Light output falls over time
Every lighting installation delivers less light as it ages. LEDs slowly lose output, dirt builds up on lenses and reflectors, and surfaces in the space get darker. A new installation therefore looks brighter on day one than it will in year ten.
If a design meets the required light level only when it is new, it will fall short for most of its life. That is why lighting standards and recommended practices express their targets as maintained values: the light level the space must still have at the end of the maintenance period.
The light loss factor
Designers account for these losses with a light loss factor, or LLF. It is a number below 1 that multiplies the initial output in the calculation, and it is the product of several smaller factors, each describing one source of loss.
Other factors may apply, such as supply voltage or dirt on room surfaces. The IES and the CIE publish guidance on choosing each value.
- Lamp lumen depreciation (LLD): how much the LEDs' own output falls by the end of the design period
- Luminaire dirt depreciation (LDD): the loss from dirt on the lens and optics, which depends on how clean the environment is and how often fixtures are cleaned
- Ambient temperature: LEDs give less light when they run hot, so a luminaire in a hot space may not reach its rated output
- Driver or ballast factor: the effect of the driver or ballast actually installed, where it differs from the one tested
Where the LED figures come from
Lamp lumen depreciation for LEDs comes from two IES documents. LM-80 is a test method: LED packages are run for thousands of hours at set temperatures and their output is measured. TM-21 then projects those measurements forward to estimate future output.
TM-21 limits how far a projection may reach, based on how long the test ran. A life claim far beyond what the test supports is an estimate, not a measurement, so ask which data stands behind any life figure.
What L70 means
L70 is the number of operating hours after which light output is expected to have fallen to 70 % of its initial value. L80 and L90 mean the same at 80 % and 90 %. These figures describe gradual dimming, not failure: the luminaire still works after L70, it simply gives less light.
When comparing products, compare like with like: the same L value, at the same ambient temperature, from the same kind of data. An L70 figure and an L90 figure for the same product can be very different.
What goes wrong without it
The aim is a realistic factor for the actual site, the actual product and the actual cleaning plan. Getting it wrong in either direction has a cost.
- Designing to initial lumens leaves the space under-lit later, often below the minimum it must meet
- Ignoring dirt and temperature brings that shortfall sooner, especially on industrial and outdoor sites
- Using an overly cautious factor over-lights the space and wastes energy and capital
Constant lumen output
Some LED drivers can compensate for depreciation. They run the luminaire below full power when it is new and raise the drive current gradually over its life, so output stays close to the design target. This avoids over-lighting in the early years and uses less energy at the start.
Constant lumen output does not replace the light loss factor. The design must still allow for dirt and temperature, and the luminaire needs enough reserve to reach the end of the compensation period.