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Thread: Par and Lux

  1. #1

    Par and Lux

    As I learn more about aquarium lighting (thanks in good part to you kind people here) my questions become more basic and, possibly, dumber.

    I have learned that both PAR and Lux refer to the amount of light energy (photons) that strike a surface a distance from a light source.

    Both refer to the energy that is produced by light in the 400 to 700 nanometer range, that part of the spectrum that is visible to people and usable to plants.

    A key point is that neither distinguishes between the energy that is produced in the red and blue parts of the spectrum, the part of the spectrum that is useful to most aquatic plants, and that produced from other parts of the spectrum. PUR refers to the blue and red parts of the spectrum.

    Both PAR meters and Lux meters measure all of the energy that comes from the 400 to 700 nm range of the spectrum. It is possible that each could give show a high, supposedly plant-friendly reading if enough energy was produced in the yellow and green parts of the spectrum, which is not as valuable to plants.

    A PUR meter would be more useful in determining the plant friendliness of a light source, but they are very expensive.

    The main reason for using PAR and par meters instead of
    Lux and Lux meters is that the former is the measurement of choice in the scientific community.

    Please tell me if any of the above is incorrect or arguable.

    Thanks.

    Bill

  2. #2
    From Wikipedia: "The lux (symbol: lx) is the SI unit of illuminance and luminous emittance. It is used in photometry as a measure of the apparent intensity of light hitting or passing through a surface. It is analogous to the radiometric unit watts per square metre, but with the power at each wavelength weighted according to the luminosity function, a standardized model of human brightness perception."

    Note, that this is a weighted measurement, weighted to correspond with human eye perception.

    PAR is not weighted, but is a measurement of the total radiation in that 400-700 namometer range. There is no conversion factor between the two measurements because different sources of light have different spectra, and would be weighted differently as lux.
    Hoppy

  3. #3
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    I read all of the scientific info on this site and form little summaries in my head to make it simple for me. Reading the posts above has made we want to verify my summaries on PAR and Lux. And, maybe this will produce at least a partial answer for Bill. OK, here is my take:

    PAR meters measure micromoles (photosythetic flux, I believe), which is the light available that the plants can use in the photosynthetic process.

    Lux meters measure "luminosity" of the light source, some of which can be used in photosynthesis and some of which cannot.

    I'm guessing that even a lamp in the 400-700 nanometer range produces some light that is not usable in photosynthesis, which is why micromoles (PAR readings) are considered a better measurement.

    Is this a good summary?
    Last edited by tedr108; 02-06-2009 at 09:51 PM.
    Regards,
    Ted

  4. #4
    I think you're getting carried away here. Whilst Par, Lux, Pur etc is very interesting it is absolutely necessary.

    Sounds to me like you are trying to go down the route of certain K rated lights being better than others !!!

    Unless you are trying an experiment to control light to the absolute necessary amount I can't really see too much reason to get so involved in light to the extent of PAR etc. At the end of the day it is much easier and cheaper to watch how your plants react to the light and gauge from there.

    We aren't after exact figures with light so a rough area is OK after all Your 4500K with your reflector with your distance from the substrate may be better than me using the same tueb with my reflector with my distance etc.

    Far to many variables to get exact guides and therefore much better to aim for a range i.e. enough to grow well, but not excessive which with T8 means between 1W and 2.5W etc.

    AC

  5. #5
    Quote Originally Posted by VaughnH View Post
    From Wikipedia: "The lux (symbol: lx) is the SI unit of illuminance and luminous emittance. It is used in photometry as a measure of the apparent intensity of light hitting or passing through a surface. It is analogous to the radiometric unit watts per square metre, but with the power at each wavelength weighted according to the luminosity function, a standardized model of human brightness perception."

    Note, that this is a weighted measurement, weighted to correspond with human eye perception.

    PAR is not weighted, but is a measurement of the total radiation in that 400-700 namometer range. There is no conversion factor between the two measurements because different sources of light have different spectra, and would be weighted differently as lux.
    Thanks, Vaughn. Does". . .weighted to correspond with human eye perception . . ." mean that all wavelengths in the visible spectrum of between 400 nm and 700 nm are weighted the same, but any radiation outside of that range is ignored? Or, are different wavelengths assigned different weightings? (That doesn't seem practical to me, but . . . )

    The Lux readings of different bulbs can be converted to PAR, and vice versa, by applying a conversion factor that is based on the specific bulb.

    Apparently the unweighted PAR is measuring total energy. Is that further evidence that total energy is more important to plants than wavelength?

    Thanks, again.

    Bill

  6. #6
    Thanks, Ted.

    Quote Originally Posted by tedr108 View Post
    PAR meters measure micromoles (photosythetic flux, I believe), which is the light available that the plants can use in the photosynthetic process.
    But I believe that all light in the 400 to 700 nm range is usable by plants, although the reds and the blues are supposedly "better". Since PAR is unweighted, I am assuming that it refers to energy, and therefore enough of, say, green light would be just as useful to plants as red or blue light. The operative word there is "enough"; probably a lot more of it would be required (meaning a much higher watt fixture.

    Is this way off base?

    Bill

  7. #7
    Quote Originally Posted by SuperColey1 View Post
    I think you're getting carried away here. Whilst Par, Lux, Pur etc is very interesting it is absolutely necessary.

    Sounds to me like you are trying to go down the route of certain K rated lights being better than others !!!

    Unless you are trying an experiment to control light to the absolute necessary amount I can't really see too much reason to get so involved in light to the extent of PAR etc. At the end of the day it is much easier and cheaper to watch how your plants react to the light and gauge from there.

