millicandelas and you!

In my previous post about LED wiring I discussed current ratings and voltage specifications, & how to wire LEDs so they don't catch fire (we hope). What I didn't talk about is what you get for all your trouble when you build a project using LEDs. Will that spotlight be bright enough to light your tombstone? Will your flying crank ghosts eyes be so bright they blind your trick-or-treaters? Hopefully this post will help us figure that out. A disclaimer - I'm no expert. If you're planning on building something critical like your own LED car headlight (please don't) then please look elsewhere for your technical guidance. This information is intended to point you in the right direction, but again I'm not an expert.

An LEDs brightness is measured in millicandelas. A millicandela - commonly written as mcd - is 1/1000 of a candela. A candela is equal to the light output of 1 candle. The light output of a lightbulb is commonly measured in lumens. 1 candela is roughly equal to 1 lumen. There are much more scientific definitions for both, but talk of steradians and 555 nanometer frequencies tends to put me to sleep. What we really need to understand is that your average 60 watt 120 volt incandescent light bulb puts out about 460 lumens:



and this LED puts out about 40,000 mcd, or about 40 lumens:



This is one LED at about 5 feet on a very dark night (sorry for the blurry pic, it's clipped from a video).

When you look at the specs of an LED you'll notice that there is usually listed an angle or viewing angle. This represents the directionality of the light output. Basically, the smaller the number the tighter the "beam" of light (think flashlight). This is important in that the tighter the beam, the smaller the effective useable area. For reference, the LED in the second picture above has a viewing angle of 12 degrees. At roughly 5 feet it effectively illuminates a 26 inch tall tombstone with very little overlap.

So basically, if you're wanting to build some cave eyes the 400 mcd LEDs from Radio Shack would work OK, but if you're wanting to illuminate your 7 foot tall Pumpkin Rot you'll probably want to look for something brighter.

There are some very good mcd to lumen calculators on line.