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LED guide: LED terms and definitions you need to know  

LED guide: LED terms and definitions you need to know  

L70

The number used to estimate LED lifetime, often on the order of 50,000 hours or almost 6 years of continuous use. The L70 number for an LED is the number of hours of operation after which the LED will have lost light output down to 70% of its original brightness. 70% is used because that’s the level of light loss that most people can recognize. Note that this “failure” is different from that of an incandescent or fluorescent lamp, which usually fails completely and burns out.

New LED components are being introduced each year, with ongoing advances in materials and manufacturing. It is difficult to extrapolate failure from shorter tests. The only real way presently to determine the lifetime of LEDs is to run them until they drop, but lighting designers can’t wait 6 years to find out exactly what the L70 number is for a new LED component.

Lumens

Lumens are a measure of how much light an object emits that can be seen by the human eye. I emphasize this last bit because it tells us a lot about why colder-CT LEDs output more lumens than warm-ish LEDs. The human eye is most sensitive to green light, specifically, light at 555nm. If a device produces 1W of light at 555 nm (green), it will be a maximum of 683 lm. Of course, few people want an intense green light – we want white light for illumination, and white light is made up of many different frequencies — a whole rainbow of them. The human eye isn’t as susceptible to these other frequencies though, and perceives them as being less bright, or fewer lumens. . This is why the first, inexpensive LED lights had a bluish cast, which is close to green — this was a practical way to make the lights appear brighter to humans.

Lumens are measured in a spherical pattern around a lamp. This is appropriate for omnidirectional light sources such as incandescent bulbs and fluorescent tubes, where the light coming out opposite the work surface usually reflects off of a lampshade (in a table lamp), the ceiling, or the top of the fixture (in a fluorescent light). It’s not the way LEDs work, however, because they are single-direction point sources of light. Since all of the light produced by the LED tube lamp is directed down onto the work surface, it takes fewer lumens to light up the work surface equally well. This is why a 1900 lm LED tube lamp produces the same useful light as a 2300 lm T8 fluorescent lamp.

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