# PAR Data - Selecting a T5HO Light



## Hoppy

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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## macclellan

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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## oldpunk78

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.)


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## Hoppy

macclellan said:


> 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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## Hoppy

oldpunk78 said:


> 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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## oldpunk78

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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## oldpunk78

if i'm using a single T5NO bulb/ballast with a high quality reflector, is there a way to use this chart to predict the light output?


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## Hoppy

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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## comatoast

Hoppy said:


> The bulb wattage isn't important. All T5HO bulbs produce approximately the same intensity at a given distance.


Hoppy- does this hold true regardless of the color temp. of the bulb? Same for 6700K as 10000K , colormax, etc.?


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## Hoppy

comatoast said:


> 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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## wkndracer

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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## DMtankd

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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## Hoppy

wkndracer said:


> 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.



DMtankd said:


> 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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## Reginald2

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.


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## Hoppy

Reginald2 said:


> 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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## i4x4nMore

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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## Hoppy

i4x4nMore said:


> 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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## DMtankd

+1 on the kudos Hoppy. This is by far the most useful and quantitative thread I've come across in dealing with lighting. Thank you very much for putting it together.


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## i4x4nMore

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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## Hoppy

i4x4nMore said:


> 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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## iliketogolf

Thanks for the information - it seems very helpful.

If my goal is to have a low tech tank (i.e. no CO2 pumped in) what would you recommend for a maximum PAR reading?


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## Captured Moments

Am I correct in assuming that these measurements are taken in the presence of water as typically in an aquarium? Wouldn't water due to it's density and of the surface reflectivity reduce these par values by a significant margin?


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## Hoppy

Captured Moments said:


> Am I correct in assuming that these measurements are taken in the presence of water as typically in an aquarium? Wouldn't water due to it's density and of the surface reflectivity reduce these par values by a significant margin?


All of the experimenting I have done with a PAR meter shows that the water has almost no effect on the light intensity. It takes a lot more water before the absorption and scattering of the light is significant. The surface reflectivity is somewhat cancelled by the focusing you get when the light goes from air to water.



iliketogolf said:


> Thanks for the information - it seems very helpful.
> 
> If my goal is to have a low tech tank (i.e. no CO2 pumped in) what would you recommend for a maximum PAR reading?


I think a PAR reading around 40-50 would be acceptable without CO2. That might be too high.


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## fishsandwitch

Gene, try starting your own thread for more responses
There is a button above the list of threads


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## gene4christ

fishsandwitch said:


> Gene, try starting your own thread for more responses
> There is a button above the list of threads


thanks for the info I moved it to its own spot sorry about that did not mean to hijack the thread guys . God Bless
Gene4Christ


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## Prime

*Starting a planted tank*

Hi
I'm starting a planted tank soon and was wondering what light would be the most appropriate for a tank of 1.6m(L)x0.70m(W)x0.50m(H)? I will most probably stagger the lights (if more than 1 is required) and have them overlap in the middle of the fixture or make some other plan to fit multiple lights. The light/s should be able to provide enough energy so that Glossostigma elatinoides will thrive. I planning to hang the light fixture from the ceiling and have it on a pulley system to be able to move the lights up and down. The tank will then obviously be open. 
What would you guys recommend?


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## GlassCat594

this should be made a sticky.


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## Hoppy

Prime said:


> Hi
> I'm starting a planted tank soon and was wondering what light would be the most appropriate for a tank of 1.6m(L)x0.70m(W)x0.50m(H)? I will most probably stagger the lights (if more than 1 is required) and have them overlap in the middle of the fixture or make some other plan to fit multiple lights. The light/s should be able to provide enough energy so that Glossostigma elatinoides will thrive. I planning to hang the light fixture from the ceiling and have it on a pulley system to be able to move the lights up and down. The tank will then obviously be open.
> What would you guys recommend?


The length of the tank, 63 inches, suggests using 80 watt T5HO bulbs. One such bulb, with a Tek quality single bulb reflector, should give you medium light intensity, if sitting right on the tank. A two bulb fixture should give you about 120 micromols of PAR, very high light, but if hung about 4 inches above the tank, should give a more reasonable 90 micromols (approximately). The 28 inch front to back depth means you really need two rows of bulbs, separated by about 12 inches, to get reasonably uniform lighting at the substrate.

So, I think I would use a pair of 2 bulb T5 fixtures (with a pair of 80 watt bulbs in each), separated by about 30 cm, and use the pulley system to adjust the light intensity until it is what you want.


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## S&KGray

GlassCat594 said:


> this should be made a sticky.


+1 I agree
:thumbsup:


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## debdp

Thanks for this. But I wish I could understand it a bit better. I have 2x24W T5HO, 28" above the substrate for a 18" deep tank. Reflectors removed. Where on the chart would that fall? The goal here is no CO2. Thanks.


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## Hoppy

debdp said:


> Thanks for this. But I wish I could understand it a bit better. I have 2x24W T5HO, 28" above the substrate for a 18" deep tank. Reflectors removed. Where on the chart would that fall? The goal here is no CO2. Thanks.


