When I’d no idea of flashlights or LED about one and half an month ago, I posted a topic raising many questions on Web forums. One of the questions was what does the word “lumen” mean. Some kindhearted friends answered me, very simply. While I’m getting interested in LED flashlights, this problem becomes more and more heavy on my mind. So I searched on forums again, without satisfactory answers. Then I searched on Google.
Now I’m going to show what I’ve learned online. I’m not sure if the information I collected compose a right point. Even if so, I don’t promise to make it clear to everyone (though I’m trying my best). I hope you would join the discussion to explain the subject fully.
I will take this LED torch (http://www.tinydeal.com/led-torch-px6pja–p-5940.html) for example. In the instructions, you can see this item “Output voltage: 5.5V/ 500MA (should be mA)”. According to it, we learn that its output power is 2750 mW (milliwatt) or 2.75W (watt) — 5.5V (voltage) by 500mA (milli). But how bright the emitted light will be ? We have to know the term of Luminous Efficacy, which is denominated by the measurement of Lumen/Watt (lm/W).
Luminous Efficacy multiplying by Output Power equal Luminous Flux, the key term on brightness. From this formula we can see that supposing Output Power is fixed, brightness would be determined by Luminous Efficacy. If you buy a flashlight online and cannot experience the light with your own eyes, Output Power and Luminous Efficacy will help you measure the brightness. Unfortunately, our website shop, like many other online shops, did not mention Luminous Efficacy — in fact, flashlight manufacturers often make mistakes on this issue, so we dare not to cite their data literally
In the meantime, the equation (Luminous Efficacy x Output Power = Luminous Flux) indicates another fact that Lumen is the measurement of Luminous Flux. So let’s see what this term means.
Luminous Flux means the light emitted by a light source per unit time, citing Wikipedia. If I was to further explain this term theoretically, I have to introduce more terms, such as Luminous Intensity, steradian, Luminous emittance, Illuminance and Luminance, each of which are also attached to sub-explanations. Obviously, I don’t have the ability to write a textbook teaching you optics, so let’s learn Luminous Flux and Lumen by experiences.
I wanna repeat that Luminous Flux is a key term on brightness and Lumen is the measurement of this term. So what we need to do is feeling the relationship between brightness and Lumen.
Then fetch a 40W incandescent bulb and electrify it under an alternating current voltage of 220V. It sends out 340 Lumen of light (a inaccurate word). As comparison reinforces cognition, we can turn on a 100W incandescent bulb under the same voltage, so that we can see what 1,300 Lumen of light is like. And a 100W incandescent bulb emits 1,700 Lumen of light in North America where civil alternating current voltage is 120V.
By the way, the theoretical Luminous Efficacy of a light emitting diode (LED) is about 400 lm/W, but the practical Luminous Efficacy is less than 100 lm/W now.
I searched all the sources of this article on the web, some of them may not be reliable. I’m not misleading friends here, so please point out any mistake whenever you find it.