# Light and Lumens

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Hey, I'm writing a book, and in it there's a sort of substance in it that's neutralized by bright light. Recently I heard about lumens, the unit of measure for light, and I was wondering if anyone knew about how lumens scale...

Like, if something has twice the lumen level of something else, would the radius of the circle of light it cast be twice as big?

How many lumens would a fire-lit torch have?

A candle?

A Heavy-duty flashlight?

The sun by the time its rays reach the earth?

A massive bonfire?

I'd really appreciate it if somebody could help me out with this.

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Hey, I'm writing a book, and in it there's a sort of substance in it that's neutralized by bright light. Recently I heard about lumens, the unit of measure for light, and I was wondering if anyone knew about how lumens scale...

It is a measure of the quantity of light per unit area. 1 foot candle is the light of one candle one foot away. 1 Lumen is 1 foot candle over one square foot.

Like, if something has twice the lumen level of something else, would the radius of the circle of light it cast be twice as big?

Roughly, yes. Precisely it means that one would receive the same amount of light per area at twice the distance.

How many lumens would a fire-lit torch have?

Not sure, found a couple vague references to 100-140

A candle?

1 as defined

A Heavy-duty flashlight?

200-300

The sun by the time its rays reach the earth?

No clue

A massive bonfire?

Stumped again.

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Thanks for the help, but I'm still quite confused... Maybe I should rephrase it in terms more useful to my book:

I've decided that the substance I mentioned is dispelled by light, and that a fire-lit torch would clear an area with a radius of about 7 feet. I'm trying to find out what kind of clearing power other light sources would have. If a candle really only has 1 lumen of lighting power, and what you said about the radius thing being proportional is true, then that would mean a candle would only clear the stuff in a field with a radius of... six tenths of an inch. That just doesn't sound right to me... am I missing something?

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The lumen is actually the unit of luminosity, which is how bright something is as perceived by the human eye, which is most sensitive around 550 nm (the same amount of green light can appear to be 5 or even 10 times as bright as the same intensity of red or blue light). What you probably want is the intensity (power per unit area)

The amount of light from a point source that hits per unit area varies as r^2, so at twice the distance, a source will be only 1/4 as bright. Or at twice the brightness, it will appear that same at ~1.4 times the distance ($\sqrt{2}$), not twice the distance, as cypress suggested,

Brightness of common situations

http://en.wikipedia.org/wiki/Lux

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Thanks for the help, but I'm still quite confused... Maybe I should rephrase it in terms more useful to my book:

I've decided that the substance I mentioned is dispelled by light, and that a fire-lit torch would clear an area with a radius of about 7 feet. I'm trying to find out what kind of clearing power other light sources would have. If a candle really only has 1 lumen of lighting power, and what you said about the radius thing being proportional is true, then that would mean a candle would only clear the stuff in a field with a radius of... six tenths of an inch. That just doesn't sound right to me... am I missing something?

I think you did your math correct. It could be that the 140 lumen estimate was way off. Or perhaps it applied to a focused beam of light from a wood torch since the site was comparing flash lights and headlights and other focused beam light sources. I don't find the source in my google search history but let's assume I made one of these two errors in quoting that figure.

as far as distance and area goes swansont is correct for radial light as per an unfocused candle. The linear formula works only for focused beams as an approximation. I neglected that consideration.

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darn it... this makes things more complicated. Okay, you know what? Maybe I should just do a test. I'll go out to as dark a night as I can manage, light various light sources, and see how far away from the source I can go and still be able to read a book by its light. Then I'd take the ratios or something, and convert them.

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darn it... this makes things more complicated. Okay, you know what? Maybe I should just do a test. I'll go out to as dark a night as I can manage, light various light sources, and see how far away from the source I can go and still be able to read a book by its light. Then I'd take the ratios or something, and convert them.

...yeah, that's great, except how am I going to get my hands on a massive bonfire...

Back to the drawing board...

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wait, actually, I reread your post, now I think I've got it... I just need to find out what lux I want to be too bright for the stuff to survive in, then convert lux to lumens, get the lumen values of various light sources, and I'm all set!

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How do we measure the intensity of unit area?

There is a measured value of sensor, 0.25W/25mm2 .

I mean, " How do we measure 0.25W?"

As there are many wave length in the light and the emitted electron number is not equal to every sensor, I think it is not easy to measure correctly.

Are there any standard instrument to do this test?

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hey, does candlepower or lumens directly convert to lux by any formula?

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there's a conversion factor.

a lumen is a candela per steridian

a lux is a lumen per meter squared

a candela is "the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10^12 hertz and that has a radiant intensity in that direction of 1⁄683 watt per steradian" -wikipedia

candlepower on the other hand has never been rigorously defined as its always based off of some standard and due to its nature could vary from place to place.

the latest definition is to tie it 1:1 with a candela but previously it was the blackbody emmision spectrum atthe freezzing point of platinum was 58.9 candles/cm^2

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