42 Partners - Maintenance Factor - LED luminaires

Lighting specifications are almost exclusively couched in terms of Maintained Illuminance.
The ruling standard for lighting is EN 12464-1:2011 because it is the latest relevant EuroNorm.
This says that:
Illuminance values given shall not fall below the value stated "...regardless of the age and condition of the installation...."
To account for the degradation of the installation a single Maintenance Factor (MF) must be calculated for "...the selected lighting equipment, environment and the specified maintenance schedule..."

Guidance on calculation of MF can be found in CIE 97-2005.
CIE 97 breaks MF down into 4 sub factors which are multiplied together to reach the final MF.
N.B. for schemes with multiple types of luminaire a separate MF must be calculated for each type.
RMF Room Maintenance Factor
takes account of the effect of dirt and dust accumulation and other degradation of the reflectivity of the room surfaces.
The main determining factor is the environment which can be classified on a sliding scale from "Very Clean" to "Dirty"
LMF Luminaire Maintenance Factor
takes account of the effect of dust and dirt accumulation on the luminaire.
Luminaires are classified according to their degree of sealing and their distribution, obviously dust accumulation on an open uplight is far more onerous than on a sealed downlight.
LSF Lamp Life Span
takes account of the effect of the failure of light sources during the maintenance period.
LLMF Lamp Luminous Flux Factor
takes account of the effect of the lumen depreciation of the light sources during the maintenance period
It is obvious from the wording and the date of issue that CIE 97 was written for environments lit by luminaires with replaceable lamps.
For GLS, Fluorescent and HID type lamps and luminaires these factors are well documented and available in a standardised form.

LEDs
It is common to see a single high value MF included in Lighting calculations for LEDs luminaires with no breakdown or justification of the MF used.
We can definitely state that under no circumstances can an MF of 1.0 be justified.
Even in the case of an intelligent driver that maintains constant lumen output for the life of the installation, and if we accept that no failures will occur, we still have to account for the RMF and LMF factors.
You can argue that the CIE 97 factors for various sealed and semi sealed luminaires using fluorescent and HID lamps will be smaller for LED luminaires.
The justification for this is that power and surface temperatures are almost certainly lower for LED luminaires.
But
There will still be degradation of the luminaire due to dirt and dust accumulation because LED luminaires suffer from a reduction in lumen output due to temperature increases in the LED junction caused by dirt and dust accumulation on the heatsink.
Maintenance Interval
The assumption in the MF calculation is that at some planned point the room and luminaires are re-lamped, cleaned and maintained; returning the room to almost initial conditions so that the Maintenance cycle can start again. This makes perfect sense for the economic life of a fluorescent lamp at 15,000 to 20,000 hours. Claiming that LED luminaires have L70 at 50,000 hours is not relevant unless you extend the maintenance interval to 50,000 hours, in which case you will have an LLMF of 0.7 to multiply by the RMF and LMF (assuming no failures) to yield an overall MF of a lot lower than 0.6. So for all practical purposes a statement of L70 50,000 hours by a manufacturer is of no use to the designer, yet this is the figure most quoted to prove how effective LEDs are over their life span.
Economic Life
(Or "To maintain or not to maintain - that is the question")
LED luminaires that cannot be maintained should be treated differently to luminaires that can be maintained.
A monolithic LED source sealed into a housing will reach a point where the installation illuminance falls below the specified level.
According to EN12464 this cannot be allowed.
As the luminare cannot be maintained at this time it must be scrapped.
A total write-off at the end of the maintenance period is the only option, with all of the associated cost of full replacement.
If we claim a maintenance cycle of 50 thousand hours, then we must accept an MF of 0.5 to 0.6.
Users must ask how this impacts on the cost effectiveness of the installation.
At 0.6 MF the scheme will be using 25% more power than a scheme that has 0.8 MF.
It should be possible to have an LED luminaire that can be maintained by using interchangeable modules of the Zhaga type.
In this case at the end of the cycle these luminaires could be "re-lamped" ("re-LEDed"?).
This would have a different impact on cost effectiveness of the installation.
It is the Lighting designers responsibility to make these effects known to the end user.
Warranty and Life
These are terms bandied about for LED luminaires, they have no bearing on Maintenance Factor.
Manufacturers offer a warranty that in some way guarantees the luminaire will be functional after a period of usage (usually with many restrictions and caveats).
This is irrelevant to the MF calculation, which is an engineering calculation designed to ensure compliance to lighting standards and also has major effect on the economics of the installation.
Manufacturers claim long life for LED luminaires, and the better ones provide some form of data to LM80.
This data presents a period of time after which the LEDs will still emit 70% of the initial lumen output.
If you use an extended maintenance period to take advantage of this "life":
Is the luminaire really still useable over this time scale?
Can you link the maintenance period to this "life"?
if so what are the economic implications of this extended maintenance cycle?
What is the economic Life of an LED luminaire? It will certainly be different for the 2 types suggested above.
Follow this link for an explanation of how Relux treats MF calculations (opens in a new window)