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Author Topic: LED-How to tell the real ligthing output of led lights?How to compare lighting p  (Read 2391 times)

haring

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I am looking at several led light options. Obviously, one of the considerations is how much light the led panel emits.
Some companies list their products specifying out put via "lux"
http://www.adorama.com/fpvl500c.html
Others list Watt, W.
http://ikancorp.com/productdetail.php?id=998
Others use lumens...

My questions: How to measure/compare the lighting output of led lights? Is there a way to convert these numbers? All I want to know how much light these lights output so I can compare them.

GrahamBy

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Lumen is the unit of measure the actual light output, whereas Lux is the unit of the intensity of that light reaching what you want to photograph:
ie if light A has 4 times as many Lumen as light B, but you place it twice as far away, the inverse square law predicts there will be the same illumination.

I've made the gross assumption of both lights having the same spread of light: a concentrated beam will yield more illumination (Lux) than a wide flood for the same total output (Lumens).

So if the output is specified in Lux, it should also specify at what distance the light is placed. If it's specified in Lumen, you're a bit stuck without knowing the beam pattern.

You can then convert Lux to Ev (for a given ISO) using the chart here:
http://www.sekonic.com/support/evluxfootcandleconversionchart.aspx
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BartvanderWolf

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My questions: How to measure/compare the lighting output of led lights? Is there a way to convert these numbers? All I want to know how much light these lights output so I can compare them.

Hi,

You can try this calculator.

I haven't tested whether it's absolutely correct, but that is basically the input needed for a conversion. Lumen is a total flux of light (in all directions, or concentrated by a reflector), Lux is the amount of light a given surface area receives or emits (usually expressed per square metre, if you want to avoid angles and distance).

One Lux is one Lumen per square meter, but for conversion you would need to know how the flux in Lumen is spread and the distance at which it is measured.

Watts only tell something about the power consumption, nothing about how efficient that power is transformed into visible light (and e.g. heat).

Cheers,
Bart

P.S. Here are some more calculators.
« Last Edit: January 18, 2016, 11:31:14 AM by BartvanderWolf »
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digitaldog

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My questions: How to measure/compare the lighting output of led lights? Is there a way to convert these numbers? All I want to know how much light these lights output so I can compare them.
You could measure the this using something like an i1 Pro Spectrophotometer from X-rite and a number of software options (BableColor). You could get this info and a lot more about the illuminant.


http://www.babelcolor.com
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BobShaw

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Measure it with a light meter at say 2m. As Metallica would say, nothing else matters.
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BartvanderWolf

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Measure it with a light meter at say 2m. As Metallica would say, nothing else matters.

Hi Bob,

Sure, that will do, but a practical issue is that you first need to acquire such a LED light, and secondly unbox and mount in a reflector, before you can measure it. That obviously won't do for comparing advertised specs, before purchasing.

And as far as specs go, the LEDs will lose a percentage of their initial Lumen output capacity over time. Not all manufacturers specify that percentage.

Cheers,
Bart
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BobShaw

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a practical issue is that you first need to acquire such a LED light, and secondly unbox and mount in a reflector, before you can measure it.
I disagree. If you are buying something that is important to your work, like lights, then you try before you buy. That's why there are still (a few) camera stores and suppliers. Buy cheap, buy thrice.
LED has yet to impress me, except for video, because you can't really use many modifiers. There are other posts her about strange colours, so it's a box of chocolates.
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BartvanderWolf

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I disagree. If you are buying something that is important to your work, like lights, then you try before you buy.

Hi Bob,

While I agree in principle, it is in practice not possible to test all that is offered.

Quote
LED has yet to impress me, except for video, because you can't really use many modifiers. There are other posts her about strange colours, so it's a box of chocolates.

I'm not impressed yet either. But the OP was more about comparing the the luminosity based on published specifications, rather than on colour. I would currently not choose LED for good color, but for other uses it may suffice.

Cheers,
Bart
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Morgan_Moore

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I mainly do video now. Still struggling to match my vivitar 285, qflashes and elly set up.

Simply I am trying before buying.

I made a test which is 1/50 shutter 800ISO, what FStop when the light illuminates an area 8ft wide?

Some spotty lights have to be far back, some soft ones close.

Certainly try.. some i tried were very green

Also google Alan Roberts, lighting and you can read up on his tests of colour quality..

S

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bjanes

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I am looking at several led light options. Obviously, one of the considerations is how much light the led panel emits.
Some companies list their products specifying out put via "lux"
http://www.adorama.com/fpvl500c.html
Others list Watt, W.
http://ikancorp.com/productdetail.php?id=998
Others use lumens...

My questions: How to measure/compare the lighting output of led lights? Is there a way to convert these numbers? All I want to know how much light these lights output so I can compare them.

What you are interested in is how much light from the lamp will fall on the subject: illuminance, which is measured as Lux (lumens/meter^20). On the Adorama site this is given under the specs tab. The falloff does not follow the inverse square law precisely, because the light is not a point source.

Bill

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