# T5HO Light Intensity



## Hoppy (Dec 24, 2005)

PAR meter data is beginning to be reported, in a usable form, especially on the Barr Report. Using the latest data from J.Downs, on that forum, plus data I already had from others, 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:









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:









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. 









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 have shows no significant difference between those.









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.


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## JamesQuall (Mar 13, 2009)

Great stuff! Bookmarked!


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## Hoppy (Dec 24, 2005)

You can make this data work for T5NO lights too. Just remember that T5NO lights give more total lumens per watt than T5HO. So, you are perfectly safe to assume that a T5NO bulb will produce an intensity equal to the ratio of the wattage times what the T5HO bulbs produce. This assumes you use the same single bulb, highly polished aluminum reflectors on both types of lights. For example a 3 foot T5HO bulb uses 39 watts, while a T5NO bulb uses only 21 watts, so it should give you at least 21/39 times the PAR you get with the T5HO bulb.


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## JDowns (Mar 6, 2008)

Wow. Great job putting this together Hoppy. Very well laid out and easy to understand. :thumbsup:

Hopefully this will help people in making a more informed decision on what lighting to purchase for a given tank, while also help the understanding on how light levels can change both vertically and horizontally along a scape.


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## Florida_Larry (Jan 19, 2009)

just how skewed would this data be for T8's? 

But great Info none the less, thanks Hoppy, JD


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## Hoppy (Dec 24, 2005)

Florida_Larry said:


> just how skewed would this data be for T8's?
> 
> But great Info none the less, thanks Hoppy, JD


This could easily be done for T8, T12, and PC (AHSupply) lights too. I have some data on AH Supply lights, but those bulbs lose quite a bit of their intensity over a year's use, so I'm not sure how typical my data is. I think it is basic to fluorescent bulb design that all T8 bulbs, for example, produce the same intensity directly below the bulb. And, higher wattage just means longer bulbs, but the same intensity below them. This breaks down when the bulb gets too short, probably because too much percentage of the bulb length is wasted on the ends, where the intensity is much less. I don't have any data to support this though. And, of course, overdriving a fluorescent bulb makes it produce more light than normally driven bulbs.

I'm still in the dark about MH/HQI bulbs.


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## defdac (Dec 28, 2003)

Unfortunately the glass+water helps reflect the light down on the bottom so the intensity is usually higher on the sides by the glass unless you have some kind of super focused reflector. Illustrated well with my movie I posted just minutes ago in this forum:
http://www.youtube.com/watch?v=z8qSy0WGZJ8

This is also illustrated when measuring the bottom of an empty tank and then by filling up the water. The intensity will rise to about double when the water is completely filled.

This makes calculating the PAR a bit easier than what is outlined here.

I've done a calculator doing precisely this and it shows that estimating the reflector efficiency will extremely crucial. I've estimated my gullwing Aquamedic reflectors to be at about 60% efficiency:
http://www.defblog.se/GTKTest/GTKTest.html

The calculator is being rewritten to account for kelvin change with depth and also illustrating the color change. Then it will also work with IE 8. I might deploy a beta of it soon, but I want some simplifications to the UI with an "Advanced" option.


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## julian_photo (Oct 6, 2008)

really it can be used for any light, the inverse square law will always apply to light in or out of the water. Now as pointed out the water it self will effect things a bit too.


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## defdac (Dec 28, 2003)

Ok, so you will still get the inverse square law only it will be offset or multiplied with some constant by the tank+water?


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## Hoppy (Dec 24, 2005)

I have measured PAR from a light over a tank, varying the height of the sensor in the tank, then varying the height of the light over the tank, with the sensor at the bottom of the tank and near the top of the tank. All of that data fall nicely on a straight line (on log-log paper) indicating that it is the distance from the light to the sensor that determines the intensity, not the amount of, or presence of water. It is certainly true that you can get reflected light from the front and back of the tank, and that does increase the intensity in the lowest part of the tank. But, that effect is dependent on how clean the glass is, and appears not to exceed about 25% or so increase. Also, the air/water interface tends to concentrate the light "beam" somewhat, very likely also increasing the intensity in the water. But, this, too, is a small effect compared to the distance from the light source.

Another small effect is the difference in the efficiency of various bulbs, and the age of the bulbs (for T5 bulbs). Those small effects add up, but, again they are not nearly as great as the effect of changing the distance between the light and the sensor.

