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Discussion Starter · #1 ·
Any light will light up the area under it with an intensity that varies approximately proportional to (1/distance squared). Over the past year I have collected data for a variety of T5HO light fixtures, with from 1 to 6 bulbs, from several manufacturers, but all with good highly polished reflectors. I used that data to make a graph of the approximate PAR intensity directly under the fixture at various distances from the fixture. For multiple bulb fixtures I just divided the measured intensities by the number of bulbs in the fixture. All of this data falls on a single line on a graph - with lots of scatter, of course. When we need to select a light fixture for a tank it makes little difference whether the fixture gives a PAR intensity of 50 or 70, as long as we know it isn't 20 or 100. So, it is possible to use such a graph to pick an appropriate fixture for any tank.

Always use a bulb that closely matches the tank length, with the bulb no more than 4-6 inches shorter than the tank. Use the height of the tank as the distance from the fixture to the substrate - this assumes the substrate height is about the same as the height of the bulbs above the bottom lip of the fixture. From the chart you can see how much intensity a single bulb will give directly below the bulb. If the tank is more than 12 inches in front to back depth you will want to use two fixtures, or a fixture with two bulbs separated by 6 inches or so, to get reasonably uniform light from the front of the tank to the back, without having too much intensity on the area between the bulbs. If two single bulbs wont give enough intensity, use 2 bulb fixtures doubling the number the graph gives as the intensity. If this gives too much intensity, plan on hanging the fixture far enough above the tank to increase the distance between the bulb and substrate enough to lower the intensity to what you want. Here is the chart:



I also have data from my own AHS light fixtures, for both 36 watt and 55 watt bulbs. That let me graph those numbers on the same page, and fit an approximate line to match those numbers. As you can see, an AHS PC fixture gives less light per bulb than a T5HO light, but not by much.

This method will not tell you exactly how much light you have or will have from any one light fixture, but it will allow you to make a good choice of fixtures to get approximately the lighting you want. And, it is far better than trying to use a watts per gallon standard.
 

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Either I am way off base, or that information is really misleading.

It implies, for example, that a 3' T5HO bulb (39w) in a good reflector has higher PAR than a 3' PC bulb (96w) in an AHS reflector(!).
A 2 bulb T5HO fixture on a 16" tank is NOT enough light for a reef. In fact, I know of no one running reefs with less than 4x-T5HO, and many use 6 or 8 bulb units, save on shallow nanos.
A 1 bulb T5HO fixture on a 16" tank is NOT "high light planted." Have you ever successfully ran a “medium" or "high" light” aquarium with one T5HO bulb? I've never heard or read of anyone doing so, and won't believe it until I see it. Two starts to seem plausible, but only in the upper reaches of the tank.

For multiple bulb fixtures I just divided the measured intensities by the number of bulbs in the fixture.
I think this is the source of your error. Multiple bulbs don't originate at a point source, they are spread over 4"-12" in a fixture. This means that dividing the PAR value by the number of bulbs for a single bulb reading will give a higher PAR value than one bulb gives in reality.

Just out of curiosity, which bulbs & what counts as a good reflector?

For the sake of discussion, here are some measurements I took with an Apogee QMSS-E last year:
Standard AGA 75g Aquarium.
Catalina Aquariums T5 Fixture w/Workhorse Ballasts.
2x54w 6700K and 2x 10000K stock Catalina bulbs, approx 4 months old.
Bottom of the bulb plane is 2” off of surface with legs.
Surface: 480
21”: 80

Standard AGA 30L.
AHS PC reflector.
96w AHS 6700K bulb
Surface: 350
16”: 55
 

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Discussion Starter · #4 ·
Either I am way off base, or that information is really misleading.

It implies, for example, that a 3' T5HO bulb (39w) in a good reflector has higher PAR than a 3' PC bulb (96w) in an AHS reflector(!).
A 2 bulb T5HO fixture on a 16" tank is NOT enough light for a reef. In fact, I know of no one running reefs with less than 4x-T5HO, and many use 6 or 8 bulb units, save on shallow nanos.
A 1 bulb T5HO fixture on a 16" tank is NOT "high light planted." Have you ever successfully ran a “medium" or "high" light” aquarium with one T5HO bulb? I've never heard or read of anyone doing so, and won't believe it until I see it. Two starts to seem plausible, but only in the upper reaches of the tank.

I think this is the source of your error. Multiple bulbs don't originate at a point source, they are spread over 4"-12" in a fixture. This means that dividing the PAR value by the number of bulbs for a single bulb reading will give a higher PAR value than one bulb gives in reality.

Just out of curiosity, which bulbs & what counts as a good reflector?

