximation
to white light – this is commonly referred to as a ‘white LED’.
Age correction can be achieved in one of two ways:
1. Measure the light output with a sensor and provide feedback to the
control system to correct for the changes; or
2. Collect data for the changes over time and correct the drive current
accordingly.
Both methods work but the second is more reliable because the sensors
used in method 1 can themselves age and thereby provide inaccurate
measurements.
So, to maintain homogeneous light throughout the cabin beyond the first
year or so of service, you should look for a system with age correction built
in to the design and a ‘data collection’ based correction is more reliable
than a ‘sensor based’ correction.
LEDs are sold according to the colour and the brightness for a given drive
current. Over time and influenced by temperature, the colour changes and
the intensity decreases. Red LEDs degrade more drastically than blue and
green and with white LEDs, the phosphor coating begins to allow more
blue light through. What you are likely to notice, therefore, is lower light
levels and a shift towards blue.
Currently, very few LED lighting systems address the effects of LED ageing.
When a unit fails (e.g. one of the units with MTBF of 100,000 hours failing
after the system has been in service for a year), the replacement unit will
be brighter and subtly different in colour from the original, and therefore
aged, units.
Since one of the attractions of LED lighting systems is the homogeneity of
the lighting throughout the cabin, selecting a system without age
correction, may soon result in disappointment with the overall lighting
effect.
© Cobalt Aerospace Ltd. 2015
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Is ‘LED binning’ enough to ensure brightness and
colour consistency throughout the cabin?
Whilst fluorescent tubes can be routinely manufactured with
negligible differences in brightness and colour, LEDs leave
production exhibiting a wide variation in both these important
characteristics. LED manufacturers test each LED and separate
them into ‘bins’ closely matched for brightness and colour. This
process is referred to as ‘LED binning’.
So, LED binning is not enough to ensure brightness and colour consistency
throughout the cabin. The solution is to manufacture lighting units using
red, green, blue and white LEDs and perform calibration to a specific
colour point during production by mixing those colours.
This colour point can then be produced repeatedly, ensuring consistency in
brightness and colour throughout the life of the lighting system.
LED manufacturers then package the LEDs onto reels of 1,000-5,000 LEDs
from a single bin. In LED cabin lighting systems relying solely on LED
binning to achieve brightness and colour consistency, the whole shipset is
manufactured using LEDs from a single reel. Reliance on LED binning has
the following limitations:
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Given the number of LEDs on a reel, the size of aircraft for which
the lighting system would be suitable is limited to the equivalent of
an A319.
If the LED binning has not been done well by the LED manufacturer,
the brightness and colour will be inconsistent within the shipset
and unfortunately these differences are easily detected by the
human eye.
Even if the LED binning has been done well and the shipset looks
good at installation, once a unit fails and is replaced, the
replacement will not have been manufactured from the same reel
as the original units and the differences in brightness and colour
will be obvious.
© Cobalt Aerospace Ltd. 2015
An example of very bad binning used to create a line of ‘white’ LED lighting
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Most food is coloured in the green and red regions of the spectrum and
few foods are naturally blue in colour. A light source with a high proportion
of blue light will interact with the natural colour of most foods to make
them look dark and less appetising. So, passengers are likely to find inflight food more appealing when viewed in light at the warmer end of the
spectrum.
Melatonin is a hormone found in animals, plants, and microbes. In animals,
circulating levels of melatonin vary in a daily cycle which influences the
circadian rhythms of several biological functions including sleep timing.
Some ‘jet lag’ remedies contain synthetic melatonin.
Natural production of melatonin is inhibited by light reaching the retina. It
is principally blue light which suppresses melatonin production and the
effect of exposure to blue light is proportional to the light intensity and
length of exposure.
Cobalt lighting units installed in the photograph above use primary (white) and secondary
(red, green and blue) LED’s to allow for accurate colour calibration as well as full mood
lighting effects and use age correction to maintain this over the life of the product.
Is blue light the right choice?
The majority of retrofit LED lighting systems are currently
focussed on bringing blue light into the cabin. Some customers
are requesting colours other than blue to match their brand
liveries and are being told that ‘blue is best’.