    We aren't after exact figures with light so a rough area is OK after all Your 4500K with your reflector with your distance from the substrate may be better than me using the same tueb with my reflector with my distance etc.

    Far to many variables to get exact guides and therefore much better to aim for a range i.e. enough to grow well, but not excessive which with T8 means between 1W and 2.5W etc.

    AC
    I agree that this technical light stuff is not absolutely necessary, but as you said, it can be interesting.

    I do have a practical reason, though. For years I have had one deeper aquarium in which I haven't been able to get good long term plant growth, at least compared to my other less deep tanks. As far as I can tell, all of the tanks have the same parameters except for light.

    At the present time I have at least 2.5 WPG of CF light on that tank. That is supposedly much too much for a non-CO2 tank, but there is little or no visible algae and the tank appear dimmer than the others. The plants grow but not at the rate of those in the other tanks. I want to see how much light is really getting to the plants. Measurement, and understanding what is being measured, will help me to eliminate that variable.

    Bill

  8. #8
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    Bill,

    I have the same exact problem with my deeper 29G non-CO2 tank -- not much algae, the plants grow, but not near as well. Something is just not quite right. Both of my 20G long non-CO2s have been so easy (great growth, no algae) that it's ridiculous -- the only difference is tank depth. Like you, I figured it's the lighting difference. The one factor I thought also might come into play on the lower tanks is that many more plants make it all the way to the surface. At that point, they get unlimited CO2, I'm guessing. Maybe that's why I have better luck with the lower non-CO2s.

    I've tried 3 different lighting setups on my 29G non-CO2. My most recent experiment (currently in progress) is more than 3wpg with an almost complete covering of floating plants! I got tired of trying the standard stuff. I have only anubias, java fern and vals below. I may end up pulling the vals because they need more light, everything else is fine. The tank is rather dark with all of the floating plants absorbing the light. Anyway, I'm just playing around and not taking my 29G too seriously at this point.
    Last edited by tedr108; 02-07-2009 at 12:26 AM.
    Regards,
    Ted

  9. #9
    My tank is a 33USG tall and I first went onto CF after upgrading from the stock lighting.

    wasn't impressed and changed to lower wattage T5HO linears a couple of months later.

    They were in there for the last 21 months with no problems

    I am not a fan of CFs to say the least

    AC

  10. #10
    Bill,
    How deep is this tank?

    One thing that can be done, borrow a PAR meter.
    I just do not even think in units other than PAR, I know what the tolerances of many species are based on PAR, not lux, or conversion factors(which add error) etc.

    The reason for this is that research uses PAR as well, so all the references for the bare min light are in such units as well.

    For our and most other plant growth purposes, comparing light no matter what source you might have Brand X, Y or Z...and they always come out with a "new" "better" bulb every 6 months(why is this??? Ask yourself that!), so knowing and converting to standard bulbs used is a losing battle.

    They will change as will the different types of systems, PC, T5, LED, HQI, various distances involved, reflectors, brands etc.

    These may influence Lux conversions, perhaps not in some cases.
    What we care about is more comparative.

    Will this amount of PAR be enough to produce a nice carpet no matter if we use ADA HQI's, Cheapo T5's, Super Coley's LED home made jalopy, my mix of PC and HQI, or more bank of different T5's?

    Then new stuff comes out, Pal in India has none of these brands, or the brands do not put out such data.....then what?

    Sure, not everyone has a meter, but as more do and they use and share them, we start having a much more comparative base.

    Light is generally the limiting factor for non CO2 systems, and it can toggle between CO2 as well. Most non CO2 plants are pretty good at dealing with low CO2.

    So early, they are light limited, maybe 1-3 hours in, some are CO2 limited or switch to KH, some are floating, no issue there, some have adapted well.

    Still, the more energy they need to put into light gathering and/or CO2 acquisition, the more trouble they will have making it and growing at depth.

    My bet is a switch to T5 would provide enough light.
    New bulbs might help a little, but not that much IME.

    As far as PUR, it too should be weighted, as the other spectral parts are in fact used, just not as much as say Red and Blue, and this varies plant to plant, algae to algae. As well as longer term adaptations.

    So even PUR should be weighted to some degree and you can get ver specific and go after the specific species of the plants if you wanted to be picky.

    The % of the PUR for one plant will be different than another species for example.
    Then find the min level of light intensity, which may change as more intensity is added and change the PUR for this species.

    There are many issues with trying to get at these questions, but you really start to get to the point of diminishing returns much beyond PAR for most horticultural issues.

    A way around it is to simply compare growth of a set of plants, same species under similar conditions and have a control, and then compare rates of growth.

    Then you can reduce the intensity down progressively over time until you get a growth rate of zero of slightly negative.

    This is indirect, but will tell you in practice what amount of PAR, PUR, Lux is required from that bulb for that species. So you do this for each bulb and for each plant species.

    There's 50 bulb types? 100?
    300-400 species of plants?

    Say you only care about 10 bulbs and say 40 species.
    That's a heck of a lot of work.
    We are going to get away with such trade offs by using a meter.

    It's just a gauge for comparisons(research, hobbyists etc).
    I list the type of light, the distance, the patterns measured, the plant species and general photo's when I measure.

    This way we have a better reference.
    40micmol is likely not a bare min, but for many species, this is fine for lower light CO2 enriched systems, non CO2 is okay here, but more plant species dependent.

    A nice HC lawn is not going to occur, but a nice Crypt lawn, or moneywort etc might.........

    Regards,
    Tom Barr

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