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Massaged PAR Data for T5HO lights

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

  1. jeremy v

    jeremy v Guru Class Expert

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    Vaughn,

    Thanks for doing all of this, it is nice to find information and talk about stuff that can lead to something real and meaningful in this hobby.

    I have a few comments on your findings and formulas. I believe that the formula you are using to determine the light intensity at distance X from a source of light is only valid for something like a light bulb sitting in the middle of an otherwise dark room. That formula would tell you what the light intensity would be per unit area on a piece of paper perpendicular to (aka tangent to the surface of) the light bulb at varying distances from the bulb. Yes the light intensity would drop very fast since the spherical area the light is trying to cover increases dramatically with distance.

    The reflector that many of us have on our aquarium lights drastically changes the rate of light reduction over distance in ways different from the formula you gave, and the better the reflector is at getting all the light produced into the tank the farther off your equation will be. It will be a big difference for a well designed reflector. A reflector focuses the light in a narrower beam that points down instead of just letting the light spread out in all directions.

    I think of it being like a laser versus a light bulb. You can have a 1 watt light bulb sitting in the desert in the middle of the night and you might be able to see the light coming from it for a few hundred yards and that's about the best you will get, but a high quality one watt laser could still put a dot of bright light on your hand at that distance. They are both only one watt and they could both even have the exact same number of total lumens of intensity, the only difference between the two sources of light is that one is allowed to disperse evenly out in all directions and the other is focused and kept from spreading to the greatest extent possible. When light is prevented from dipersing it can travel very long distances with little reduction in intensity.

    I would think that a reflector (like an Icecap SLR for instance) would fall pretty much right in between a laser and a lonely light bulb. That is a well designed aquarium reflector and it has only a very slow increase in the dispersion of the light beam as the light is raised.

    With that being said, I think that your equation only works if people have t5ho lights without reflectors of any type on top of their tanks. Then the light will disperse like a light bulb would in a dark room and lose intensity rapidly. That is assuming of course that you also ignore the way that light refracts and becomes more vertical when it shines down into an aquarium and hits the air/water interface. That refraction will alter your equation as well, because assuming that the water was infinitely clear (and there were no losses due to floating particles in the water blocking the path of light), the refraction just due to the change in media density will help to focus more light in the downward direction and cause more light to hit the aquarium substrate than would end up hitting the substrate of the tank if the infinitely clear water were not there at all and the light only traveled through air. That factor alone would lessen the drop in lighting intensity over distance to some extent versus your formula.

    Have a good one,
    Jeremy
     
  2. jonny_ftm

    jonny_ftm Guru Class Expert

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    Hi,

    This raises another question I was getting to ask: those measurements to calibrate T5HO bulb were made with a reflector or without? Indeed, if the last graph is based on a non reflector measurement, than things can go drastically far from the theorical data with the new optimized T5 reflectors.

    In anycase, as VaughnH said, these data should be only considered to give a different thought on lights, away from the watt/gallon rule and to accomodate more tank depths. It can help for an estimation, but, with plants growth and aquascapes, things can varry even more than with a reflector

    I personally find VaughnH data great as a new way to look at our T5 fixtures and better understand the impact of fixture height above water. Before he publishes his data, I took the decision to put my luminaire from 11 to 14in heigher based on my plants aspect and a very confined spot of recurrent spyrogyra algae on the most exposed part of the tank. I also was driven by the fact that I changed most my stems to crypts and swords. In 1 week the results were evident and no more of this annoying stuff coming. Now, with VaughnH data, I know I can even put them heigher

    In the end, when I begun, I never thought I'd be able to observe plants behavior in my tank and adapt their specific place to the amount of light they could receive, shade them if needed or expose them. It takes time, but with constant observation and tries, you'll find that plants aspect and growth is the best indicator of where you should place the plant and how you could adapt your light fxture or even aquascape
     
  3. VaughnH

    VaughnH Lifetime Charter Member
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    I struggled with trying to understand this too. My data is from typical T5HO lights, with the typical reflectors that those fixtures use. I didn't expect to see a drop off of intensity that followed the inverse square rule, but much more nearly a linear drop off. But, there are no focused reflectors possible with bulbs of a size that isn't extremely small compared to the reflector dimension. Focusing only works for sources located at the reflector focal point, not focal area, but focal point. With a linear bulb, that would require a T.001 bulb, or some equally tiny diameter bulb. A 5/8 inch diameter bulb in even a 10 inch wide reflector cannot be located at the focal point of the reflector. So, in the real world, all reflected beams are divergent. And, all divergent beams lose intensity by an inverse square rule. If the intensity is "A" at 10 inches from the source, it will be A/4 at 20 inches from the source.