For the sake of discussion, here are some measurements I took with an Apogee QMSS-E last year:
Standard AGA 75g Aquarium.
Catalina Aquariums T5 Fixture w/Workhorse Ballasts.
2x54w 6700K and 2x 10000K stock Catalina bulbs, approx 4 months old.
Bottom of the bulb plane is 2” off of surface with legs.
Surface: 480
21”: 80

Standard AGA 30L.
AHS PC reflector.
96w AHS 6700K bulb
Surface: 350
16”: 55
A 3 foot long T5HO bulb with the typical great reflector does produce more PAR than a 3 foot long AH Supply light. All PC bulbs with the same reflector produce the same intensity directly under them, just as all T5HO bulbs do. This doesn't mean they produce exactly the same PAR, but within reasonable scatter they produce the same. You have to measure your specific fixtures output to get exact numbers. Different bulbs, different reflectors, different bulb ages, different cleanliness of the reflector, etc. all have an effect on PAR to some degree.

I concluded that multiple bulbs give give the same multiple of PAR as one bulb from the data I have. Again, it isn't exact, but it is close enough for making decisions about which fixtures to use. And, it is a rare fixture that doesn't place the bulbs as close together as they can be. DIY fixtures are whatever we want them to be, but when you buy a manufacturers fixture, the bulbs will be crowded together. Also, once you get closer than 16 inches or so from that fixture the relationship does break down, but more than 4 bulb fixtures are not used on planted tanks that are 16 inches deep, unless someone is looking for algae instead of plants.

I hope someone will attempt to massage the available data and come up with a better way to predict what different fixtures will do on top of various tanks. Until they do, this is my effort to do so.
 

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Discussion Starter · #5 ·
so using this data, how do i figure out how high i should hang my 2x24w t5ho fixture for low levels? (sorry , i couldn't figure it out. math was never my subject.)
The bulb wattage isn't important. All T5HO bulbs produce approximately the same intensity at a given distance. So, with two bulbs you just look at the chart for 1/2 of what you want. So, if you want 40 micromols of PAR, you look for 20 micromols on the chart, which would put the bulbs at about 34-35 inches (off the chart). If your tank height is such that the fixture sits with the bulbs 22 inches above the substrate, you would want to raise the fixture 12-13 inches to get 40 micromols at the substrate. The advantage of hanging the fixture above an open tank is that you can then adjust that height until you get the results you want. The chart just tells you that you will have much too much light compared to what you want, if you place it on top of the tank.

Raising the fixture also allows the light to spread out and give more uniformity of intensity at the substrate from front to back, as well as side to side. And, the bonus is that the intensity will not be nearly as high at the top of the water as it would be with the fixture sitting on the tank.
 

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thanks hoppy. it will be a nice test to see how this works out. i just moved my fixture up to about a foot off the tank, 30" above the substrate. hahaha - my living room is a bit brighter now.

Edit - woop's... i read that wrong. my fixture is now 34" above the substrate.
 

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Discussion Starter · #8 ·
T5NO bulbs have been reported to produce light more efficiently, in lumens per watt, than T5HO bulbs do. But, T5NO bulbs use a bit more than half the wattage that T5HO bulbs use. So, a usable approximation is to assume that 2 T5NO bulbs will produce a bit more light than one T5HO bulb, assuming equal quality reflectors are used. Since the reflectors for T5NO are probably not as good as for T5HO, a good guess is that 2 T5NO bulbs produce the same intensity as one T5HO bulb. Until someone takes some good PAR measurements and reports them I think this is about as good an estimate as you can get.
 

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Discussion Starter · #10 ·
Hoppy- does this hold true regardless of the color temp. of the bulb? Same for 6700K as 10000K , colormax, etc.?
I don't know for sure, but the bulbs I tested, which were 6700, 10,000 and 9325K gave approximately the same results. I'm sure there are differences with color temperature, as well as with manufacturers, and with ballasts, but I don't believe those differences are very significant. A lot more testing would be needed to find out what the differences are.
 

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Hoppy, If I'm understanding your posted information correctly would 4xT5HO @ 25" from bulb surface to substrate equate to 160 micromols on your PAR chart?
 

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So if I have 2 T5NO bulbs using a single reflector, would you recommend using the 'T5H0 Fixture - 1 Bulb' line on the graph as a reference?

Assuming that is the case, then I should hang the fixture about 11" above water surface to keep from having algae issues at the surface? And that would then leave me at about 35 micromols at the substrate?

So my tank would run the spectrum from very high light to low light. Based on the proportional drop off, if I want moderate light throughout, I should hang a lot of T5HO light well above the tank?
 

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Discussion Starter · #13 ·
Hoppy, If I'm understanding your posted information correctly would 4xT5HO @ 25" from bulb surface to substrate equate to 160 micromols on your PAR chart?
Assuming the bulbs are fairly close together, a couple of inches or so, yes that is what I would expect to get.

So if I have 2 T5NO bulbs using a single reflector, would you recommend using the 'T5H0 Fixture - 1 Bulb' line on the graph as a reference?

Assuming that is the case, then I should hang the fixture about 11" above water surface to keep from having algae issues at the surface? And that would then leave me at about 35 micromols at the substrate?

