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PAR and Lux

Discussion in 'Lighting' started by aquabillpers, Jan 24, 2009.

  1. aquabillpers

    aquabillpers Lifetime Charter Member
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    I have a question about the practical application of these two concepts as far as aquatic plant husbandry is concerned.

    Both refer to the amount of light energy (photons) that fall upon a surface a certain distance from the light source. Right?

    Given a broad spectrum "sunlight" bulb with a color temperature of 5500 to 6500 Kelvin, and assuming that the light emitted is suitable for aquatic plants, would both PAR and Lux both provide a reasonable view of the amount of light energy reaching the plants at various distances from the light source?

    Assuming that an aquarium was successful at a certain level of Par or Lux, could either be used to "calibrate" the lighting required for a new aquarium of a different size, with the same bulb(s)?

    Thanks!

    Bill
     
  2. SuperColey1

    SuperColey1 Guru Class Expert

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    From what I know (and I have no equipment. lol) PAR is the definitive measure used at the moment to determine the actual light that is reaching to where you measure it.

    Lumens/Lux is the output of the light in what it emits and therefore in no way tells you how much will reach a certain area.

    You can have 2 tubes with a measured exact same Lumens yet one may be better at penetrating, similar to using LEDs with or without collimaters/lenses.

    So Lumens is a little better than Watts to gauge the light but just as watts gives you a very rough guide Lumens will not tell you how far or how wide the light is going.

    Light is one of those subjects where people tend to get highly involved in where I think it's best just to take rough estimates rather than go the route of paying megabucks for expensive equipment!!!!

    AC
     
  3. ntino

    ntino Guru Class Expert

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    Thats not entirely true...
    Lux or is light measurement, as in light hitting surface. Lux is lumen per square meter. Likewise, because of various reasons the rated Kelvin rating of bulbs means nothing as it really tends to stray alot from facts.

    However, total lux or lumen measurements do not matter for planted tanks, because plants can only use light radiation in a certain spectrum, from 400 to 700nm- which is measured in PPFD or Photosynthetic Photon Flux Density.
    This is what a PAR meter measures.
    So lumens dont mean anything, whereas PAR is the only valid measurement for plants. BTW, the human eye sees even less, so the lumen measurements you get when buying a bulb are BS.

    Hope this helps.
     
  4. aquabillpers

    aquabillpers Lifetime Charter Member
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    Yes, thank you. But a question remains.

    If one is comparing different bulb/reflector combinations to see which are best for aquatic plants, certainly PAR is the proper measurement.

    But if a bulb is known to produce most of its radiation in the 400 to 700 nanometer range, as do the so-called "sunlight" bulbs, then from a practical view wouldn't whatever radiation is measured by definition be roughly the same using either PAR or Lux?

    I agree that lumens is a poor measure of anything except of the amount of energy is emitted at a source. That's why the supposedly energy-efficient spiral screw-in bulbs that claim "33 watts same as 100 watts" is so misleading. If it wasn't supported by the government that claim would be prosecuted as consumer fraud.

    Thanks again.

    Bill
     
  5. ntino

    ntino Guru Class Expert

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    well, unfortunately, bulbs DO NOT emit all of their energy at the 400-700 range, and every bulb will have a different actual light output depending on the chemical composition and manufacturing process. The ballast is what is rated at the wattage - and the actual power consuption will also vary.
    Also, different bulbs will work differently when driven by different ballasts, and reflectors can make a huge difference in actual light hitting a certain point.

    You really need a meter to test to discuss most anything as far as light goes, otherwise you are just guessing.
     
  6. aquabillpers

    aquabillpers Lifetime Charter Member
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    Thank you, ntino.

    I believe that the kind of bulb that I used as an example does emit almost all of its energy in the 400 to 700 nm range. Below 400 the light is “ultraviolet”, of little if any value to aquatic plants and invisible to the naked eye; above 700 the light appears very red and soon becomes “infrared” which is also invisible.