LED light, even when it doesn’t look blue, contains a fair portion of the
blue spectrum. This is the ‘blue light hazard’ referred to today to describe
the increasing amount of blue light we are being exposed to by LED
lighting, computer monitors, tablet computers, smart phones etc. and the
effect this is having on people’s sleep patterns.
So, there is increasing awareness that high levels of blue light are likely to
make passengers feel wide awake. If your passengers would benefit from
sleeping whilst on board, for example during long-haul overnight flights,
you should consider an LED lighting system which enables a reduction in
the proportion of blue light to which passengers are exposed.
In fact, blue light may not be the right choice because it can have a
detrimental effect on the appearance of in-flight food and on the circadian
rhythms of passengers.
© Cobalt Aerospace Ltd. 2015
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Summary
Well-designed LED cabin lighting products can be installed easily
and with no changes to aircraft wiring and procedures. The very
best LED lighting systems can also deliver benefits in terms of
passenger experience, exceptional reliability, reduced
maintenance costs and fuel savings. However, some
manufacturers are building more into their designs than others
and, to ensure that you invest in the right LED cabin lighting
system, you should consider our answers to the following
questions:

Is an MTBF of 100,000 hours as good as it sounds?
This is realistic for a well-designed product but, given the large
number of units in a shipset, you should still expect one or two
failures per year. Be sceptical if you are quoted MTBF in the region
of 200,000 hours, it may be that telecoms reliability standards have
been used in the calculation; this is not appropriate and should not
form the basis of your system comparison. Remember also that
MTBF does not provide information about the expected life of the
product – ask about the wear out point. Also, the rate at which an
LED ages, is influenced greatly by temperature and drive current.
Ask what steps have been taken to manage the heat output of the
LEDs and how close to the LED current rating the LEDs are being
driven. It is not recommended to exceed 50% of the rated current
for the device and having more LEDs driven at a lower current will
prevent thermal hotspots.
© Cobalt Aerospace Ltd. 2015
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Should the system I choose include built in age correction?
Yes, you definitely need built in age correction. Without feedback and
correction, brightness and colour consistency will be lost when failed
units are replaced. Ideally, look for a system which incorporates data
collection for the changes over time and corrects the drive current
accordingly.
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Is LED binning enough to ensure brightness and colour consistency
throughout the cabin?
No, LED binning cannot provide brightness and colour consistency
throughout the life of the lighting system. You need lighting units using
red, green, blue and white LEDs and for the manufacturer to have
performed calibration to a specific colour point during production by
mixing those colours. Calibration derived consistency, in conjunction
with built in age correction, enables the maintenance of brightness and
colour consistency even after the replacement of a failed unit.
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Is blue light the right choice?
Not necessarily. Passengers are likely to find in-flight food more
appealing when viewed in light at the warmer end of the spectrum.
Furthermore, if your passengers would benefit from sleeping whilst on
board, you should consider an LED lighting system which enables a
reduction in the proportion of blue light to which passengers are
exposed. Ask which colours are available and whether the units are
supplied with multiple colour options built in which would enable you
to make a change later. Consider a full mood lighting system if you are
interested in influencing passenger behaviour.
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About Cobalt Aerospace
Cobalt Aerospace is a supplier of design and production services and is an
Original Equipment Manufacturer (OEM).
We provide products and services to airlines, design organisations, OEMs,
integrators and VIP aircraft completion centres throughout the world, as
well as designing, certifying and manufacturing our own product range for
retrofit to commercial and military aircraft under a variety of STC and
minor change certifications.
Cobalt Aerospace has well designed LED lighting systems certified and
available for retrofit to the following aircraft:
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Airbus A320 family
Airbus A330 family
Airbus A340 family
Our systems are compatible with both classic and enhanced CIDS and
available from white only to full mood ‘drop-in’ solutions.
For further information:
www.cobaltaerospace.com
[email protected]
+44 (0)1603 327757
© Cobalt Aerospace Ltd. 2015
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© Cobalt Aerospace Ltd. 2015
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