The chart is for T5HO with individual reflectors on each bulb. And, it gives the approximate PAR that you should get with one bulb, at that distance from the light. If you use T5HO bulbs with no reflectors, I can only guess that you have somewhere around 1/3 of the amount of PAR the chart shows, and with two bulbs you would get about twice that, so you could expect about 2/3 of what the chart shows for one bulb, or about 20 micromols, for your example. This is an extreme extrapolation of the data, and is little more than a guess. But, I really doubt that it is possible that you have as much as 50 micromols, or as little as 10 micromols.


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## Hoppy

The chart shows about what you should get from one T5HO bulb with a reflector comparble to Tek reflectors, at that distance from the bulb. With 2 bulbs you would expect twice what the chart shows, unless the bulbs are several inches apart. With no reflectors on those bulbs I can only guess that you would get somewhere around 1/3 the light shown for each bulb. So, for your case, you could expect around 20 micromols, and I doubt that you could possibly have as much as 50 micomols or less than 10 micromols. This is little more than a rough guess, though.


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## debdp

Thanks Hoppy. At least it's a reasonable guesstimate of keeping my tank in the low light area... and if it's too low I can simply lower the lights. Great help considering the WPG really can't apply with so many different variables with types of lights and reflectors.

Thanks again!


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## fishyjoe24

how light will 2 - 96w pc corallife sunpaq 6,700k produce on a 55g 21 inches high.


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## Hoppy

The chart shows AH Supply PC light kit results, which are similar to Coralife lights, but with much better reflectors. About all I can do for Coralife lights is guess, knowing they will produce less light than AH Supply lights do. We really need someone to test their Coralife lights with a PAR meter to get more data.

So, my guess is that you will have medium light with two Coralife Sunpaq lights on a 55 gallon, 21 inch high tank. It is very unlikely that you will have either low or high light.


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## zavikan

*intensity? Plus another question that requires a graph.*

Looking at what Tom Barr said, ADA suggests FAR less lighting then previously thought. Maxing at around 50 on your scale.

Using this approach, my 38 gal would take a 36" t5HO which is approx 39watts.

Meaning, to copy ADA, I would place a single 39watt over my tank at 22" off the surface. or to translate, approx 1WPG :eek5: (speaking of which, what kind of photoperiod do they use?)


I think another good test would be what the curve is for losing light intensity as you go from directly under the bulb, to the edges of the tank. of course I suppose that would all change depending on how high the light is....


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## Hoppy

As you move away from directly under the bulb the distance from the bulb increases with the distance away from under the bulb. The equation is X^2 = H^2 + Y^2 where X is the distance from the bulb, and Y is the distance away from under the bulb. The intensity drops off roughly proportional to one divided by X^2. As the angle between the light beam and the water surface goes from 90 degrees to a smaller angle, more light reflects off the water surface, further reducing the intensity. But, that loss is partly made up by light striking the inside of the front/back glass and reflecting back to the substrate. You can ignore those last two things, and just assume the light drops off per the first equation. What this means is for a tank with a front to back dimension that is less than the height of the tank, or for lights hanging several inches above the top of the tank, you don't lose much light that way.


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## zavikan

Interesting then. I am going to suggest something that most would find not logical, but by Hoppys figures (and Tom Barrs ADA comparisons) seem to be supported.



43"L X 18"W X 24"T = 80 Gal tank.

Aiming for Approx 60 PAR spread fairly evenly across the large 43"x18" foot print, I could use a 2x39w 36" t5HO fixture at 30" off the substrate. (.9 WPG lol. WPG is such crap).

By being 30" off the substrate, the fact that the bulbs intensity has to spread over 18"W shouldnt be a problem, yes? (considering that a single light source can do 12"W tanks just fine at MUCH less then 30")

Having a slim single light source over tanks that large would be a MAJOR plus for the 'look' of the tank. 


This is not theoretical exactly... Point being, if this is a supportable arguement, I'm planning on doing exactly this.


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## Hoppy

Zavikan, I think that arrangement would work well, giving you medium light intensity. There are some advantages to having the bulbs further apart, primarily to keep tall plants from shading low plants quite as much. And, as always, hanging the light makes adjusting the intensity pretty easy.


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## reybie

Sorry for necroposting but I just happened on this thread. I'd like to say thanks Hoppy for the great info, I can't wait to get home and check my numbers against the chart.

Edit: hah, just saw the more updated thread after posting here... will continue my post over on that side.


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## barbarossa4122

Thanks a lot for your work/research Hoppy.

According to this chart I have 60 micromols of PAR on my 29g and 70 micromols of PAR on my 55g which means medium light.


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## barbarossa4122

Now, since yesterday I am almost 30" from substrate on both tanks, medium low lighting, I guess.


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## Bilbo

Not to nit pick, but...
I know this is old and no one is probably reading, but the 1/r^2 only applies to point light sources not to "bar" sources. In the middle of the bar, the straight down drop is more like 1/r and tappers off at the edges but is never 1/r^2
If the data presented is interpolated via this equation, the results are off.


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## Hoppy

I used to believe as you do, but measuring the intensity versus distance for many different bulbs has proven to me that the inverse square relationship works very well until you get too close to the bulb. The easy way to check for such a relationship is to plot intensity vs distance on log-log graph paper. If the data follow a straight line that has a downward slope of 2, it is an inverse square relationship. The data I get doesn't follow it exactly, but it is close enough for what we need. For typical 2-4 foot long bulbs, once you get around 10 inches from them, the relationship breaks down and gets closer to a slope of -1. But, at 16 inches and more, it is inverse square.


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