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What Is a Lumen and What is it Doing in my Flashlight?

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When I was growing up, it was pretty easy to understand how light was measured. A 25 watt bulb cast just enough light to see the cellar stairs. Three 40 watts over the bathroom mirror gave just the right light for doing your makeup, and if the 75 watt bulb in the reading lamp wasn't bright enough, you could just replace it with a 100 watt and you'd be good to go.

Those were the days when most home lighting was incandescent, except the fluorescent ceiling lights in the kitchen – which we all knew were brighter than incandescent light bulbs, but somehow used fewer watts to make more light. Then in 1971, the U.S. government started requiring that all light bulb packages add another type of measurement – lumens – so that we could start being more energy conscious and comparing light sources by the amount of energy it took to produce a certain amount of light.

Which brings us to lumens – precisely what is a lumen, how is it different from a watt and how does it relate to the measurement of light?

What is a Lumen?
A lumen is a unit of measurement that is used to express how much illumination a source of light provides. We could get all technical here and go through the entire history of lighting measurement starting with whale blubber candles and making our way forward to LED lights, but it's really far simpler than all that. A lumen is about the equivalent of the amount of light put out by a single birthday candle if you are one foot away from the candle. A lamp that puts out 1 lumen of light is as bright as one birthday candle a foot away. A lamp that puts out 100 lumens is as bright as 100 birthday candles a foot away from you.

Differences Between A Lumen And A Watt
So what about this lumens per watt thing? A watt is a unit of electrical energy. We measure electrical usage in watts, and pay for our electricity by the number of watts we use. When we talk about lumens per watt (lm/w), we are measuring how many lumens of light are produced for each watt of electrical energy used. It's how we measure the efficiency of a light source – and now we're getting to LEDs and how they differ from incandescent and fluorescent lights.

Incandescent vs Flourescent vs LED
We measure the efficiency of a light source by the number of lumens put out per watt of energy consumed. Incandescent light bulbs can reach as high as 17 lumens per watt. Compact fluorescent lamps (CFLs), which are typically used to replace incandescent bulbs in the home, run 35 to 60 lumens per watt. As of October 2007, white LED lights typically achieved 25 to 64 lumens per watt, and the US Department of Energy's long-term research goals call for LEDs with 160 lm/w output by 2025.

While LED lights are still considered too expensive for most typical home lighting, manufacturers have been using LED lamps in battery operated lighting for a number of years now. As of October 2007, the DOE web site noted at least one privately held corporation that was making LED lamps with lm/w efficiency of 100 lm/w. They noted that those lamps were too expensive to be used for typical home lighting, but were being routinely used in flashlights.

Flashlight Lumens
There. I promised you we'd get to flashlights, didn't I? In home lighting (and city lighting and office lighting, but that's a whole other story), the lm/w rating is important because the higher efficiency of LED lamps saves money and energy. In flashlights, LED lamps serve an even more important purpose. Because LED lamps consume far less electricity than other types of flashlight bulbs, your batteries last longer – which can be pretty darn important when you're counting on your flashlight as your only source of light.

In addition, LED lamps offer a higher flashlight lumen output than standard filament flashlight bulbs. As an example, if you replace the filament flashlight bulb that comes in the Mini Mag light with an LED lamp, you'll get three times the battery life and double the brightness from the two AAA batteries. Depending on the LED flashlight that you choose, you can get hundreds of hours of use out of your flashlight before you need to replace the batteries – and the lights themselves will last years before dying on you.

The era of LED lighting is well on its way into our lives. Already, Australia has banned the sale of incandescent bulbs after 2010. The US Department of Energy envisions affordable, efficient LED lamps in everyday home use by 2025 at the latest. Cities around the globe are saving millions by replacing fluorescent and incandescent lights with state of the art LEDs.

~Ben Anton, 2008

http://www.lightsandknives.com/led_lumens_flashlight.htm


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