# Effect of Water on PAR



## Hoppy (Dec 24, 2005)

I have been doing lots of measuring of PAR from various lights, under various conditions, but almost always with no water in the tank. This has caused some disagreement, because of my contention that the water has no effect on the PAR unless you have far deeper water than we ever use. Today, my curiosity got to me, so I decided to do some more testing to see what effect water really does have.








I used my 10 gallon tank, just cleaned, inside and out, and my adaptation of Home Depot work lights for a light, with the light mounted 19 inches above the sensor at the bottom of the tank. First I measured PAR at various spots on the bottom of an empty tank, then repeated most of the measurements with about 5 inches of water in the tank. I found that water has only two effects on the PAR readings: first it tends to focus the light a bit, so the center of the tank PAR readings go up slightly. Second, the reflection of the light off the sides of the tank, when there is water in it, increases the PAR readings near a glass side or end considerably, enough to almost even out the intensity distribution across he tank.

The graphs below show my readings:
First, the readings along the center of the tank from end to end:









Then, the readings at the center of the tank from front to back:









This is with the tank about half full of water. I'm not sure if the effect would be greater or less with the tank full, but I don't dare completely fill the tank, since I removed the rim. I may decide to try it about 3/4 full, and if I do I'll report on that too.

This data is in pretty good agreement with the data on http://www.plantedtank.net/forums/lighting/85667-par-data-spiral-power-saver-bulbs-5.html#post904062 which is pretty encouraging too.


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## Lil' Swimz$ (Jul 24, 2010)

What does PAR mean?


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## Hoppy (Dec 24, 2005)

Lil' Swimz$ said:


> What does PAR mean?


PAR is *P*hotosynthetically *A*ctive *R*adiation, the total radiant energy that reaches a specific area and is within the spectrum that plants respond to. It is analogous to Lux, which is light intensity weighted to match human eye sensitivity.


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## Lil' Swimz$ (Jul 24, 2010)

Oh okay, thanks for clearing that up Hoppy.


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## macclellan (Dec 22, 2006)

That's interesting, but doesn't do much to address the disagreements (some coming from me  ). It would be nice to know the effects, if any, in more typical situations: i.e.:
water 16"-24" deep, not 5"
actual aquarium water with surface movement (with fish pee, ferts, food, other DOCs, tannins, etc.) not fresh tap water - I know my tank is visibly darker after about 10 days, I think this is from the aquasoil.
substrate, especially darker substrates, not what's effectively another reflector on the bottom of the tank.

I might give this a shot when I borrow the local club's meter again.


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## Hoppy (Dec 24, 2005)

The dilemma with light measurements is that the intensity varies all over the tank. It varies with distance from the light, a major effect, with shading from other plants, also a major effect, with water contamination, with glass cleanliness, with age of the bulbs, with cleanliness of the reflector, etc. So, it will never be possible to use a single number to specify exactly what the intensity is in an aquarium. The best we can do is to specify the intensity at the substrate level, at the center of the tank, with no shading, with reasonably new bulbs, with a typical reflector, clean tank glass, and clean water. For any specific aquascaped aquarium you would need to use a PAR meter to check the range of intensities at various locations in that particular tank with that particular light fixture. But, that doesn't help anyone when they are looking for the "right" fixture for their tank. For that we have to use a specific location in an empty tank - with water, but nothing else. Just painting the back glass has a significant negative effect on the light intensity near the back. With all of those uncertainties, the variation caused by having water in the tank is trivial. (My opinion.)


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## macclellan (Dec 22, 2006)

It's true that there are many factors affecting light, and water in the tank may be trivial in comparison, but that's different from saying it has no effect. 

Honestly, I don't think PAR data helps much in deciding what T5 fixture to get for a particular tank. The proof is in the pudding: look at successful tanks similar to what you want to achieve, see what they are using, and get what they have. In most cases, there isn't much of a real choice - 2 bulb fixtures for low light, 4 bulb fixture for high light. 

