Monday, October 8, 2012

Steve K Provides Some Enlightenment

In my recent whine about current (no pun intended) LED headlights, I released an unabashed troll attempting to lure friend, bike dude, and soldering-iron gunslinger Steve K to talk about lights. Being a swell fella, he's obliged with perhaps one of the more concise and readable cut-through-the-marketing explanations descriptions of how lights are measured and how those measurements translate into reality. Since I'm learning (slowly) that this blog improves when I shut up, I'll let Steve talk. Take it away, Steve!
The biggest confusion is just about the way light is measured... most often in lumens, but sometimes in lux, and sometimes the manufacturer just says "it's a 3-watt LED!!!" Well, there are 3-watt LEDs and then there are 3-watt LEDs!

The quick explanation is that lumens are a measurement of the total flow of photons out of the light. Sort of like saying that a pump can output 2 gallons an hour.

By contrast, lux is a measure of the intensity of light hitting the target. To quote the Wiki folks: "It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface."

The thing that converts the lumens to lux, so to speak, is the optics or reflector of the light. If your light's reflector (or lens/optic) is designed for mountain biking, it'll produce a wide beam and scatter the precious photons over a large area. As a result, the light at any given spot will be dimmer (i.e. a lower intensity) than what you would get with a road bike headlight that had a tight beam that spreads the light over a relatively small area. Same number of photons hitting the ground, but producing a higher intensity since they aren't being spread as thin. Simple, eh?  Same number of lumens in each case, but the lux will be lower for the wide beam relative to the narrow beam.

The business about wattage is confusing because it is usually a measure of electrical power, not light power. Most modern LEDs will produce about 100 lumens per watt, so a nice Cree XP-G being run at 3 watts will produce about 300 lumens, and can make a decent road headlight. A cheap LED might only produce 70 lumens per watt, which will be barely noticeable as being dimmer than the Cree XP-G. Human vision is logarithmic, so a big change in lumens really doesn't look like a big change to us.
So there you have it. The introductory lecture in our Lights 101 course. Please address your questions to Professor Steve in the comments section. And big ol' thanks to the professor for contributing his astute guest lecture.

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