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Massaged PAR Data for T5HO lights

Discussion in 'Lighting' started by VaughnH, Jul 19, 2009.

  1. VaughnH

    VaughnH Lifetime Charter Member
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    The PAR meter data from J.Downs, plus data I already had from others, is finally enough that I think it is now possible to predict the light intensity you will get for any size aquarium for any size T5HO fixtures.

    To start with, light intensity drops off with the square of the distance you move from the source. This illustration shows why that is true:
    [​IMG]

    Using that proportionality between light intensity and distance from the source, you can plot data from a variety of T5HO fixtures, and, from that determine how the intensity varies at various places on the substrate, and in the water column. For example:
    [​IMG]

    This shows why the intensity near both front and rear walls of the tank is less than in the middle, and by about how much. Plus it shows why the intensity drops at the two ends of the tank, even though the bulb is the same length as the tank.

    For years we looked for "watts per gallon" as a measure of the needed amount of light. That is a useless criteria, unless you consider only tanks of standard proportions, as well as with the same type of light fixture. If you take any light fixture and suspend it above a very large tank of water, that fixture will give about the same intensity directly under the bulb as when it is over a tank only 12 inches front to back.
    [​IMG]

    From that it should be clear that it is only the height of the tank, the distance from the light to the substrate, that determines how intense the light is directly under the bulb. And, the intensity at the front and back of the tank is determined by the front to back dimension of the tank. The bulb length has no effect on intensity directly under the bulb, at least for 24 inch to 60 inch bulbs, other than helping to maintain the intensity near the ends of the tank.

    So, it is possible to use the PAR data now available to determine with reasonable accuracy how much light you will get at the substrate level, for any height tank. This data is for T5HO lights of a quality comparable to Tek or Catalina fixtures. The data I saw showed no significant difference between those.
    [​IMG]

    If one T5HO bulb, or row of bulbs, doesn't give you the intensity you want, you can use a second bulb, fairly close beside the first one. And, if you want better uniformity of the intensity over the front to back span of the tank, you can use two bulbs, separated by about 1/3 of the tanks depth.

    Once you pick a light to get the intensity you want at the substrate, you are stuck with whatever intensity you get in other parts of the tank, except that you can raise the lights higher above the tank, which reduces the intensity at the substrate, and more importantly, reduces the extreme difference in intensity between the water surface and the substrate. So, the ideal light setup for most tanks we would use for planted tanks, would be 2 rows of T5HO bulbs, separated by about 1/3 of the tank depth, and raised enough to reduce the resultant intensity at the substrate to the level you want.

    Cross posted on The Planted Tank Forum.
     
  2. SuperColey1

    SuperColey1 Guru Class Expert

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    superb info Vaughan. Nice to see

    AC
     
  3. dutchy

    dutchy Plant Guru Team
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    Great info. Finally an outcome based on research.
     
  4. jonny_ftm

    jonny_ftm Guru Class Expert

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    Thank you VaughnH for those info,

    I have some questions/clarifications though.

    If I get it right, the last graph you provide is to help us have the PAR measurement of a T5HO light for our specific substrate to luminaire height. And based on the PAR plot in your graph, we can calculate the PAR for any point in the tank. If we have "y" bulbs, we just should multiply the first reading in your graph by the number of bulbs, "y"

    I still didn't get the formula for the calculation well. Is it as this:

    [​IMG]

    Where:

    A and B are the points as shown in your figures
    Intensity A: is read directly in PAR on your last graph using the vertical height "H"
    H: is the vertical distance from light source, also used to read Intensity A in PAR from your graph
    X: is the distance from point A to B, as shown in your figure

    Am I getting it right?

    A concrete example would be:

    I have a 2x54W T5 bulbs at 30in height from substrate (H = 30in). My tank is 14in large from front to back glass.

    The intensity of light at the substrate (Intensity A) under my lights would be read in your graph: approx 2x30 PAR = 60 PAR

    At the front and back glass (Intensity B), that is 14in /2 = 7in (since the lights are centered on the tank width) ---> X = 7in

    It gives me Intensity B at front/back glass is: 60 * [Sqr30 / (Sqr30 + Sqr7)] = 40 PAR

    Am I getting you right?


