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LUX Measurement & Ideal Ranges

Discussion in 'General Plant Topics' started by dapellegrini, Aug 27, 2008.

  1. dapellegrini

    dapellegrini Lifetime Charter Member
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    I am not having a lot of luck finding information about LUX Measurements - including how to take them (top of water, bottom of tank, etc) and what they mean (what ranges are ideal in a planted tank).

    Here is what I have found:

    Direct sunlight reads between 75k - 120k LUX at midday over the ocean. At a depth of 30ft you still see about 10k of LUX in the ocean.

    I have a (cheap) Milwaukee SM700 LUX meter and have run some tests on my lights, but I don't really have anything to compare them to, or even any clear testing procedures.

    Any good links or experience with LUX in the planted tank?
     
  2. ceg4048

    ceg4048 Lifetime Charter Member
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    Hi,
    LUX, or Luminance is a measurement based on a standardized model of human brightness perception. Since humans are more sensitive to green wavelengths, light sources containing these wavelengths appear to be brighter. As a result of this subjectivity LUX is not a reliable indicator of light energy within the context of photosynthesis.

    Photosynthesis is a process dependent on incident radiation within all wavelengths in the visible spectrum. The incident radiation carries with it photons which collide with chlorophyll molecules to liberate electrons. The measurement of this activity is referred to as Photosynthetically Active Radiation, or PAR. The relative growth rate and net carbon assimilation rate of plants depends in part on the plant's intercepted PAR also referred to as Photosynthetic Photon Flux Density (PPFD which can be thought of as the number of photons crossing a unit area during a unit time period) and the pathway the plant uses to assimilate the carbon.

    Therefore, instead of a LUX meter it is suggested to use a PAR meter. A review of PAR meters can be found here: http://www.advancedaquarist.com/2005/7/review/view

    A discussion of PAR measurements and suggested levels in planted tanks can be found in this thread: http://www.barrreport.com/articles/4265-light-intensity-aquarium.html?highlight=PAR+meter


    Cheers,
     
  3. ceg4048

    ceg4048 Lifetime Charter Member
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    Oops, double post.
     
  4. Tom Barr

    Tom Barr Founder
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    Stay away from Lux and move over to PAR, this is plants, not cameras/people photosynthesizing. While the meters run 75$ or so, the PAR meters run about 250-300 for the cheaper Apogee's, still, not bad and it tells you precisely(depending on how you use it) what you are interested in.

    Lux is an antiquated measurement but is used for office, home bulbs etc.
    You can simply use you camera's f/stop an convert to lux.

    It is not used in botany, at least not in any real meaningful way for the last 3 decades..........

    Use PAR if you plan on measuring light.
    Reef folks do.

    Regards,
    Tom Barr
     
  5. dapellegrini

    dapellegrini Lifetime Charter Member
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    Thanks... I guess this LUX thing was a bit of a waste. I appreciate the links - that is exactly what I was looking for.

    I did a quick measurement before replacing the bulbs in my 96w Tek light - LUX before change from the bottom of the tank was 5,500 - with new bulbs, same spot it measured 9,500. The bulbs were 12 month old T5HO's.

    Could this be utterly meaningless in reference to how much PAR has diminished over the same period and how much effective light is reaching the plants?
     
  6. Tom Barr

    Tom Barr Founder
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    It might be wise as you are in a plant group there, to have the group do a few plant sales drives and buy some good test kit equipment such as a PAR meter, about 250$, a decent set of Lamotte Test kits for NO3/PO4 or a colorimeter from Hanna for both of these, maybe K+ meter from Hanna(170$). A 2000$ CO2 meter might be a bit much though:) A LDO meter might also be worthwhile, used off ebay, they can be had fairly reasonably.

    Same for a Hach 890 colorimeter.

    The light meter is easy to use and folks really do not need it much typically, but they can do a number of test throughout their tank/s and then have a good idea about the light.

    Regards,
    Tom Barr
     
  7. ceg4048

    ceg4048 Lifetime Charter Member
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    Well, it's no secret that bulbs get dimmer as they age so the LUX readings simply confirm that. It follows that the photon flux that a bulb is able to produce will also diminish as the bulb deteriorates. Since you are comparing the same bulb over time this is logical, but since you have no direct correlation between LUX value and PAR value there is no way to determine absolute values of the change in PAR. In other words you have no way of knowing how many photons are striking 1 square centimeter of a plant surface at coordinates X-Y-Z distance from the centerline of the bulb. You simply know that the number is lower now than it was a year ago.

    In this sense it's not completely meaningless but since you have no meaningful numbers as regards PAR you are simply confirming qualitatively what you already know.

    Where it really becomes meaningless is if you were for example to compare the LUX values of several bulbs of different spectral characteristics. The LUX meter would give you a higher reading for the bulb containing the higher green content whereas higher photon flux is likely to emanate from the bulb with more blue content simply because a higher frequency wave will deliver more photons to a surface per unit time compared to a lower frequency wave. Also a bulb with a broader spectral curve (having more area under the spectral curve) will generate more photons even though it may not "look" more luminous. So there are many ways in which the LUX meter readings will simply be rubbish and will be no more meaningful than what you can see with your own eyes.

    But even this is muddied by the fact that at a given X,Y,Z coordinate in the tank there is reflection, refraction, attenuation and a host of factors that affect exactly how many photons per second actually reach that X,Y,Z point, and only a PAR meter will tell you that. Even with the PAR meter it's necessary to build a 2D or more accurately a 3D photon flux profile across the dimensions of the tank to have an idea of the photosynthetic energy distribution of the tank using a specific bulb set.

    As Tom says, it's better to sell the LUX meter to someone who hasn't read these threads and to get the PAR meter.:cool:

    Cheers,
     
  8. Tom Barr

    Tom Barr Founder
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    An interesting side note here: as the tank is newly set up, over time you can see how the light drops off, often by quite a bit with some of the older T12's etc, remove a reflector etc from the equation, allow the plants to grow 2x closer to the light...........


    This is why folks had issues when the tank was new vs 12-24 months later and when they switched bulbs, they saw dramatic changes(not always good ones).

    I typically tell folks to use less light, if they had started with low light , then they often would have an easy time managing things and less pruning.

    Light drives all the growth for algae, plants etc. Best to understand it well.

    Regards,
    Tom Barr
     
  9. Gerryd

    Gerryd Plant Guru Team
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    Tom,

    Thanks for this info. Saves me a future thread and some research as I wanted to get some reasonably priced test equipment, but wasn't sure what to get.

    Yes, the c02 meter is out of my reach, but the others are not.

    I would like a PAR meter, good quality tests for N03, P04, and PH. What about a TDS meter? Will this be of any practical use?

    If you have specific models that you recommend, go for it lol

    What is an LDO meter?

    I would be interested in knowing 'more' about my tank specifically in regards to these measurements.
     
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