The lighting industry has many standards governing the safety and performance of lighting products. When assessing the 'useful life' of a bulb, manufacturers must follow strict guidelines.
Every type of lighting, whether it be fluorescent, incandescent or 'solid-state,' has its own distinct test procedure, which can be found in official publications from the Illuminating Engineering Society of North America (IESNA).
For example, LM-49 and LM-65 define the test procedures for incandescent bulbs and CFLs (Compact Fluorescent Lamps) respectively. LED Lighting has its own test procedure called LM-80.
Unlike incandescent and fluorescent lighting, the useful lifetime of an LED cannot be fully tested. This is largely due to the fact that LEDs last a very long time. By the time a thorough test were even close to being completed, technology would have already overtaken the bulbs being tested.
The effectiveness of an LED bulb is instead extrapolated by testing its 'lumen maintenance.'
What is lumen maintenance?
Like all light sources, LEDs gradually lose their luminosity over time. This phenomenon is called lumen depreciation.
Over the course of its life, 1000 hours or so, an incandescent bulb will lose between 10-15% of its light output. A CFL will become up to 20% dimmer.
In LEDs, lumen depreciation is caused primarily by heat generated at the diode's p-n junction. You will find that most LED bulbs are designed with a heat sink, which carry heat away from the diode and thus prolong the life of the bulb.
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 Lp of X hours, where L is the initial light output and p is the percentage of light retained over X number of hours.
For example, if a light source has an L50 rating of 10,000 hours, after 10,000 hours of use the light source will be operating at half of its initial light output. Put another way, if a light source produces 1000 lumens when it is brand new and 700 lumens after 50,000 hours of use, it has a L70 rating of 50,000 hours.
The Significance of L70
So far so simple. But there's a problem. Assessing the useful life of an LED bulb based on the rate of lumen depreciation begs the question; what is an acceptable level of light? Obviously this will vary depending on the application and specific requirements of the user.
The Alliance for Solid State Illumination Systems and Technologies (ASSIST) have deemed L70 a suitable level for most commercial LED bulbs, with the reason being that lumen depreciation only becomes noticeable when a light source has lost 30% of its initial luminosity. For decorative lighting, L50 is considered acceptable.
The amount of time it takes for a bulb to reach this point varies based on the conditions in which it is used and the quality of its components. Naturally, a well-built bulb will remain brighter for longer than those with poor quality components.
So, the next time you are buying LED bulbs, make sure you are comparing the L70 rating as well as other factors such as price and efficiency.