PAR meters are useful for other things: deciding when to get new bulbs, corroborating what we already know about what amount of light leads to a successful tank, simply for the sake of knowing, etc.


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## Hoppy (Dec 24, 2005)

More knowledge is always helpful. And, at a time when so many people are hooked on watts per gallon, or on using really bight lighting, thinking it has to be the best thing to use, knowledge is all that will keep them in the hobby. Otherwise after several months of having a tank full of algae, or non-growing plants, they will just give up and "grow" plastic plants. I have been there.

Look at the number of posts from people with 40 breeder tanks and 4 tube T5HO lights, to get the "right" watts per gallon, then disappointed because nothing seems to work right.


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## houseofcards (Mar 21, 2009)

macclellan said:


> That's interesting, but doesn't do much to address the disagreements (some coming from me  ). It would be nice to know the effects, if any, in more typical situations: i.e.:
> water 16"-24" deep, not 5"
> actual aquarium water with surface movement (with fish pee, ferts, food, other DOCs, tannins, etc.) not fresh tap water - I know my tank is visibly darker after about 10 days, I think this is from the aquasoil.
> substrate, especially darker substrates, not what's effectively another reflector on the bottom of the tank.
> ...


+1

That's the problem with these 'experiments' they very rarely take into account the 'real world' variables that everyone has in their tanks not to mention tall tanks that really require a completely different lighting scheme to grow plants well. Many tanks based on certain parameters do require high light and the aquarist must work the tank through religious maintenance, low stock, good consisent co2 to be successful. There is too much emphasis on 'if I reduce light, I'll reduce my problems' I'm not saying that's not true many times, but it's not that black and white.


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## Hoppy (Dec 24, 2005)

houseofcards said:


> +1
> 
> That's the problem with these 'experiments' they very rarely take into account the 'real world' variables that everyone has in their tanks not to mention tall tanks that really require a completely different lighting scheme to grow plants well. Many tanks based on certain parameters do require high light and the aquarist must work the tank through religious maintenance, low stock, good consisent co2 to be successful. There is too much emphasis on 'if I reduce light, I'll reduce my problems' I'm not saying that's not true many times, but it's not that black and white.


I have never accepted that there are problems that don't have solutions, where only some mysterious unknown forces control what happens. Light is a very basic subject with pretty simple physics behind it. If we can find a method for arriving at a good light setup for any tank it does a great service to others entering this hobby, or having difficulties even after months in the hobby.

That's why I enjoy working with light. A hobby that is built around "watts per gallon" demonstrates a massive need for more knowledge.


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## houseofcards (Mar 21, 2009)

Hoppy said:


> .. If we can find a method for arriving at a good light setup for any tank it does a great service to others entering this hobby, or having difficulties even after months in the hobby...


Not saying it's not a worthy endeavor, but the above is tough to do since so many factors go into whether a particular light will work for one's tank.


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## RipariumGuy (Aug 6, 2009)

I think most of the reason people are hooked on the WPG rule, is that it is simple and easy to work with. With Par and what you are working on Hoppy, (Great stuff!) there is no set rules. So people go for the simpler and easier approach. Just my thought.

Is there any set Par levels which separate high, medium, and low light levels? Or is that just a bunch of nonsense?


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## csmith (Apr 22, 2010)

JakeJ,

http://www.plantedtank.net/forums/lighting/105774-par-vs-distance-t5-t12-pc.html

Should help.


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## hbosman (Oct 5, 2006)

JakeJ said:


> I think most of the reason people are hooked on the WPG rule, is that it is simple and easy to work with. With Par and what you are working on Hoppy, (Great stuff!) there is no set rules. So people go for the simpler and easier approach. Just my thought.
> 
> Is there any set Par levels which separate high, medium, and low light levels? Or is that just a bunch of nonsense?