So my tank would run the spectrum from very high light to low light. Based on the proportional drop off, if I want moderate light throughout, I should hang a lot of T5HO light well above the tank?
Yes, I just use the one bulb of T5HO as the approximate light intensity from a 2 bulb T5NO fixture. But, when you get as close as about 10 inches the chart becomes less accurate, for various reasons. All tanks that don't have the light hanging high above the top of the tank have much higher light at the water surface than at the substrate. You can easily see this by watching how much faster plants grow as they approach the water surface. Also, any filter or other hardware that is near the water surface is hard to keep algae free, due to the high light intensity there. That is one of the good reasons for hanging a light well above the top of the tank.
 

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Discussion Starter · #15 ·
would it be too much trouble to ask you for the original numbers (the averages that you used to create the graph)? I would love an opportunity to play around with them.

Thanks,

P.S. Thanks for doing all of this.
It took me awhile to find the original numbers, but I finally did. I got them from JDowns on The Barr Report, and they are in an Excel Spreadsheet. PM me your email address and I will send you the spreadsheet. This was several months ago, and since then I have added a few more data points from others, and I used PC data from my own lights, plus, as I recall, some data from a couple of other sources. Lots of data massaging went into this.
 

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Yay, Hoppy!

Even with the inherent approximations and/or errors in assumption... this is still probably much better than the WPG approximation for those using T5HO. And I agree that plus or minus 20 micromoles is not going to be a deal breaker except if you get near the low end and are targeting 30 micromoles for some reason.

I personally think that the divisions in your chart between low/med/high PAR values are shifted a little low. But it doesn't matter so much because the associations of actual plant growth characteristics to PAR values isn't widely available (studied?). The kinds of correlations that I sought out in the beginning were, for example: in what PAR range will glosso grow flat and compact, in what PAR range will ludwigia repens turn red, in what PAR range does riccia thrive attached to rocks at the bottom of the tank, and so on. I'm sure light alone does not affect all these factors, but it is a predominant force. And knowing these kinds of relationships is one of the strengths of working with PAR values. An aquascape can really be fine tuned knowing these things.

I appreciate the work you've put into assembling this data. I'm sure this will help a lot of people who use (or want to use) T5HO.

Cheers!
 

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Discussion Starter · #17 ·
Yay, Hoppy!

Even with the inherent approximations and/or errors in assumption... this is still probably much better than the WPG approximation for those using T5HO. And I agree that plus or minus 20micromoles is not going to be a deal breaker except if you get near the low end and are targeting 30micromoles for some reason.

I personally think that the divisions in your chart between low/med/high PAR values are shifted a little low. But it doesn't matter so much because the associations of actual plant growth characteristics to PAR values isn't widely available (studied?). The kinds of correlations that I sought out in the beginning were, for example: in what PAR range will glosso grow flat and compact, in what PAR range will ludwigia repens turn red, in what PAR range does riccia thrive attached to rocks at the bottom of the tank, and so on. I'm sure light alone does not affect all these factors, but it is a predominant force. And knowing these kinds of relationships is one of the strengths of working with PAR values. An aquascape can really be fine tuned knowing these things.

I appreciate the work you've put into assembling this data. I'm sure this will help a lot of people who use (or want to use) T5HO.

Cheers!
Thank you! I really appreciate that.
 

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Hoppy,

I was doing some spot checking to verify the values in your proposed T5HO PAR chart. I measured the following:

2x24W T5HO; 22" long bulbs / 24" wide tank, GLO brand fixture
Height 14 inches, from bulbs to substrate

With a meter, I measured a max of 85 micromoles at the bottom of the tank with this configuration. However, using the chart and multiplying by 2 (for the two bulbs), the chart would have indicated 190 micromoles which is double the actual value.

Has anyone else tried using the chart and comparing with actual meter readings?
 

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Discussion Starter · #20 ·
Hoppy,

I was doing some spot checking to verify the values in your proposed T5HO PAR chart. I measured the following:

2x24W T5HO; 22" long bulbs / 24" wide tank, GLO brand fixture
Height 14 inches, from bulbs to substrate

With a meter, I measured a max of 85 micromoles at the bottom of the tank with this configuration. However, using the chart and multiplying by 2 (for the two bulbs), the chart would have indicated 190 micromoles which is double the actual value.

Has anyone else tried using the chart and comparing with actual meter readings?
You got about twice what I expected. The data I used gives 100 micromols at 13 inches with two 24 watt T5HO bulbs, but an Archaea fixture. And that data point was way off from others, so I assumed that the reflector in the Archaea fixture wasn't a good one. Did you use single bulb fixtures, or a single fixture with two bulbs. If it was single bulb fixtures, the greater distance between the bulbs would reduce the amount of light. Or, if it was a two bulb fixture the reflector may not be as effective as typical single bulb reflectors are.
 
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