    This is true, and it means that the intensity at the 10 inch distance may be 10A instead of A, but again, at 20 inches it will be 10A/4, so the inverse square rule still applies.

    What bothered me too, is that you can convince yourself that for a very long tubular source the intensity must drop in a linear way, because the source is effectively infinite at the close distances we use. But, then I realized that for every infinitestimal element of the bulb, the loss of intensity is still proportional to the square of the distance, and it is only when you are very close to the bulb that the distance increases at a rate slower than the distance directly below the bulb. This effect is why the inverse square relationship breaks down for small distances from the bulb, but still holds after you get 10 inches or so from the bulb.
    Before I started this exercise, your analysis is exactly what mine was. But, the data convinced me that I was wrong. And, I found this same inverse square relationship for PC bulbs too - always with a reflector.
     
  4. Gilles

    Gilles Lifetime Members
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    Hey Hoppy, does this mean that it does not "mather" if you add reflectors or not?
     
  5. SuperColey1

    SuperColey1 Guru Class Expert

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    Of course not ;) What we are saying is that the reflecter 'recaptures' light that was directed away from the tank but they do not act as a 'focus'.

    Its just a redirection of light. Its pretty impossible (probably is 'possible' but not for the majority of us) to focus the light with reflectors on a fluorescent because of the nature of its design. being a long tup it would need to be stood vertically rather than horizontally to use a full wrap aroudn reflector to get it to focus. Even then it wouldn't be too efficient. Focused lights tend to be sources where the reflector can be behind the whole light source and then all around it too. like a cone sort of thing. I guess in theory a CF could be hung from its end cap with a very large cone around it but why? It misses the point of this research which is to gain spread rather than bright spots and dead spots. We are trying to spread the light rather than focus it :)

    Not to use a reflector means that if you take a tube and then 2 rulers. rest each ruler on the tube and the tank's edge and you have a very crude judgement of how much of the tubes light is going into the tank. Not scientific but a rough guide as it assumes all the light is directing outward. probably about 30% of the tube diameter will be between these rulers (this crude test falls apart as you raise the tube because the section of dismeter reduces)

    Add the reflector and it recaptures a lot of the light that was outside of this 30%. I would suggest more than the 'double output' statements the reflector manufacturers state even with the simple single bend ones.

    So you use 100% electricity and without reflectors get (example) 30% usable and with reflectors (example) 70%. :)

    The high power LEDs are virtually 100% because they normally have a reflector fitted behind the actual die and all the light faces downward anyway. their collimators and lenses then gather all this light and push it into a focused beam. Again not what we are after. To uselenses and collimators and at the same time have a good even spread means you need loads of LEDs in close proximity like the ones made for reefers and even they have been proven too much for corals. That then means we are adding silly light again which is kind of what we are trying to move away from.

    We are trying to get good spread, good coverage and at the same time use less light to do so than has been thought 'needed' for so long.

    All this is purely the amount of light from the source remember. What Vaughan and the others are measuring is light received at the target :)

    AC
     
  6. Tom Barr

    Tom Barr Founder
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    I'll have some data from ADA's HQI 150 W bulb using a different fixture, then an E ballast vs a magnetic and coralife 10,000K and 8000K watergrass bulb to compare soon.

    Both bulb and fixture data needs taken, not just the brand of fixture.
    As well as HQI, T8 and so on. Then combine the data into and large table, that gives the lighting data.

    Start with what you have, your group, your LFS's that will let you measure thigns etc, folks that will let you borrow a meter etc.

    Vaughn, you might consider a picture of the light sampling rod that you made, posted here. I have the meter but it went out to do the ADA and HQI measurements.

    This will give an idea how to DIY an easy to make lighting rod to move the probe around in the tank easily. we have one at the lab made from PVC pipe and simply strapped the Apogee to the LiCOR to compare the light reading differences.