    If yes, than your data is really a gift for us
     

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  5. Philosophos

    Philosophos Lifetime Charter Member
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    Vaughn, you've done a great job once again on creating good visualizations for lighting principles. While I understand simple formulas, this is something much easier to look at, and something much easier to use to explain to other people. I also haven't seen some of the application of inverse square that you've added in.

    I have bookmarked this thread and I'll probably be posting the odd link to it, as I do some of your other posts.

    -Philosophos
     
  6. VaughnH

    VaughnH Lifetime Charter Member
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    No, the equation is:
    [​IMG]

    So, your intensity at the front/back glass is 60*[900/949] = 57
    But, this is not an exact correlation, so for all practical purposes, the intensity at the glass is 60, and it may be as much as 75 due to reflected light off the glass. (assuming the glass is clean both inside and outside)
     
  7. Philosophos

    Philosophos Lifetime Charter Member
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    If I had to add anything in terms of critique here, it would be relfectivity. This gets mention in favored books, and people like their argumentum ad verecundiums; these books don't even need good sources. Your contributions give something aspiring to peer review; perhaps in order of a text book given how little most of us understand.

    How much is lost from obscure angles from reflection of still water? What's the base formula so we can all play with geometric designs, and approximate light loss? If negligible, then we could all stop caring; that'd be nice.

    Thanks again for your contributions.

    -Philosophos
     
  8. Gilles

    Gilles Lifetime Members
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    But what if i have a canopy which is (for instance) 60cm in depth, i have 5x T5 HO lights:
    1) at 10cm
    2) at 20cm
    3) at 30cm
    4) at 40cm
    5) at 50cm

    This would mean that i have 5cm space around the lights, except off course the front and back light which are 10cm from the edge. Given this information; how can your information help me? (sorry that i don't understand, i am from the Netherlands and have not used a light meter before).
     
  9. jonny_ftm

    jonny_ftm Guru Class Expert

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    Many thanks VaughnH for clarifying it.

    Now, what those micromoles do mean in term of light intensity?

    I remember Tom saying that 40 is supposed to be low light, yet ok to grow most plants. Is 60 too much? Could you provide some ranges of micromoles for:

    - low light/low tech
    - ideal minimum to grow almost any plant, yet keeping light low for better CO2 /plant growth control
    - high light
     
  10. Tom Barr

    Tom Barr Founder
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    I think once we get enough data from various brands of lights, and types of bulbs, types of lights(T5, HQI, PC, T8 etc), we can build a good model to predict the light in a certain sized aquarium.

    It is also useful for comparing brands and their cost vs what they actually produce in terms of lighting and how it's distributed.

    This seems where most hobbyists want to head with PAR data and light meters, basically to revise the watt/gal rule. This is not my personal intent, but I am supportive of folks doing this.

    My view is more looking at individual species and their tolerances under controlled conditions with and without CO2 enrichment etc. What are some minimal ranges for specific species etc, how canopies form and affect the light, shading and scaping issues with light etc.

    There are many ways to go, but for new folks, I think the light calculator will be useful.





    regards,
    Tom Barr
     
  11. VaughnH

    VaughnH Lifetime Charter Member
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    Your aquarium is 24 inches in depth - front to back. That is a big enough dimension that you would need at least two, probably 3 lights spaced evenly across the top, to get uniform light intensity at the substrate. But, the length of the bulbs (wattage) you would need depends on the length of the tank and the height of the tank. It depends on the length because if the bulbs are significantly shorter than the tank, the ends of the tank will not get enough light. It depends on the height of the tank because the intensity that each bulb contributes depends on how far away from the bulb you want that measurement.

    Lets assume your tank is 24 inches high (60 cm). If so, each bulb, if located right at the top of the tank would give you about 40 micromols per square meter per second of PAR. You have 5 bulbs, so you might have as much as 200 micromols of PAR at the substrate at the center of the tank, which is very high light. If that isn't acceptable to you, you could turn off 2 of the 5 fixtures or bulbs, and have about 120 micromols maximum of PAR, which is much more reasonable as high light intensity. Or, you could just raise the lights higher above the substrate, to about 35 inches from the substrate, about 10-11 inches above the top of the tank, and have about 20 micromols per bulb, 100 micromols max total intensity at the substrate in the middle of the tank.

    That is how the data, charts, etc. can help you to decide where to start - how many of what bulbs to use, and how high to put them above the tank.

