If you've ever been in the market for lighting, then you've probably heard of at least one of these terms: candela, lumen, or lux. All of these terms help us quantify the output of a light source, but each term has their own unique meaning. Many flashlight makers try to dazzle us by throwing out huge numbers (1,000,000 candle power!) that blow our minds! But what do these numbers actually mean? Let's find out!Credit: http://urbanvelo.org/candela-lumen-lux-what-do-they-mean/
Candela is the SI unit of power emitted by a light source in a particular direction. The particular direction must be defined in order to make sense of the number. Also, this doesn't take into acount the amount of light being given off to the sides. Think of it like this, when you use a flashlight, it helps you see things in front of you, but ambient light can help you see off to the side as well. The candle power of the flashlight will be refering to the power of the center of the beam.
A lumen is the SI unit of total amount of light being radiated in all directions. This quantity is not particularly useful if you do not know the shape of the beam. Say you had a 1000 lumen flashlight with a roughly cone shaped beam. You would perceive this to be much brighter than a lightbulb with with same number of lumens. This is because the light is spread over a much larger area.
Lux is the SI unit of light emitted over a given area. It is equal to one lumen per square meter. This is probably the most useful measurement of light because it tells you how bright a light beam will be over a given area. If you have a beam with a wide angle, the lux will decrease the further you are from the light source. If you have a beam with a very small angle (like a laser), lux will not vary by much unless you are extremely far away from the source. If you are looking for something like a bicycle light, lux will be most useful when determining which product to buy.
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