let's Talk About Lumen, Seriously.

[thatsok]

When I'd no idea of flashlights or LED about one and half an month ago, I posted a topic raising many questions on this forum. 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 this forum, 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 LINK REMOVED for example, again. 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, this shop, like many other online shops, did not mention Luminous Efficacy -- in fact, flashlight manufacturers often make mistakes on this issue.

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.

[Neil]

How convenient that the example you post in your first post is from the company you work for.

Message incoming and link removed.

[thatsok]

How convenient that the example you post in your first post is from the company you work for.

I it was my company, I would suggest they add the Luminous Efficacy and modify the MA to mA, at least.

[Neil]

I would like to point out that we can see the IP address from which you post.

[thatsok]

And surely you will find my IP adress and the company's are very different. I don't wanna explain any more, if you think my post is rubbish (very possibly), just ban me.

[Neil]

OK

[NutSAK]

I'm not sure what the point is that you're trying to make, but, if I have this straight, you're implying that we should be able to look at an incandescent bulb, with a known value of luminous flux, and somehow compare that to another light and guess at that light's luminosity?

Yes, I would say that's rubbish.  The difference even between even ten lumens output is too minute for our eyes to accurately detect a difference.  That difference becomes even more difficult to compare as the total flux output increases.

[sawman]

First it's 2 week overnite fedex Atwood whistles, now a 500 word essay on flux capacitors...  how do these nuts find us 

[Mr. Whippy]

New batteries.

I find new batteries make the biggest difference.


 

[SlackOne]

I guess this would be OK to guess at the rough lumens of a light...if you knew what resistance was being provided by the driving circuit.  Finding the luminous efficacy of a particular, say, Cree LED bin wouldn't be all that hard, but I have no idea what they're putting between the LED and the battery which means I have no idea of what the current going to the light actually is.

The light bulbs were just an example, I think.  The luminous efficacy of a 100W tungsten bulb is around 17.5 lumens/watt, resulting in an average of 1750 lumens emitted.  Of course, doesn't mean a thing in practical life.

[John]

Yeah lets talk about a lumen, seriously  1 lumen= 1 lumen right? and it gives out light, so what else is there to talk about 


What I really want to know is light any brighter in a vacuum deep in outer space? 

[Charger]

im going to miss him

[J-sews]

Well that soks





(heehee, sometimes I just slay me!)

[nuphoria]

So, if a flashlight with 1750 lumens turns on in the middle of a forest where no-one can see it, is it still bright?

 

[Sazabi]

So, if a flashlight with 1750 lumens turns on in the middle of a forest where no-one can see it, is it still bright?

 

It'd probably be detectable from orbit... and nuked by SAC as a hostile alien staging point or something. 

[sawman]

So, if a flashlight with 1750 lumens turns on in the middle of a forest where no-one can see it, is it still bright?

 

No-one, not even the creatures of the forest? 

[John]

So, if a flashlight with 1750 lumens turns on in the middle of a forest where no-one can see it, is it still bright?

 

No-one, not even the creatures of the forest? 

No! not unless they've access to internet and twitter,I doubt we'd ever hear from them so actually we'd be left in the dark 

[VicX]

So, if a flashlight with 1750 lumens turns on in the middle of a forest where no-one can see it, is it still bright?

 

Ah, flashlight philosophy. I like it.
I wonder how Aristotle would answer your question.

[Neil]

So, if a flashlight with 1750 lumens turns on in the middle of a forest where no-one can see it, is it still bright?

 

Quit getting all anthropic on us; remember the HiveStar^TM sees all .... yes Chris, including what you did out back next to the raised potato bed 

[sawman]

So, if a flashlight with 1750 lumens turns on in the middle of a forest where no-one can see it, is it still bright?

 

Quit getting all anthropic on us; remember the HiveStar^TM sees all .... yes Chris, including what you did out back next to the raised potato bed 

[Essexman]

remember the FiveStar^TM sees all

Fixed that, you mean the naff 80's Jackon's like band from Romford?      Or am I showing my age?

[Neil]

Age and bad taste

[nuphoria]

Oddly enough, we were just discussing the potato bed

I have promised to go and dig some up tomorrow.... I wonder what else I shall find