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Old May 27th 2016, 12:20 AM   #1
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How to Convert Lumens to Watts

Conversion from Lumens to Watts is an important solving problem in physics syllabus. My question paper doesn't come without this problem, but I could not able to understand the problem. When i was surfing last night I found the solution for the conversion in this website. It gives the clear idea using formula.

But, it has been given a static value for the categories of light source. Will it be static? Else we can choose any other value for the light source.
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Old Aug 30th 2016, 09:58 AM   #2
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I certainly would not consider "Conversion from Lumens to Watts" be an important problem in physics. As you give here "One lumen is equal to 1/683 of watt light power."
So given light with intensity N lumens divide by 683 to get the intensity in watts, N/683 watts.

Now, what do you mean by "static"? Yes, we can "choose any other value for the light source", with a different intensity in Lumens then we will get a different value in watts.
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Old Aug 31st 2016, 10:20 AM   #3
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I had a look at the indicated online calculator.
The confusing piece of additional information seems to be the "luminous efficacy" term, which is not defined at all.

I am guessing that this term includes the device efficiency

So that the Watts calculated is actually the Electrical power required to produce the required lumens from different devices.
Not the actual "light power" produced.

If my interpretation is correct, the actual light power in Watts is given by entering the "luminous efficacy" as 683.
(This would be equivalent to a perfect light bulb, converting 100% of the electrical energy).

It is quite informative to try different devices with 900 lumens.
This is equivalent to a 60W standard tungsten filament bulb.
Most of the devices are woefully inefficient at converting electricity into light.
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Old Aug 31st 2016, 11:46 AM   #4
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"Lumens" is a measure of the brightness, the amount of light being given off or received. "Watts", on the other hand, is a measure of "energy per time". They do not measure the same thing so there is no one way to convert from one to the other. You gave the link to the page that gave the figure I used before.

To get light, measured in lumens, from, say, a lamp, you have to feed electricity (measured in Watts) to it. But the conversion from electricity to light will depend upon the lamp itself. That is the "device efficiency". The more efficient the device is, the more light you will get for a given amount of electricity.
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