I do believe eventually if, enough people do what Hoppy is doing, we can generalize on PAR numbers that would equate to low, medium and high. Just like WPG generalized on low, medium and high in the T12 era, PAR numbers could do it in the T5HO era if, enough people document there findings. We need more people that do have access to PAR meters to document their findings so we can get to that point.

That being said, a full tank of water would have been nicer. :hihi:


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## malaybiswas (Nov 2, 2008)

Agree with what everybody has mentioned about the usefulness of the data for more practical purposes like choosing the right light for a tank. However simple experiments are necessary to gather some data before you dive into complex ones, so what Hoppy is doing is probably a baby step right now but in a right direction. 

Also, no bit of information is really "useless" to everybody. Somebody can make sense of it so whatever information can be gathered with whatsoever limited capacity we have will help us at least in future if not now.


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## Hoppy (Dec 24, 2005)

I'm a little bit surprised that everyone but me already knew that you get brighter light at the bottom of a tank with water in it, than in an empty tank. And, while I knew that light reflects off the outer surface of glass, at the glass-air interface, back into the tank, I didn't realize that everyone else knew that it was enough reflection to have a significant effect on the uniformity of the light at the substrate. I'm glad I got up to speed with the rest of you.

Yes, that is snarky. Sorry about that.:hihi: But, I will continue to document what I learn, so I can refer back to it by searching if I forget this stuff at some time.

I have a 15 high tank, also de-rimmed, that I can get much more water depth in, with more effort. I'm thinking of trying that today sometime. And, all of this isn't even what I was trying to learn. I picked up the PAR meter to measure the effect of using ordinary window screening to reduce light intensity. That is still to come.


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## plantbrain (Dec 15, 2003)

houseofcards said:


> Not saying it's not a worthy endeavor, but the above is tough to do since so many factors go into whether a particular light will work for one's tank.


Well, you have to start somewhere if you want to test a model system. Hoppy is using an ideal system which is also something folks can duplicate and mimic as standard. 

If you had to pick or chose, what would you use?

1. I think a clean glass, good reflector, new bulb, empty tank is a good starting point. *You need some reference to compare things to*.

2. From here, we can go back and measure real world tanks using the meter, and compare the effects from the ideal model.

-How much difference does tannins make? How do we measure those tannins in concentration?
-How much difference does glass vs lexan make? 
-How much difference does Brand X reflector vs Mylar or Brand Y make in PAR?
-How much difference is there with PAR evenness on the bottom of the tank when water is present vs not?
-How does clean glass vs algae fur coat glass affect PAr at different depths?
-How does age affect most brands of bulbs with respect to PAR vs time?
-How does ballast type/brand affect PAR?
-How does the combo of brand X and the reflector fair compared to brand Y?

I think a PAR meter can really answer many things if you use a reference, whether it is a real world reference such as a nice top scape or an ideal system like a bare tank and new equipment. I also think it makes a good reference to compare our real world aquariums to and adjust accordingly.

Hoppy's data can illustrate many of the "hows" in our lighting system and where we should focus and could improve upon. It is obvious that these are not real planted tanks, but it does not negate the reference point. They are ideal maximal PAR for those test. 

The other end might be a reference tank with real plants and nice scape, but has dirty bulbs and old reflectors with film etc. This might lead us to think those lights are ineffective, but if clean etc, they might be as good as say another brand/bulb type. So we might reach an incorrect conclusion about them without a good reference point.

Aquarist are good at not using references, true for light, for ppm's of any number of nutrients, including CO2 more pervasively.

We start simple and then build on the complexity from there.
But.......we should start somewhere with fewer interactions that can confound things.

Same deal with testing Ferts, you have to have good stable independent CO2/light before those can be tested critically, likewise, for testing light and CO2, nutrients must be independent as well. Testing all three at once is a lot harder than only one thing at a time.

Are all aquariums independent nutrient wise?
No.

Same deal for CO2, and also..........light.

Regards, 
Tom Barr


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## plantbrain (Dec 15, 2003)

Hoppy said:


> I'm a little bit surprised that everyone but me already knew that you get brighter light at the bottom of a tank with water in it, than in an empty tank.


