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Thread: Instrument To Measure lumens?

  1. #1

    Default Instrument To Measure lumens?

    Hello all, I'm putting together a website to do apples-to-apples comparisons of lights (mainly bike lights).



    I want to perform two tests: light color and light intensity. The color part is easy. The intensity seems more complicated/expensive.



    I'm trying to find an instrument to measure lumens. Lux seems to be easy but it's a measure of light over an area and I'm trying to get light intensity at different distances.



    What instrument can I use? Someone suggested using a calibrated integrated sphere. Those are $1000+



    Any suggestions would be highly appreciated.

  2. #2
    Flashaholic*
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    Default Re: Instrument To Measure lumens?

    Intensity (brightness) is measured in Lux. Lumens is a measure of light output. What do you want to measure?

  3. #3
    *Flashaholic* kramer5150's Avatar
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    Default Re: Instrument To Measure lumens?

    you'll need an integrating sphere to measure Lumens. You can buy one for $1000's as you have already noted, but even the "best" commercial sphere is only as good/accurate as the calibration method used to set it up. CALIBRATION (and the 100s of associated man hours is what its all about), if you are after "absolute" lumen numbers. "This light is 110 Lumens."

    If you just want "relative" lumen numbers, "Light A emits more lumens than light B." then calibration is not as important.... and I think this is what most CPF'ers are set up for.

    Heres a pic of the BC sphere used to test 100s of lights from across the country. IIRC Jose spent countless hours in an optics lab with MrGman (when he was here in the bay area).... testing and calibrating this sphere against some really high end equipment.


    You can DIY one using readily available materials for under ~$300 as many CPFers have done.
    Last edited by kramer5150; 07-09-2011 at 11:18 AM.
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  4. #4
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    Default Re: Instrument To Measure lumens?

    The thing with Lumens is that it is a measure of output as opposed to Lux which is a measure of intensity (or brightness).

    Comparing the OTF output (in Lumens) of a flashlight is like comparing the RWHP (in horsepower) of a car. It tells you how much each device outputs but gives you no indication of how either performs. For a light, you need to know how bright it is and whether it suits your application. This is where the Lux comes in and is why all specs for lighting intensity (workplace, hospitals, schools, etc.) is specified in this unit and not in Lumens. Same thing with a car - you want to know how quick it is (acceleration), what kind of mileage you will get, how much can it tow, etc.? Two cars with identical horsepower will have very different characteristics. Two lights with identical OTF lumens will also exhibit very different brightness characteristics.

    BTW, Lux is defined as Lumens per sq. meter.

  5. #5
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    Default Re: Instrument To Measure lumens?

    Why not use a photographic light meter set in continous mode?

    the photo sensitive eye is sensitive enough to detect 1/10ths of a stop of light.
    http://www.sekonic.com/ for starters but they're many others and cheaper on ebay.

  6. #6

    Default Re: Instrument To Measure lumens?

    What will you be using to measure the "light color"? Are you refering to color temperature and will you be able to generate full spectral flux data? If you are planning to use a spectrometer for this test then it will not be cheap but it can serve both purposes of measuring the color and output. The input aperture of the fiber of the spectrometer can be mounted outside of your integrating sphere and the light under test can be mounted on a rotation stage to make a basic goniometer.

  7. #7
    Flashaholic Dr.Jones's Avatar
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    Default Re: Instrument To Measure lumens?

    As rmteo already said, intensity and 'lumens' are very different things.

    The luminous intensity of the spot determines the throw of the flashlight; you can even calculate the throw from the measured spot intensity (see here).
    Measuring the spot intensity only requires a lux-meter.

    A proper measurement of the luminous flux (or luminous power; NEMA calls it "light output") requires either an integrating sphere or a gonio-photometer, both calibrated and with spectrometer for spectral corrections, like kramer5150 said.
    For less accuracy, use a lux-meter and somehow calibrate that yourself, again as in kramer5150's post.
    For even less accuracy, one could use ceiling bounce or a light box.

    Both values are helpful performance data for flashlights, that's why ANSI/NEMA FL1 specifies both.

    Harold: It's more like using either an integrating sphere or a goniometer, not both in the same setup... Both can be built with a spectrometer (for spectral corrections) or with a simple lux-meter; these have filters to mimic human light perception (they approximate the CIE luminosity function), but are less accurate.

  8. #8

    Default Re: Instrument To Measure lumens?

