Life and Light output
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Life and Light output
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Life and Light output

What are “cold” and “hot” lumens?

These terms do not have any photometric or engineering meaning. However, "cold lumens" is the light output of the LED chip alone when it is first switched on. "Hot lumens", refers to the light output of the LED when it is fully warmed up in the luminaire. The hot lumen value may be 30% - 50% lower than the cold lumen value.

Why are LEDs considered more efficient than conventional light sources?

When comparing the lumen output between LEDs and conventional light sources, LEDs may have lower lumen value in many cases. However LEDs are directional light sources, all the lumens emitted from an LED are directed towards the task area. Conversely, conventional sources emit light in all directions. The light is then modulated in a given direction with optical systems like reflectors and lenses. The amount of lumens that falls in the intended task area from an LED light source is greater than that of a conventional light source.

Does increasing the wattage of LEDs increase their output?

This may occur if you are using the same product from the same brand, with the same optics and hardware. However, in general, the nature of the components (like the optical system, the heat sink, the LED chip, and the driver) affects the output more than the wattage does. A 3watt LED luminaire from one manufacturer will have a different output to a 3watt LED luminaire from another manufacturer, even if the same LED chip is used. Hence, using a high quality chip alone does not guarantee better performance. Note that as the wattage increases, the efficiency drops slightly. An LED driven at 3W will emit slightly less than three times the output of one driven at 1W.

What is the best way to compare the output of LEDs with other light sources?

Sometimes simply comparing the lumen output of LEDs and conventional light sources may not be adequate. The amount of light falling on a specific task area (the lux) gives a more realistic comparison. You should also consider the amount illumination visible on the walls. This helps identify applications where LEDs offer better solutions than other light sources.

How much longer does an LED last than a conventional light source?

Typically, an LED will last four times longer than a CFL and 25 times longer than an incandescent source that puts out the same amount of light.

Is lifespan reduced if the LEDs are frequently turned on and off?

Unlike discharge lamps, LEDs are semiconductors and their life span is not affected by the number of times they are turned on and off.

What are the factors that affect the lifespan of the LEDs?

The thermal management of the LEDs. If LEDs come on a standalone chip, appropriate heat sinks have to be designed to prevent premature failure of LEDs.The electrical stress: Running LEDs at currents higher than specified make the LED run hot. This can happen with wrongly matched drivers. For example, if the driver produces 700mA but the LED needs 350mA, this will put stress on LED and reduce its lifespan.Higher ambient temperatures than the ones that the LED is rated for will reduce its expected life.

Why is the life of LEDs measured as lumen depreciation?

Unlike conventional light sources that reduce in output and eventually fail, LED products do not normally suddenly fail. Instead, the light output reduces over time.The normal convention is to measure the life from when the output has reduced by 30%, i.e. when there is 70% light output remaining. This is often quoted as the L70 life and is measured in hours.

What does a life of 50,000 hours mean in the case of - LEDs?

50,000 hours would imply 5.7 years if the light is operated for 24 hours in a day, 7.6 years if the lights are on 18 hours per day and 11.4 years for 12 hours a day.