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Thread: Converting lux to lumen

  1. #1

    Default Converting lux to lumen

    Being a photographer, I have a professional light meter which also measures light in luxes. How should I multiply or divide the quantity of lux units in order to get lumens.

    1 lx = 1 lm/m2

    But no datasheet of LEDs shows lumens per square meter, there are just lumens.

    I have found this conversion but I also get weird results, no matter what I convert to.
    http://www.unitconversion.org/unit_c...umination.html

    For example, my room is lit by a 23W Philips bulb (tungsten). The meter shows 12lx from a 1m distance. When I try to convert it, I get:

    12 lumen/m2
    0.0012 lumen/cm2
    etc...

    My room is quite big and I don't believe that a 12lm/m2 LED will replace my 23W bulb (equivalent to a 100W filament bulb).

    So what's the deal with this conversion? I would like to perform some experiments but I simply don't know what are these lumens without any suffix. All the lumens I get are 'per square something'

  2. #2

    Default Re: Converting lux to lumen

    Speaking of...I noticed the other day at the store that they are starting to put lumens ratings on light bulbs. They had one...I think it was a 100W bulb that said 3000 lumens, or something ridiculous like that.

    I figured I'd just send a batch of thse to the Army, so they could save money on those Surefire Hellfighter lights.

    Maybe they use a similar conversion?

  3. #3
    Flashaholic
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    Default Re: Converting lux to lumen

    Lumens is a light quantity, and lux is a light quantity over an area (luminous flux.) They are two different measurements and there is no direct conversion.


  4. #4
    Flashaholic* Nereus's Avatar
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    Default Re: Converting lux to lumen

    You can find good info here.

    -N
    My mods. Please post questions about my mods to the corresponding thread: like that all CPFers can get the info - thanks!

  5. #5

    Default Re: Converting lux to lumen

    1 Lux = 1 lumen/square meter.

    To do a conversion, you essentially need to take a bunch of lux measurements over a given area and multiply by the area to calculate the given lumens. I.e. say a wall is 4*4 meters. You could break that into 16 1m*1m squares and measure the center of each square. Multiply that by 1m*1m to get the approximate lumens in each square. Add up all the 16 numbers to get the total lumens hitting the wall.

    Now on your example below, do you mean 23W flourescent (to be the same as a 100W incandescent?).

    If you are measuring the output of a bulb that approximately radiants spherically, then what you could do is calculate the area that would be covered at the distance of your measurement.

    Areas of a sphere = 4*pi*radius^2 or in your case 4*3.14*1*1 = 12.6 approximately.

    You measurement 12 lux for a total lumen output of about 150.....which tends to tell me you read your meter wrong or it is broken.

    I flourescent bulb is certainly not a perfect spherical source, but not too bad on say a 270 degree radiation pattern. That bulb should be about 1,500 lumens, so I am wondering if you had the scale wrong and you really read 120 lux?

    Hope that helped.

    Semiman


    Quote Originally Posted by MikePL View Post
    Being a photographer, I have a professional light meter which also measures light in luxes. How should I multiply or divide the quantity of lux units in order to get lumens.

    1 lx = 1 lm/m2

    But no datasheet of LEDs shows lumens per square meter, there are just lumens.

    I have found this conversion but I also get weird results, no matter what I convert to.
    http://www.unitconversion.org/unit_c...umination.html

    For example, my room is lit by a 23W Philips bulb (tungsten). The meter shows 12lx from a 1m distance. When I try to convert it, I get:

    12 lumen/m2
    0.0012 lumen/cm2
    etc...

    My room is quite big and I don't believe that a 12lm/m2 LED will replace my 23W bulb (equivalent to a 100W filament bulb).

    So what's the deal with this conversion? I would like to perform some experiments but I simply don't know what are these lumens without any suffix. All the lumens I get are 'per square something'

  6. #6
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    Default Re: Converting lux to lumen

    Quote Originally Posted by Delta View Post
    Speaking of...I noticed the other day at the store that they are starting to put lumens ratings on light bulbs. They had one...I think it was a 100W bulb that said 3000 lumens, or something ridiculous like that.

    I figured I'd just send a batch of thse to the Army, so they could save money on those Surefire Hellfighter lights.

    Maybe they use a similar conversion?
    I'd guess that a 100W lightbulb could be up to 3000 lumen. (30 lumens per watt)

    Notice how the whole room lights up when you turn on a 100w lamp - an effect that can only be replicated with a bounce test from a very powerful light.

  7. #7
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    Default Re: Converting lux to lumen

    For decades they have been putting lumen ratings on light bulb packaging. Takes a pretty powerful incan flashlight light to equal the output of a 100 watt light bulb, and I believe that those are accurate lab figures for light bulb lumens.

    Bill

  8. #8

    Default Re: Converting lux to lumen

    Likely that 100W bulb is more like 1,500 lumens.

    Semiman

  9. #9
    Flashaholic* Calina's Avatar
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    Default Re: Converting lux to lumen

    Quote Originally Posted by SemiMan View Post
    Likely that 100W bulb is more like 1,500 lumens.

    Semiman
    Yep! Phillips rates theirs at 1600 initial lumens.
    Health is merely the slowest possible rate at which one can die.

  10. #10
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    Default Re: Converting lux to lumen

    Go find a package of 100 watt light bulbs. The lumen rating will be on the packiage. It may be 1500 lumens, I'm out of spare bulbs, but that 1500 lumens, or so will sure "light up a room". Cheap too. Not too portable.

    Bill

  11. #11

    Default Re: Converting lux to lumen

    There has been lots of talks about LEDbulbs in the market now. In my opinion its not possible to make good LEDbulb yet.If its has to give same kind of light as incandescent.