I didn't.



> And, while I knew that light reflects off the outer surface of glass, at the glass-air interface, back into the tank, I didn't realize that everyone else knew that it was enough reflection to have a significant effect on the uniformity of the light at the substrate. I'm glad I got up to speed with the rest of you.


I didn't either, glad I'm now up to speed also:tongue:



> I have a 15 high tank, also de-rimmed, that I can get much more water depth in, with more effort. I'm thinking of trying that today sometime. And, all of this isn't even what I was trying to learn. I picked up the PAR meter to measure the effect of using ordinary window screening to reduce light intensity. That is still to come.


Asking the basic questions and then seeing if they are really true or not is a good thing. Why assume when you can confirm and reference against a known standard?

It may seem obvious once you have tested, have some in depth understanding and can see the issue, but till you do, you are guessing and assuming based off someone else's data, which may or may not exist/be real etc. Maybe it just sounded good and they read it off some hokey web site or read in a book(must be true if they found it on the web or written.....in a book).

Who knows and does it apply to us and if so, how much difference is there?
Blessed are the ignorant, for they question everything, much like the 4 years old who asked me "What is a plant?" "What is water" etc.........dang kids will get you:hihi:

We actually know far less than we like to think. I do anyway.

Regards, 
Tom Barr


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## jjp2 (May 24, 2008)

Hoppy said:


> I'm a little bit surprised that everyone but me already knew that you get brighter light at the bottom of a tank with water in it, than in an empty tank.


 
I would have thought an empty tank would be lots brighter. Reefers use artinic lights because blues penatrate deeper while the others disappate with depth. Artinics and high K bulbs seem dimmer to my eyes than a daylight bulb.

All the data, you've collected and analyzed is great. I beleive what really matters is pointing people in a direction on how to choose a light. There are lots many variables in a real setting to adjust and collect data for for a truely accurate picture, unless its your own and you have a PAR meter.


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## Hoppy (Dec 24, 2005)

As I understand it, reefers use actinics to simulate the available light at the water depths their particular corals, etc. usually grow, not because it penetrates the 2 feet of water we normally use. But, I am not, nor have I ever been a reef tank person.


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## macclellan (Dec 22, 2006)

hbosman said:


> I do believe eventually if, enough people do what Hoppy is doing, we can generalize on PAR numbers that would equate to low, medium and high. Just like WPG generalized on low, medium and high in the T12 era, PAR numbers could do it in the T5HO era if, enough people document there findings. We need more people that do have access to PAR meters to document their findings so we can get to that point.


Or just look at successful high and low tech tanks driven by T5s, and do what they are doing...in fact we already have a pretty good idea of what works. XXX PAR at X distance from the bulbs is pretty abstract. "Get a 4x54w fixture if you want high light on a 21" tank" seems much more practical. Of course, these two lines of evidence will agree. That doesn't mean that both are needed to make practical purchasing decisions.



Hoppy said:


> I'm a little bit surprised that everyone but me already knew that you get brighter light at the bottom of a tank with water in it, than in an empty tank. And, while I knew that light reflects off the outer surface of glass, at the glass-air interface, back into the tank, I didn't realize that everyone else knew that it was enough reflection to have a significant effect on the uniformity of the light at the substrate. I'm glad I got up to speed with the rest of you.
> 
> Yes, that is snarky. Sorry about that.:hihi:


Snarky indeed, and I didn't know either of the above. But you tried to sell this set of measurements as if this would resolve the disagreement, but with the testing conditions, it has not done so.

Hoppy and Tom - if pure baseline data is the point - wouldn't it be best to use the PAR meter and light ONLY? All tanks are different in terms of dimensions (reflecting light), which impacts the findings like any of the other findings discussed above in other posts.


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## plantbrain (Dec 15, 2003)

macclellan said:


> Hoppy and Tom - if pure baseline data is the point - wouldn't it be best to use the PAR meter and light ONLY? All tanks are different in terms of dimensions (reflecting light), which impacts the findings like any of the other findings discussed above in other posts.