    Here is a link to the spectral charts of two bulbs, a Phillips “daylight” bulb and a Grolux bulb. From the charts it is obvious that almost all of the radiation is in the 400 to 700 nm range. The Grolux is heavily weighted toward the red end – hence its pinkish color – but probably 98% of its radiation is still at less than 700 nm.

    http://www.aquaticplantcentral.com/forumapc/lighting/41510-lighting-spectrum-5.html

    It is certainly true that other variables can greatly affect the amount of energy that gets to the plants. I tried to phrase my question in such a way as to eliminate those variables and focus :) only on the bulb.

    What prompted my question was my interest in whether I needed to buy a PAR meter in order to get a feel for the effective light energy that my plants were getting in different sized aquariums with the same bulbs, or whether I could get by with a much less expensive Lux meter.

    I haven't seen anything yet that would lead me to believe that I need a PAR meter.

    Thanks again.

    Bill
     
  7. ntino

    ntino Guru Class Expert

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    Well, here is the thing as far as I know it -
    Marketing companies and charts lie - a lot. For example, they routinely advertise bulbs to be 15000k in color, when actual combined spectrum output could be 9000k, also, the charts are often off, sometimes by a lot. Also, certain waves reflect differently, and your lux measurements are going to be effected a lot by ambient light.

    If lux meters would work, every reef person would use them as high light is much more important in reefs.

    The bulb chart wont even say what ballast is used, which is a huge factor for both MH and T5.

    Google up PFDD measurments in reef tanks and you will find that different bulbs with the same wattage and their spectrum appearing within the 400-700 range produce very different numbers in the same fixtures.

    Just using an 80w bulb doesnt mean that you will get the same amount of PAR as using a different 80w bulb.

    All of this applies even more to MH light, where you can go from a 250w to 150w but using a different reflector you can have more light from the 150w going to the bottom of the tank while using the same bulbs 150/250w difference, but same brand and spectrum.


    In the end, light doesnt really matter all that much for planted tanks, as long as you dong have too much, but its really nice to be able to fine tune it, and make sure you have good spread throughout the tank - this is easier to do with an adjustable high fixture such as an MH pendant
     
  8. VaughnH

    VaughnH Lifetime Charter Member
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    For a different opinion: If you are using a Lux meter to help you decide how much light you will get on a 90 gallon 4 foot tank versus a 55 gallon 4 foot tank, both using the same brand of bulbs and fixtures, I think it would work fine. For example, you have a 55 gallon tank with a single tube 54 watt T5HO bulb fixture and have found that all plants grow at a good rate, including HC. Now you want to figure out what will happen with the higher and deeper 90 gallon tank with 2 or 3 of those same fixtures. You could use the lux meter to get a reading in the 55 gallon tank, then raise the fixture so it is the same distance from the substrate as it would be on a 90 gallon tank and make another reading. Then place a second one of those fixtures about as far from the first as you would have it on the 90 tank and measure again. You would have a good idea about whether you would expect the same results from the 90 with 3 of them as you had with the 55 with one.

    But, if you decide to save money and get cheaper bulbs for the 90 gallon tank, you wouldn't be likely to get much accuracy using the lux meter to guess how that would work.
     
  9. Tom Barr

    Tom Barr Founder
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    Are your light bulbs "hot"? Then they emit other wavelengths than 400-700nm.

    Lux meters can be used, but I really do not know the light used vs what reesearch uses since they rarely ever use Lux anymore.

    Advanced Aquarist's Online Magazine - Product Review: Lighting for Reef Aquaria: Tips on Taking Light Measurements

    There is a conversion factor for EACH type of bulb, so it's not like you can use a Lux meter for every type of bulb, they are different and you need to use the appropriate conversion for your bulb.

    This is nothing new, the issue is that we keep getting new types of lighting and bulbs all the time...........so rather than doing conversions over the entire scale, I just spend a few more $ and get the PAR meter. Then there's less question.

    Camera light meters, internal or hand hled use lux so they can be used for lux meters also. So if you are cheap........
    But PAR meters are much better for most situations and not that much costlier.
    Still, you can use a lux meter which is better than nothing.