    Some models i have seen ,example:
    8w e27 up tp +400lm.Manufacturer is saying that is the same lightoutput as 40w incandescent.Well..it might almost be,BUT..Its about 140deg angle.So you cant really compare that to incandescent, because of that angle.And...that lumen is from the LEDs not from the bulb,so there is difference.

    This 8w e27-model has 3pcs of CREE XR-E.Ithink this UP TO.. is meaning if you drive every LED with 700mA. But they are not driving it with 700mA.In the datasheet they tell that LEDs are 3x2w.So 2w is wasted in the other parts in the bulb.If they drive every LED with 700mA, it should be more than 10w..

    I think there is lots of different kind of E-base LEDbulbs coming to the markets soon. So, im affraid the marketing its going to be once again, with wrong informations. This wrong kind of marketing is problem in the MR16 now.If this continues,there is a hard way for the products ,in the general lighting

  12. #12
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    Default Re: Converting lux to lumen

    Quote Originally Posted by SemiMan View Post
    Likely that 100W bulb is more like 1,500 lumens.

    Semiman
    Unless it is a Mercury Vapor bulb (100W rated at 4300 lumens):

    http://www.prolighting.com/h310mevabume.html

    but we digress....

  13. #13

    Default Re: Converting lux to lumen

    I think it is "possible" to build a reasonable LED based bulb with today's technology. The question is, is it economically feasible. How would I do it?

    - I would probably start with Rebels for two reasons. One is they have a much wider pattern than Cree which will make the need for secondary diffusers less. Two is they stand up to heat much better than XREs or Seouls and a bulb is going to have a limited amount of area for heat sinking.

    - Thinking about an Edison bulb..... likely need on the order of 8 LEDS arranged in an offset circular pattern ... two rows of 4, 90 degrees apart with each row 45 degrees offset. One row pointing upwards, one down.

    - I still think I will need some diffusion.... I need to be careful in the design...how do I get the diffuser in while keeping the heat sink clear to air flow?

    - If I am shooting for efficiency, I could go cool white 100 lumen parts, 350mA. That would give me probably 700 lumens or so at the LED when warm. Add in say 10% loss for some sort of diffuser and I am probably looking at 650 lumens. I will assume 85% efficient power supply, so that 8 watts out is going to take 9.4 watts in. That results in about 70 lumens/watt. That would be highly competitive. Problem is, today, that would run me about $40 to manufacture I would guess. I could go with cheaper 80 lumen rebels but that woul drop me to about 500 lumens or 53 lumens per watt.

    - Esthetically I may want to go neutral white

    - I could try pushing the LEDS harder, however, I am not sure how much heat sinking capability I can get out of a heat sink that roughly fits the form factor of your typical Edison bulb. Perhaps I could reduce the number of LEDs get the same light output, reduce the efficiency and the cost?

    At the end of the day, probably a nice market. There are not may who are going to pay $100 for a 40 watt LED equivalent bulb, however, if you have a rough service location, i.e. outdoors, high vibration, or just hard to get to, a 50,000 hour bulb that is reasonably efficient for $100 may seem like a good deal.

    Semiman

  14. #14

    Default Re: Converting lux to lumen

    We have diverted a little bit from the main subject, but I will tell you what I am planning to do.

    I needed a simple conversion to lumens in order to test the real output of different LEDs. What people claim is one thing and the truth is another. Having the possibility to measure light would let me do some sort of comparison.

    So far I only see lux meters all around but no lumen meters. So how do they measure the output at the factory?

  15. #15
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    Default Re: Converting lux to lumen

    They most likely use an Integrated Sphere calibrated for accuracy by a reputable lab. SureFire does this. Arc does this. Not sure about other companies. If you can capture all of the lux of a light in a lightbox such as made by Quickbeam at flashlightreviews.com you can get an approximate value for lumens if you work out a formula based on the lumen output of a known light. People who were involved in the Lightmeter Benchmark Testing thread (see threads of interest in battery forum) were able to receive lumen info on the passaround light from a certified lab using an IS, and those using lightbox or other contraptions to test approximate lumen output were able to use this info. Using that info people could work out a formula to get approximate lumen output info for various lights that they test.

    Of course if we all had IS's we would be more accurate, but even IS's have to be calibrated every so often for accuracy.

    Bill

  16. #16
    Flashaholic Yenster's Avatar
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    Default Re: Converting lux to lumen

    Quote Originally Posted by MikePL View Post
    We have diverted a little bit from the main subject, but I will tell you what I am planning to do.

    I needed a simple conversion to lumens in order to test the real output of different LEDs. What people claim is one thing and the truth is another. Having the possibility to measure light would let me do some sort of comparison.

    So far I only see lux meters all around but no lumen meters. So how do they measure the output at the factory?
    As noted already, there is no accurate way to convert Lux to Lumens because these are two very different dimensions and quantities.

    Maybe a photographic analogy....

    Take a picture with studio lights in your room. Set your digital camera on ISO 100, F11, with shutter speed 1/125 sec. Now adjust your studio lights so that you get a good exposure, either by adjusting the output of the lights and/or moving the lights closer to the subject. Now go outside in the sunlight with that same subject and take a picture with the same settings...ISO 100, F11, 1/125 sec. and you get a proper exposure.

    Both pictures look good because they both have the same Lux (light per area). But I'm sure you'll agree that there's no way you're studio lights has the same Lumens (total output) as the Sun.

    So is there a simple conversion....the answer is no.
    Is there a way to measure Lumens...the answer is yes...BUT...it'll is very difficult to get accurate measurements and it would take a lot of very precise and expensive equipment to do it...and then it would have to be adapted to the different characteristics of various LEDs
    Last edited by Yenster; 10-27-2007 at 12:09 AM.

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