I think the size and shape of the tank would make a difference, certainly the depth. Still, chose a 20 Gal, a 40 Breeder, 55, 90 Gal any standard sizes.
So yes, good point.

You are not going to get the model so close, but rather, a fairly reasonable predictor of general light spread and intensity for an average tank. I think some view this as a watt/gal replacement, perhaps it might one day be so.......

I'm not particularly privy to this goal however, never have been. I prefer measuring the direct effects in a tank with a more plant specific question. I think Hoppy is advocating more why the light is different under ideal conditions, what effects does reflectance have, spread, distance etc, the mechanics of light in our tanks, with and without water. I do think the general shapes of the data curves will be similar, but a little different with different sizing of tanks. But they should be somewhat close, so we can generalize some here. Be nice to have a dozen standard tank sizes. Hoppy does not have that many available.

But some do.
Likewise, most do not have 6 ADA tanks in display shape to measure either, I am lucky there.

These are more physical light properties. They do play an additive role to aquariums and plants, even though the questions are of less interest, passion to me, Hoppy has enough motivation to test them.
My curiosity is different, as is likely your own. Perhaps more interested in the end resultant, the plants. 

Going with what Hoppy has shown, would plants on the rear, and sides where PAR is less, but be able to grow better due to reduced CO2/nutrient demand than the higher light plants?


Regards, 
Tom Barr


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## snafu (Oct 9, 2004)

Hoppy said:


> I'm a little bit surprised that everyone but me already knew that you get brighter light at the bottom of a tank with water in it, than in an empty tank. And, while I knew that light reflects off the outer surface of glass, at the glass-air interface, back into the tank, I didn't realize that everyone else knew that it was enough reflection to have a significant effect on the uniformity of the light at the substrate. I'm glad I got up to speed with the rest of you.
> 
> Yes, that is snarky. Sorry about that.:hihi: But, I will continue to document what I learn, so I can refer back to it by searching if I forget this stuff at some time.


i think the main difference is the relative refractive index of water and air. since water is closer to glass, it tends to bend the light less towards the surface normal at the internal tank interface. for the air case, more light is transmitted out. the light that does reflect off the external glass/air interface, the larger critical angle of the water/glass lets more light back into the tank. i suppose the best to way to understand is using this cool website. (http://interactagram.com/physics/optics/refraction/)


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## trackhazard (Aug 24, 2006)

I'm wondering about this: is there a distance at which you'll have the most even distribution of the greatest intensity? If you drop the lights all the way down, for example, you get hot spots of intensity but poor coverage. If you mount them way up, then your coverage increases and the average intensity across that coverage area stays more consistent but then you lose overall intensity in the tank since your lights are so high. Not that I'm wanting you to test as this would require access to quite a few different setups but these are just some things that are rolling around in my head.

The fact you are using point source of lights might also have an affect on critical angles and angles of incidence especially on the extreme edges of the tank. Wondering how using a linear bulb that spans the width of the tank might affect that.

Thanks for your work Hoppy.

-Charlie


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## Hoppy (Dec 24, 2005)

Today I emptied my 15 High tank, and did some PAR measurements with it. One big difference from yesterday's data is that any water above the top of the PAR sensor dropped the reading, until the water depth exceeded about 3 inches. I'm still thinking about how that could be true, but in any case, here is the data I got:










Then because you can sometimes see relationships between variables better on a log-log plot, I replotted the data:









The increase in light intensity isn't much, but for me it proves that you can test lights for PAR at various distances from the fixture with or without water without worrying about the effect of the water. There are so many variables involved, that have small effects on the PAR that worrying about what water does to it isn't fruitful. At best you can rely on data like this for guessing the approximate intensity you would get with a fixture where it sits or is hung at some specific distance from the substrate.