    It's a trade off.
    But I read research based on PAR mostly,. so I stick with those units.


    Regards,
    Tom Barr
     
  10. aquabillpers

    aquabillpers Lifetime Charter Member
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    Thank you, Vaughn,

    That's what I plan to do, use the Lux meter to try to come close to the reading in a tank that works well.

    The same bulb in different reflectors would also yield a valid comparison. I would expect the reading to be higher in the more efficient reflector.

    I have a feeling that comparing similar bulbs of different brands, like the GE Chroma 50 "Sunlight" brand and the Phillips equivalent, with about the same Kelvins, would also yield usable results.

    Bill
     
  11. aquabillpers

    aquabillpers Lifetime Charter Member
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    Tom,

    If I were doing research or had a lot of tanks and bulb types, I have no doubt that PAR measurement is the way to go.

    But all I want to do is to grow plants in a low tech environment, with the same kinds of bulbs that I have used for a decade.

    BTW, I don't use "hot" bulbs and I'm not either, so it is a good match. :)

    Thanks.

    Bill
     
  12. ntino

    ntino Guru Class Expert

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    your bulbs dont get hot? those must be really good bulbs :)

    If all you want to do is a low tech tank, why bother? just use 0.8wpg of t5 with individual reflectors and you should be good, it wont matter what color bulbs you use.
     
  13. SuperColey1

    SuperColey1 Guru Class Expert

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    How dare you say 0.8WPG T5 is low light. That super highlight in mine!!!

    Just kidding ;) WPG rule can be used for low light basically stay under 1WPG of T5 and under 1.5WPG of T8 and you won't be far off. lol

    AC
     
  14. Tom Barr

    Tom Barr Founder
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    Well, if you go to the Lux thingy, you might as just as well use the Camera's f/stop.

    Likely would do okay, better than Watt/gal etc, but I cannot compare everyone's bulb and new bulbs are made all the time, this cuts through all that.

    200$ vs 80$ is not that big of a deal since there's no conversion factor and no drop off, decline etc, other possible issues over time with the Lux and conversion fatcor.

    But you can if you do the table and know the conversion factors for every bulb type, but that's a PITA and folks use different bulbs as time passes.
    What's good today will be fairly worthless tomorrow.

    Stick with a simple standard unit, then such info will be useful 20 years from now, regardless of what goes on with watts/efficacy of bulb types, reflectors, distances, heat, color temps etc, Brands that come and go.

    It's just more practical if you plan on spending time/money on the issue.
    I'd argue you do not "Need" either type of meter, but it can answer a lot of things about CO2/nutrients and other issues.

    So you could save yourself 80/200$ and argue for that too.

    Regards,
    Tom Barr
     
  15. Tom Barr

    Tom Barr Founder
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    Not according to the table in the above link I posted, the difference is quite large for similar kelvin values. I'd be nervous about making that assumption.

    Regards,
    Tom Barr
     
  16. SuperColey1

    SuperColey1 Guru Class Expert

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    All this talk about PAR but do we not measure PUR?

    AC
     
  17. ntino

    ntino Guru Class Expert

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    we use PAR, pur is usable radition, vs par which is avaliable radiation.
    We use PAR because different plants have different pigments and adapt to different things.
    So its PAR.
     
  18. Tom Barr

    Tom Barr Founder
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    The cost of a PUR meter is a lot, the PAR is cheap.

    If you know the PUR of the bulb type and assume it is the same over the life span, then you can convert it. But you need to know the bulb's rating there.

    Or measure each bulb type and make a big old list.
    I do not think there's that large a difference between conversions between PAr and PUR, there are large differences between the Lux and the PAR.

    I've never seen good evidence that this is a large difference between PUR and PAR and in any growth studies that filtered out certain wavelengths in aquatic plants.
    We can can see evidence between PAR and Lux issues in the table above however.

    Greener bulbs will weight more to the lux, and red/blue mixes will rate more to the PAR. PUR and PAR are much closer however.

