When selecting an LED, you might like to know just how bright it will be when you get it built into your circuit. And the package tells you -- it might say something like 10,000 millicandelas. But how bright is that?
When we ask how bright something is, it helps to have something to compare it to. We measure illumination in units called lux. Walking outside in the sun, you are experiencing between 30,000 to 100,000 lux. In a well-lit office, you are in about 1,000 lux. Walking in the moonlight, you are getting only about 1 lux.
If you are trying to light your office with a lamp, and you want 1,000 lux on your desk, you will need to know how much light the lamp produces, and how far away from your desk it is. We measure of how much light a lamp produces in lumens. A 60 watt incandescent bulb might produce 850 lumens. A 100 watt incandescent might produce 1,700 lumens. To get an illumination level of 1,000 lux on your desk, you can put a 60 watt bulb about 12 feet away.
An LED might put out a about a half a lumen. That doesn't sound like a lot. But the LED has a little lens in front of it, that focuses the light in one direction. So now all of light is falling in a smaller area, giving better lighting to that area. To take into account that focusing of the light, we measure LEDs in candelas (or millicandelas), which is lumens focused on a spot.
All of this sounds complicated. So I have built a little gadget to help out.
In the picture below, you can move the sliders back and forth to play with the different lighting concepts. You can change the amount of light, how far away it is, and how focused it is. At the right, you will see the resulting illumination, in lux, and the size of the illuminated spot.