    Regards,
    Tom Barr
     
  7. VaughnH

    VaughnH Lifetime Charter Member
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    I concur with what AC said:D
     
  8. SuperColey1

    SuperColey1 Guru Class Expert

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    Oooh Tom gets dangerous using abbreviations sometimes. lol.

    Those both could be called 'E' ballasts as a Magnetic ballast should really be called an Electric.

    Gets confusing when one is electric and one electronic. lol

    AC
     
  9. VaughnH

    VaughnH Lifetime Charter Member
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    I forgot to take any photos of that sensor holder, but I might remember the details enough to make a sketch of it. It is a collection of various acrylic pieces I had on hand.
     
  10. JDowns

    JDowns Lifetime Charter Member
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    This is the holder I made out of an old Kent Scraper. I used 1/8" plastic I had left over as the insert into the scraper, drill a hole and mount. I also heated the elbow and increased the bend. A couple velcro wraps and ready to measure.

    [​IMG]
     
  11. VaughnH

    VaughnH Lifetime Charter Member
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    Here is a sketch, as best I remember, of the sensor mounting wand that I made. None of the dimensions is critical and were just what the scraps I had available were.
    [​IMG]
     
  12. jonny_ftm

    jonny_ftm Guru Class Expert

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    Hi,

    Are you planning to make a table for the PC lights, U shaped, 9-11-13W for example? I find nano light completely different from the bigger aquariums, usually much dimmer
     
  13. VaughnH

    VaughnH Lifetime Charter Member
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    I would like to make some kind of table for PC lights, but first I need some good PAR data, which I don't have. The only PC light I now use is a 36 watt AHS light. Also, short fluorescent tubes are another group I have no data for. And, that includes PC versions.
     
  14. Tom Barr

    Tom Barr Founder
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    We might want to make tables for each light type, PC, HQI's, T5's, T8's and T12's.
    LED's etc.

    But the need for data is high!

    Regards ,
    Tom Barr
     
  15. jonny_ftm

    jonny_ftm Guru Class Expert

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    I have another question:

    On Apogee Quantum site, they provide conversion tables from Lux to PAR and inversely. Are those tables really of any interest? Since Lux meters are much more affordable
     
  16. VaughnH

    VaughnH Lifetime Charter Member
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    Lux involves adjusting the intensity at various wave lengths to match the human eye sensitivity - low sensitivity for red, high for blue. PAR just uses the intensity as is, but limits it to the wave lengths that plants respond to. With that, I can't see how there can be a conversion factor for lux to PAR. You can use such a conversion factor to get somewhere in the right ballpark for what lighting is needed, knowing the lux rating for a source. But, that's about all.
     
  17. jonny_ftm

    jonny_ftm Guru Class Expert

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    Thanks for having cleared it
     
  18. meatus

    meatus Junior Poster

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    This thread is perfect timing for me. I'm going to start making a new stand for my standard 75 gal. I'm going to hang the lights and was curious what your guy's recommendations are for height and placement of the lights in the hood. I have 2 of the A&H Supply 2X55W T5 kits so a total of 220 watts. Should I use all 4 bulbs? With 2 ballasts I'm able to set each kit on a separate timer if needed or even be flexible with using 3 bulbs or whatever will be best.

    I'm going to make my own pendant/hood to hang so now I can make it so it will work best for my setup. I don't have access to a PAR meter right now (hopefully will be one of my purchases in the next little bit) but was hoping one of you guys could make a good suggestion.

    Thanks so much for everyones help on this site. This is my first post but I've pretty much had most of my questions answered already just by searching around here.
     
  19. Tom Barr

    Tom Barr Founder
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    My old 75 Gal had 330W of A&H supply PC lights, I only needed 220W however and used the outer banks only.

    It was still a very high light tank even with 8" above the water's surface.

    [​IMG]

    [​IMG]

    This tank has about 1/2 the light:

    [​IMG]

    Same size almost

    Regards,
    Tom Barr
     
    Nikolyator likes this.
  20. meatus

    meatus Junior Poster

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    So then staying with the 220W T5's should I have it higher than 8" to go along with the lower light and better spread? Around 10" - 12" or more?
     
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