Friday, 5 December 2014

Evaluating Light Output: How many Watts is that LED?

When looking to define the output of a light source most people will look for its wattage. Traditional incandescent lighting has made it a rule that a 100 watt lamp will output more light than a 60 watt lamp. This is because all incandescent lamps use the same filament material which is heated to the same temperature, so the only way to increase the light output of these lamps is to increase the wattage. This is one of the main reasons why incandescent lighting wastes so much energy. 
However, LED light sources are much efficient at converting watts to lumens. Each material which can be used withing LED sources themselves has its own light extraction efficacy. For this and other reasons, two different LED sources can consume the same wattage but different lumen outputs.  Therefore watts can not be used as an index of light output in LED light sources. 
Evaluating the 'brightness' of LED sources for each situation requires you to think differently about lighting. A 60 watt incandescent lamp emits a total light output of roughly 800 lumens, but the light is equally emitted in all directions. When you're reading at your office desk, your book does not receive all the 800 lumens from your desktop lamp, nor should it need to. 
The most important measurement is delivered light. According to the IES ( Illuminating Engineering Society) serious reading requires 500 lux on the page. Many linear LED under-cabinet fixtures and task lights can deliver this level of light whilst consuming far less than 60 watts. 
So, wattage is no longer a relevant measurement to understanding the light output of a lamp, you might want to consider this when choosing which LED  lamp you require. 

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This blog post has been adapted from Philips' evaluating light output: technical brief. 

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