    You can debate all 3 measures, but PAR seems to be the better unit given the trade offs. There is no definitive answer as much as we might like there to be:cool:

    Camera's light meter, Lux meter, PAR, spherical probe, PUR analysis etc.
    You can also measure the light indirectly for PAR vs PUR using O2 evolution from the plants.

    See if there's enough differences in O2 produced for different bulb types.
    All in all, it depends on what question you are asking and want to know.
    It also involves adaptation of plants and how they shade eachother, as well as CO2, plant biomass etc.

    So a few micro mols difference often does not make much difference in a tank.
    A few extra PUR is not that helpful, and likewise, if you are little off with Lux conversions, might not be a big deal, but if you get the conversion wrong, or do not know it for a given bulb, you can end up making large errors that are significant.

    So for the trade offs based on Cost, errors, and to compare with other folks, PAR meters are pretty good overall.




    Regards,
    Tom Barr



    Regards,
    Tom Barr
     
  19. Gerryd

    Gerryd Plant Guru Team
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    Just my 2 cents, but for all of the $ I have spent on this hobby over the last 10 years alone, the PAR meter was one of the better investments....

    I am sure some of us have spent the cost in test kits, excel, prime, etc alone LOL

    If you can beg or borrow one do yourself a favor and do so.

    Knowing your actual PAR values is extremely useful at least it was for me.

    Also the sooner we can use PAR as our comparison basis the better off the hobby will be............
     
  20. Tom Barr

    Tom Barr Founder
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    I've always gotten my $ worth from it to answer questions that folks tend to speculate, rather than critically measure and test, and to answer specific questions.

    CO2 is quite another matter.
    Still, I have not made or reversed engineered a couple of designs for gas measure of dissolved gases in water. Some are beyond my ability technically.

    Some are not.

    But I'd rather have a nice small simple, easy to use device that I could use that does not require recalculations, scaling, conversion factors etc.

    Light is a bit different, once you decide ona good bulb, then you have it for years.
    It's stable. However with light, I tend to try a number of different bulbs colors to get a nice look.

    I managed to find some nice 8800K bulbs and they really bring out the Green in the plants.

    Water is less yellow, they look better than the 10,000K and much better than the 6700K. I managed to find the 96w and the 65 W square pin CSL(no longer around, but the bulbs are new "old" stock).

    HQI have limited color temps, but still better than the 96w. I focus almost entirely on aesthetics, then I adjust the height and angles to dial in the PAR I want for intensity, they set duration for the PC's and the HQI's.

    Some folks might only have 1 bulb or maybe 2, so they are looking to get the most out of each watt for PUR. Not me however.

    Others might like a red plant bulb color, mixed with a blue white color, or 9235 GE's straight pine, or the T5 Gieseman(I like their colors and mix).

    Some bulbs are not available that you want, or you simply cannot get them where you live. So often times that's what you have to work with, and you have few choices, then a meter can come in pretty handy.

    Still, there are trade offs and arguments for camera's, light meter, A Lux meter and also a PAr meter as well as PUR, and you can take PUR a couple of more steps and measure actual in situ flourescence from different light sources and PAR, and alos measure total dry weights produced and pigment % content between light sources via chromography or plant metabolmics, yes...........I could keep going and ask one question farther down the line and never stop.

    So at some point you have to stop and say this is reasonable and we will accept this as a comparative unit. PAR's not bad really.

    But it's not perfect either.
    And that is where light debates and makers get folks on marketing.

    If I'm really interested in saving $ on electric and getting the most, I'd use LED's.
    Maybe later once the cost come down and they make them more commonly like they did with FL's, then PC's, then MH's, then HQI's, then T5's.

    It's only a matter of time.

    Then we can do some really cool color changes and manipulation with LEDs.
    I would like to change the color and scale the intensity up and down without having to buy another bulb or use a series of them etc.

    That will happen in the future, it has already, but the damn things cost way too much.

    Regards,
    Tom Barr
     
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