    "Harold: It's more like using either an integrating sphere or a goniometer, not both in the same setup... Both can be built with a spectrometer (for spectral corrections) or with a simple lux-meter; these have filters to mimic human light perception (they approximate the CIE luminosity function), but are less accurate."

    Perhaps in my desire to be ecomical with text I didn't communicate my intent. I understand that an integrating sphere and goni-photometer are two seperate devices. We have them set up so that we can use the spectrometer either with the integrating sphere along with the filtered detector or as a measurement tool for the goniometer. We can gather off-axis data on LED sources outside the sphere that cannot be collected due to the nature of the device (integration is what the sphere is all about). I hadn't really considered flashlight specs since the OP is measuring bike headlights but then I didn't consult SAE specs either. My first approach would be to minimize cost of the set up while gathering the most useful data. A spectrometer is much easier to calibrate for color standards than a detector with a filter stack.

    Another suggestion I have for the OP is to search for a copy of "The Light Measurement Handbook" by Alex Ryer and "A Guide to integrating Sphere Radiometry and Photometry" offered by Labsphere. Both should be available in pdf on the web.

  9. #9
    Flashaholic Dr.Jones's Avatar
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    Default Re: Instrument To Measure lumens?

    Ah, I see... Interesting setup. Have you perhaps measured a full beam profile (intensity versus angle) of a flashlight? I really would like to see such a plot. At what distance do you measure? Should be at least 2 m.

    I was planning a goniometer myself, with servo motor, a simple photo-diode and a micro-controller, but I was unhappy with the servo's accuracy and didn't try my other ideas yet - and nowadays, with the XM-L's strong angular tint shift, a 'photopic' or spectroscopic sensor would really be needed. Hm, maybe the cheapest way is to get one from a lux-meter...

  10. #10

    Default Re: Instrument To Measure lumens?

    We are using the goniometer for measuring HB LEDs, inparticular our own product that has been in development. The primary use is for measuring color seperation which is significant for consumer applications or applications where the LED will be used with a reflector. We can also use the data to derive the Polar or Cartesian plots though it is more of a relative measure than an absolute. This allows us to confirm our software models. Considering what we are trying to measure our distance of .5M is plenty. When we need data from a third party we send it to a lab that is a couple hours away and they measure on a full scale LM79 certified gonio-photometer (it's not cheap so we don't do it often!).

    The positioner on our set-up is actually operated by an acceptionally bright physicist even though he is much more expensive than a motor driver! We also built an integrating sphere but have found that the effort required is nearly as costly as buying a good set-up. We are buying a second sphere.

  11. #11

    Default Re: Instrument To Measure lumens?

    Testing Lumen Output?

    Anyway to test on a light actual lumen output - as opposed to stated by manufacurer?

    Thread Merge - Norm
    Last edited by Norm; 12-12-2011 at 10:20 PM.

  12. #12
    Flashaholic* yifu's Avatar
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    Default Re: Instrument To Measure lumens?

    A ceiling bounce test is often the best way to judge relative outputs and is usually quite accurate as well as easy to set up. The actual ratio might be closer to an exponential graph but it is close enough.

  13. #13
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    Default Re: Instrument To Measure lumens?

    Quote Originally Posted by yifu View Post
    A ceiling bounce test is often the best way to judge relative outputs and is usually quite accurate as well as easy to set up. The actual ratio might be closer to an exponential graph but it is close enough.
    That's my belief as well, even if it's a discussed issue. I think it works very well providing the light will be held close enough to the ceiling so differences in spill widths don't matter.

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    Default Re: Instrument To Measure lumens?

    Quote Originally Posted by Harold_B View Post
    What will you be using to measure the "light color"? Are you refering to color temperature and will you be able to generate full spectral flux data? If you are planning to use a spectrometer for this test then it will not be cheap but it can serve both purposes of measuring the color and output. The input aperture of the fiber of the spectrometer can be mounted outside of your integrating sphere and the light under test can be mounted on a rotation stage to make a basic goniometer.
    Few method,
    Most expensive one is colorimeter used in Anal. Chem.
    Or a cheaper but very subjective one is grating, which works like a prism but by interference of light
    ^ Simply: turn a multicolored light source into fraction of color beam
    (Even CD with silvery surface peeled off is able to do this, but of course a grating is more effective)

  15. #15
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    Default Re: Instrument To Measure lumens?

    BTW, the use of integrating sphere is to diffuse the focused beam into a evenly distributed one?

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