You can decrease the huge increase in intensity as you move up in the water towards the light source if that source is evenly distributed over the top of the tank, as with multiple LED lights. That is because the closer you get to the lights the less the distance from the farther away LEDs changes - with a 24 inch wide array of LEDs, for example, when you are 6 inches from the plane of the LEDs, the outermost LEDs stay about 12 inches away, even as you move closer to the plane, so their contribution to the intensity doesn't increase.

This has started me to thinking that the ideal aquarium light might be a single super bright LED, pointed back at a parabolic reflector, which converts that light to a near parallel, nearly uniform, tank footprint sized beam. The ultimate DIY project.


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## plantbrain (Dec 15, 2003)

Hoppy,

Exotic sells those LED standard sized bulbs(24-36-48") bulbs that fit into regular FL fixtures.

Cost a bit, but could fit the bill there and look nice.

Regards, 
Tom Barr


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## defdac (Dec 28, 2003)

Agree with everything there Hoppy. Distance and reflector efficiency is where you get most bang for the buck.

I'm seeking a simple mathematical model. Would it be ok to say that most hobbyists more or less strive for spreading out all the intensity of the bulbs down in the tank instead of beside the tank on the floor?

In that case you can say that a percentage of the total radiance of the bulb (bulb lumens) will be reflected down in the tank more or less evenly distributed?

Is it then true that you could calculate the lumens spread out over the tank area with a certain percentage being the reflector efficiency? (getting the Lux over the surface)

If that is so could you then use the inverse square law to calculate the Lux on the bottom?


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## Hoppy (Dec 24, 2005)

defdac said:


> Agree with everything there Hoppy. Distance and reflector efficiency is where you get most bang for the buck.
> 
> I'm seeking a simple mathematical model. Would it be ok to say that most hobbyists more or less strive for spreading out all the intensity of the bulbs down in the tank instead of beside the tank on the floor?
> 
> ...


Yes, the goal is, or should be, to get all of the light from the source down into the tank, preferably evenly distributed over the water surface. And, any light that hits the floor isn't helping light the tank.

You can estimate what percentage of the light from the source actually enters the tank, for example, by measuring the area lit up around the perimeter of the tank. But, you can't assume that the light intensity is even close to the same all over that reflected light. In fact it won't be at all uniform unless you use a lot of effort designing the reflector to do that.

Yes, you could take the total lumen output of the source and the percent of that light that actually makes it into the tank, and from that say how many lumens you have entering the tank. But the intensity distribution is still unknown for real world lights.

No, I don't think you can calculate the lux at the substrate level, because it will vary widely over that surface, unless this is a specially designed reflector/bulb combination, that successfully evens out the distribution. What you could do is measure the lux at the water surface, then scale that to the substrate level using the ratio of the distances squared. That would work easily for the point directly under the fixture center, but calculating that for every point in the tank would likely not work well. Instead, it should give you a good estimate about how the intensity varies over the substrate, just not the actual numbers. Then, by plotting experimental data, it might be possible to use that combination of data and calculations to make a pretty good prediction about what a different light would give. I don't think it would be simple, but it might be.

It would be very interesting to see you attempt this. At this time I think it is hard enough to get a usable estimate for the light intensity directly under the fixture at the center of the tank vs. the distance to the fixture. Expanding that to include the distribution of intensity throughout the tank could be very helpful.


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## Hoppy (Dec 24, 2005)

plantbrain said:


> Hoppy,
> 
> Exotic sells those LED standard sized bulbs(24-36-48") bulbs that fit into regular FL fixtures.
> 
> ...


You could turn the bulb over so it shines up into the reflector, and possibly get a relatively uniform beam of light close to the size of the tank footprint. But, as I recall, those multi-LED, tubular "bulbs" don't yet use the high wattage LEDs that would be needed to get enough intensity for planted tanks. That is a certainty because the higher wattage LEDs have to have a good heatsink and a way to transfer the heat from that heat sink to the air, or they just burn out. And, the super output LEDs need the heat sink even more than the 1-3 watt LEDs do. I don't know how the maker of those 10 watt+ LEDs expects people to cool them sufficiently.


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## legomaniac89 (Mar 16, 2008)

Wow, very interesting. Great job Hoppy!

This needs to be a sticky.


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## plantbrain (Dec 15, 2003)

Hoppy said:


> Yes, the goal is, or should be, to get all of the light from the source down into the tank, preferably evenly distributed over the water surface. And, any light that hits the floor isn't helping light the tank.
> 
> You can estimate what percentage of the light from the source actually enters the tank, for example, by measuring the area lit up around the perimeter of the tank. But, you can't assume that the light intensity is even close to the same all over that reflected light. In fact it won't be at all uniform unless you use a lot of effort designing the reflector to do that.
> 
> ...


