one candlepower equal 12.57 lumens

I have problem with this, and with some of the explanation on how lumens are meassure here on this page.
LINK

I explain in writing in another Forum that candlepowers is a meassure of the light intensity at the hot spot and that lumens are a better meassure because is the meassure of the total light emitted by a source.
I was rebuked that it is the opposite and shown this page??

Anybody care to comment, its my explanation faulty?
Juan C.

I don't think that you can link lux with lumens, at least in the flashlight world. With light meters, we're not measuring lumens per square meter because we're reading only the hot spot of the beam. The hot-spot reading will partly be a function of the sensor on the light meter (the width of the beam that it measures).

Say you put a Mag85 on flood and measure the lux at one meter. You could find a tight LED beam that would show the same lux, but still have way less lumens (total light).

But tighten that Mag85 beam, and suddenly the lux goes way up but lumens stay the same.

That page could have been made a lot shorter, and a lot less confusing. With flashlights, there are two major parameters that describe the light output in a useful way.

The first is luminous flux (measured in lumens), which is the total quantity of light emitted regardless of focusing. The second parameter is colloquially called "throw" on this forum, but it basically comes down to the beam-center illuminance at a given distance, measured in lux. The distance must be the same when comparisons are made, since illuminance decreases greatly as the distance increases.

The only company I know of making flashlight bulbs that gives total light output in candle power is Carley Lamps. However they us MSCP (mean spherical candle power). 1 MSCP = 4pi Lumens = 12.67 Lumens. Candle power and MSCP are completely different animals, and there is no conversion between the two.

hmmm, 12.57 lumens huh?

I have always wondered that.
its fairly high, I wouldnt be surprised if it was higher than that.

But Josey is right,
You cant really compare Lux to Lumens, its just not the same.
candlepower sorta translates someties, but I dont think your really supposed to do that either

I had typed a fairly lengthy article on this at one time, but I'll try to condense it a little here.

First, candlepower is a description of the light at the source. As in, "that lighthouse produces one hundred million candlepower". It is useless in our world of flashlights, as witnessed by the Thor and it's "10,000,000 candlepower".

Second, lumens and footcandles are equivalent. They are the a measurement of the amount of light as it falls on a one square foot surface.

However, there is a problem measuring how much light is produced by a light source. Most light sources are roughly spherical, they might be a little longer in one axis than another, but they throw light off in all directions. Thus, you must measure ALL of that light if you wish to know how many candlepower the light produces, and once again, that figure is useless for us, since we are going to redirect it and lose some of it anyway!

So what measurements mean something, then?

Really only two pertain specifically to our world. Lumen production out the front of the light, basically defined as the total output that comes out of the light, and Lux output, which allows us some measurement of the intensity of that output.

Lux is the measurement of how much light falls on a square meter. By common use, we typically position the source 1 meter from the surface we illuminate, then measure the light with a light meter at its most intense spot, what we call the "hotspot." This creates a false Lux reading since the meter is assuming even distribution of light over a square meter, but the number can be used, judiciously, to compare one hotspot with another.

Is this really the measurement of Lux? No, because generally the area we are measuring is much smaller than one meter! But it gives us an idea of the brightness of the light in comparison to others measured similarly. To be fair, we should state something like this, "the hotspot measured 6 inches in diameter, and had a lux reading of 8000 across the spot." Then when someone says, "hotspot measured 12 inches in diameter and had a lux reading of 8000 across the spot" we would know that though the hotspots were of similar intensity, one lamp was producing MUCH more light than the other, at least in their respective hotspots. (When you add spill in you create a whole new ballgame!)

So the simple answer is this as used in the flashlight world here on CPF:

Lumens are used to measure the total output of a light, Lux is used to measure the intensity of that output at a given spot. However, even that measurement of Lux is somewhat different than the actual scientific definition.

Hope that helps!

Bill

Since there is so much confusion about this topic, I'll try to explain once again:

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juancho said:
I explain in writing in another Forum that candlepowers is a meassure of the light intensity at the hot spot and that lumens are a better meassure because is the meassure of the total light emitted by a source.

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Candela (a synonym for Candlepower) is not necessarily measured at the hotspot, candela is just a measure for how much light is emitted in a particular direktion. You normally take the hotspot for your measurement, because you are most interested in how bright the hotspot of your flashlight is, but you can also measure the candela value of the sidespill.

Lumens is a measure for the total output of the light, but you can't say that it's better than candela, it depends on what you want to know about the light.


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Josey said: With light meters, we're not measuring lumens per square meter because we're reading only the hot spot of the beam.

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Yes we are. Lux is Lumen per square meter and that's what the common lightmeters show us.


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Josey said:The hot-spot reading will partly be a function of the sensor on the light meter (the width of the beam that it measures).

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No. The sensor of a normal light meter is much smaller than the hotspot you want to measure. So if you light meter works correctly, your Lux readings are independant of the sensor you use.


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asdalton said:The distance must be the same when comparisons are made, since illuminance decreases greatly as the distance increases.

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Either that or you just convert the readings to make them comparable. At twice the distance, the illuminance is reduced to 1/4.
I agree with everything else you said.


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jtice said:
You cant really compare Lux to Lumens, its just not the same.
candlepower sorta translates someties, but I dont think your really supposed to do that either

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If you want to compare Lux to Lumens, you have to know how the light is distributed. Converting candela to lux is easy, you just have to know the distance. One candela in a distance of 1 Meter is exactly one lux.


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bwaites said:
As in, "that lighthouse produces one hundred million candlepower". It is useless in our world of flashlights, as witnessed by the Thor and it's "10,000,000 candlepower".

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The candela claims are only useless because the manufacturers are cheating on you. I measured some of the x million candela myself, the true readings are about 1-5 percent of the values claimed on the package. If you want to know how much you can see with such a spotlight in a great distance, you have to know hou much lux it produces, the lumen values are useless here.


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bwaites said:Second, lumens and footcandles are equivalent. They are the a measurement of the amount of light as it falls on a one square foot surface.

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That's wrong. I don't know about footcandles (and I think noone should use it) but lumens has nothing to to with surface. It is just a measure of how much light is produced.


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bwaites said:Thus, you must measure ALL of that light if you wish to know how many candlepower the light produces,

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That's also wrong. What you describe is how lumens are measured. To measure candlepower a person has to measure how much light the source emits in a given direction. He or she normally does that by measuring how much light falls from the source onto a surface in a known distance and convert these readings into Candela.


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bwaites said:By common use, we typically position the source 1 meter from the surface we illuminate, then measure the light with a light meter at its most intense spot, what we call the "hotspot." This creates a false Lux reading since the meter is assuming even distribution of light over a square meter

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No, you don't get a false Lux reading. You don't have to light up a whole square meter just to calculate the Lux (Lumens per square meter). If you drive in you car and look at your speedo and it shows 60 miles/hour, then this reading is correct, regardless of how long you drive. You don't have to drive a whole hour just to calculate the speed. The same is true for the Lux.