    A complication not really discussed yet is where in the aquarium should we arrange for a given PAR intensity to be. For example, if we say 50 micromols is our criteria for low light, does that mean at the substrate level, which means the middle of the tank has a lot more intensity than that, or at the middle of the tank, which means the substrate has a lot less than that. I don't know the answer to that. Tom very likely has a well reasoned opinion about it. I started assuming that it is the light at the substrate that is most critical, since the hardest to grow plants seem to be the carpeting plants, like HC and glosso, so it is there that we would want to be sure not to have too little light.
     
  12. Tom Barr

    Tom Barr Founder
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    I'd pick the sediment for the minimal lighting intensity.
    Easy choice since it would represent the lowest intensity, the min for that tank's size, bulb no#, dimension etc and plant choice.

    Low light might be 25-40, 40-60 for med light and 60-80 for high/med, 80-120 micromol for high light.


    Regards,
    Tom Barr
     
  13. dutchy

    dutchy Plant Guru Team
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    I'm confused. If I apply this to my own tank (32 x 18 x 25) 52 gallon of water, I should use 2 tubes of 29 inch (35W each) which would give me around 80 mmol of light at the substrate, which is medium to high light. But if I apply to Watt per gallon rule it's 2 x 35Watt = 70 Watt or 1,35 Wpg, which is low light.

    At the moment I'm using 4 tubes with a total of 118 Watt. 2 x 24W (OEM) and 2 x 35W, which are 3 inch above the water surface. Am I overdoing it? Better use just the 2 x 35 Watt tubes?
    I'm not using any floating plants.
     
  14. JDowns

    JDowns Lifetime Charter Member
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    I think the benefit as we get more data is that people will be able to make a more educated decision on purchasing a fixture. And the understanding of how much light approximately they are getting. We can get much much closer then "wpg", IMO the "wpg" rule is completely broken.

    Hopefully we also start to get a much better understanding of how raising and lowering a fixture can adjust PAR levels, and how raising and lowering effects overall PAR both vertically and horizontally.

    We can also get away from these ridiculous recommendations for plants at 3wpg and more. If we have reliable information on a species growing well at "x" umols, then we can apply that and rule out light being inadequate.
     
  15. jonny_ftm

    jonny_ftm Guru Class Expert

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    Also remember that those data are for T5 HO and not T8, so could be much more micromoles than with classic T8
     
  16. dutchy

    dutchy Plant Guru Team
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    I have T5, so it's applicable to my setup and it seems like two bulbs would be sufficient.

    But....how much light (mmols) would the sun give at any point of day at a certain depth and how is this comparable with what we measure?
     
  17. Gilles

    Gilles Lifetime Members
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    Great! That is data i can use! But what about this one;

    My new tank will be: 180cm width by 70cm depth by 60cm height. I am planning on lighting this tank with 80w T5, which are 149cm in width. Since they are 30cm short, i am planning on placing them (from back to front) to the right side of the canopy, next to the left and so on, so cross-wise.

    Since there are more people here who calculate with inches:
    Width: 70.86"
    Depth: 27.56"
    Height: 23.62"

    Lights will be: 57.04" without caps, 60" with caps

    So i am missing 10"; part of this is solved by the sides of the tank which are (since it is plywood) 0.74" meaning i have 1.5" less to worry about. Since i am going to make my own background with epoxy, this adds another 3.15". All in all, this means i have to cross-place each light 6.5" from the other light. I hope you know what i mean.

    How many lights would i have to use for this tank? I am guessing 5x80w but maybe 4x80w will do...
     
  18. jonny_ftm

    jonny_ftm Guru Class Expert

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    Much more than any of your bulbs. But there's no point in achieving sunlight intensity. You'll never be able to catch up with good CO2 levels under those conditions
     
  19. dutchy

    dutchy Plant Guru Team
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    Im done trying to find out what's right or not. I'm going to buy a PAR meter.
     
  20. Tom Barr

    Tom Barr Founder
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    :p

    Gilles:
    Based purely on experience alone on that size of aquarium, 4x 80W should be adequate and about the same what I have for my 180cm 60x60 aquarium. I have the 4x 96W but they are PC lights and they are 30cm away from the water's surface.

    For those interested in the light meter, I can see if we can get a discounted group buy from Apogee. They ran about 200$ each with the probe for the sunlight and electric light source with a 1 meter waterproof probe.

    Regards,
    Tom Barr
     
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