The lighting industry has many standards governing the safety and performance of lighting products. When testing the life-expectancy of a bulb, manufacturers must abide by these standards.
There are, for example, testing procedures for CFLs (Compact Fluorescent Lamps) and Incandescent bulbs, which have the respective LM-65 and LM-49 designations. LED lights have their own separate test procedure, called LM-80.
Unlike older forms of lighting, which are tested on their rated lamp life, the mean time to failure, LED bulbs are tested by something called their lumen maintenance life.
So What Is Lumen Maintenance?
Like all light sources, LEDs gradually lose their light output over time. This phenomenon is called lumen depreciation. In LEDs, lumen depreciation is caused by heat generated at the LED’s positive/negative junction, which is why LED bulbs often include a heat sink to keep the unit cool and to prolong its life.
Lumen maintenance measures the amount of light produced by a source when it is brand new and compares it with its light output at a specific time in the future.
Lumen maintenance is expressed as Lpof x hours, where the ‘L’ value relates to the initial light output and the ‘p’ denotes the percentage of light maintained over x number of hours.
For example, if a light source has an L90 rating of 10,000 hours, after 10,000 hours of use, the light source will be working at 90% of its initial light output.
Put another way, if a light source produces 1000 lumens when brand new and 700 lumens after 50,000 hours of use, it has a L70 rating of 50,000 hours.
The Significance Of L70
Judging the useful life of an LED bulb based on its light output raises a further question; what is an acceptable level of light?
Obviously the reply will vary depending on the application and specific requirements of the user, but L70 is the widely-held standard for LED lighting in domestic situations.
Why L70? Research performed by ASSIST (the Alliance for Solid State Illumination Systems and Technologies) has found that lumen depreciation only becomes noticeable when a light source has lost 30% of its initial luminance. For decorative lighting L50 is considered an acceptable standard.
LED bulbs that are manufactured using high quality components will generally have a much higher L70 rating than those made more cheaply. So, the next time you’re buying LED bulbs, make sure you compare the L70 rating.
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