I think if you use the model that Defdac suggest and the real world data and tweak and adjust the model, you'd get fairly close.

You'd need more data, more types of and brands to compare for.
Factors like Time, bulbs ages, tank depth/height etc, reflector types etc.

But it could be done and tweaked to adjust the model to match the real world measures. It would take a fair amount, but might be worthwhile in looking at how light is spread evenly throughout a tank with the minimal wattage.

Seems that is a good goal to compare various brands, bulbs, reflectors, tank sizes, various fixtures against.



Regards, 
Tom Barr 

Regards


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## plantbrain (Dec 15, 2003)

Hoppy said:


> You could turn the bulb over so it shines up into the reflector, and possibly get a relatively uniform beam of light close to the size of the tank footprint. But, as I recall, those multi-LED, tubular "bulbs" don't yet use the high wattage LEDs that would be needed to get enough intensity for planted tanks. That is a certainty because the higher wattage LEDs have to have a good heatsink and a way to transfer the heat from that heat sink to the air, or they just burn out. And, the super output LEDs need the heat sink even more than the 1-3 watt LEDs do. I don't know how the maker of those 10 watt+ LEDs expects people to cool them sufficiently.


Well , take a look at the Exotic and see the corals they are growing under term.
the 900$ versions of the LED's make good use of what you are speaking of, but 900$..............

The T5 ATI fixtures also add a wrench into comparisons since the cool operating temp of the T5 bulb results(claimed) in a 50% increase, which is a huge amount.

So comparing those vs say a Tek vs a LED and the variation incantations........will need to be factored in and compared. 

Lots of different light set ups and they all are different, so with data, you can hit these various shops and get it, then enter it, then graph and compared.

Little by little, you/we can pick away at it and slowly get somewhere.

I'll take the meter down to Exotic sometime in the next few weeks and see.
ATI I'd love to measure, that's got my curiosity right now.

Defdac is interested in the PUR vs PAR issue, but if I can knock out 50% more PAR, that slight difference in PUR is peanuts, and I can measure the PAR easily. I think better T5s and LED's are on the way, as well a decrease in cost certainly with time. Good testing and methods will help when they do and to make better comparisons.

Be nice to have a good PUR meter, but not cheap.

I'm working with Neptune to get a CO2 probe for their AC3 pro controller.
This way it will be much cheaper to measure CO2 accurately and it'll have a nice chamber to do very accurate small volume batch sampling.

Add that with good PUR meter readings, nutrients are easy.........then you have excellent data logging and estimation of the parameter.

Regards, 
Tom Barr


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## defdac (Dec 28, 2003)

> What you could do is measure the lux at the water surface, then scale that to the substrate level using the ratio of the distances squared. That would work easily for the point directly under the fixture center, but calculating that for every point in the tank would likely not work well.


Great, that is what I want to do. I'm fairly uninterested in exactly how the light varies at surface level and more of an average intensity that you can use compare with others averages.

Getting into the more exact geometry of reflectors etc to be able to select a good reflector and how to place the bulbs over the tank is also very interesting but not something I've planned to do. That sounds like a very good application for raytracing with the bottom of the tank acting as viewport.


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## Hoppy (Dec 24, 2005)

defdac said:


> Great, that is what I want to do. I'm fairly uninterested in exactly how the light varies at surface level and more of an *average intensity* that you can use compare with others averages.
> 
> Getting into the more exact geometry of reflectors etc to be able to select a good reflector and how to place the bulbs over the tank is also very interesting but not something I've planned to do. That sounds like a very good application for raytracing with the bottom of the tank acting as viewport.


I have been assuming that the hardest plants to light well enough to let them grow well are the "carpet plants", like HC and glosso, so I have been trying to characterize light by the PAR at the substrate level. That means the light half way up the tank will always be close to "high light". Is it a better idea to concentrate on the PAR at the midpoint of the tank, where most plants spend much of their time? So, a low light level would be 50 micromols at the midpoint, which is much less at the substrate?

When it comes to how well the plants grow and how poorly the algae grow, is it better to pick an intensity by mid-tank value or substrate value? This is no trivial question - my tank has about 50 micromols at the substrate level, low light, but at the mid tank level it is about 100 micromols, definitely well above low light. But, I want low light.


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## defdac (Dec 28, 2003)

The only plants I've had been trouble growing because of light limitations is HC and P. helferi so to me the big question have always been what kind of intensity is on the substrate level.

I see your point though.


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