I'm concentrating only on PAR at the substrate level because that is the lowest PAR in any tank, forgetting the shading effect of the aquascape. If that PAR is too low, plants will likely refuse to start growing when really small plants are planted, or with HC or other "ground cover" plants. The top of any tank, with the light sitting right on the tank, will have high light intensity just because it is so close to the light fixture, but the substrate level is another story. I still believe the idea lighting setup is a hanging fixture high enough over the tank that the intensity variation from top to bottom is small.

One last comment: using a PAR meter is really tricky. With a distributed light source, like a long tube or multiple well reflected spots of light, you have to be extremely careful not to block light by holding the PAR sensor. I found the only way to get really consistent data is to just lay the sensor in position and keep my hands out of the tank entirely. Significant ight comes to every spot in the tank from every direction imaginable. Also, when the water is sloshing back and forth a bit, the sensor output follows the "wave", making getting a steady reading still harder.


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## plantbrain (Dec 15, 2003)

trackhazard said:


> I'm wondering about this: is there a distance at which you'll have the most even distribution of the greatest intensity? If you drop the lights all the way down, for example, you get hot spots of intensity but poor coverage. If you mount them way up, then your coverage increases and the average intensity across that coverage area stays more consistent but then you lose overall intensity in the tank since your lights are so high. Not that I'm wanting you to test as this would require access to quite a few different setups but these are just some things that are rolling around in my head.
> 
> The fact you are using point source of lights might also have an affect on critical angles and angles of incidence especially on the extreme edges of the tank. Wondering how using a linear bulb that spans the width of the tank might affect that.
> 
> ...


This is an optimalization question.
The answer is yes, there should be such a point.
A light meter can tell you this range for most any tank.
I'd say a range of 10+/- micromoles is an acceptable range of variation for good spread.

If you get higher than say a 50micromol range, then this less important, if you go less, say in the 15-30 range,m then the spread evenness will be more critical since you are approaching the LCP's for many plant species.

But which factors are also dependent?
Reflector type and angling?
Spacing between the bulbs?
no# of bulbs?
Bulb type?
Glass, acrylic, lexan covers?
Effects of dirt, lime etc(is this distributed evenly?)? 
Several variables there to contend with with respect to evenness.

Regards, 
Tom Barr


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## Hoppy (Dec 24, 2005)

snafu said:


> i think the main difference is the relative refractive index of water and air. since water is closer to glass, it tends to bend the light less towards the surface normal at the internal tank interface. for the air case, more light is transmitted out. the light that does reflect off the external glass/air interface, the larger critical angle of the water/glass lets more light back into the tank. i suppose the best to way to understand is using this cool website. (http://interactagram.com/physics/optics/refraction/)


That interactive graphic site is one of my favorite places to play with optics. And I think the explanation for why having water in the tank can increase the light intensity at the bottom of the tank can be seen with that graphic. Also, it shows why the bottom corners of the tank get more light than other areas, as long as the glass is clean inside and out. (Light absorption on inside surface, and reflectance on outside surface of glass)

This is good news too, because these effects help to make the light intensity much more uniform across all of the substrate, letting a single or double tube light do a good job lighting a tank with what seems to be too much front to back depth.


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## macclellan (Dec 22, 2006)

Thanks for following up, Hoppy.

I'll be borrowing the club's PAR meter again soon and will try the same on a 75g.

EDIT: Your readings are higher in the corners? Every time I measured PAR on an AHS/PC combo the light was significantly less there. That was 96w on a 30L.


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## Hoppy (Dec 24, 2005)

macclellan said:


> Thanks for following up, Hoppy.
> 
> I'll be borrowing the club's PAR meter again soon and will try the same on a 75g.
> 
> EDIT: Your readings are higher in the corners? Every time I measured PAR on an AHS/PC combo the light was significantly less there. That was 96w on a 30L.


I certainly didn't expect to see the increase that I saw. The only explanation I can see for having higher intensity in the corners is reflection from two glass surfaces. And, we already know that that reflection disappears if the outside of the glass is painted or has anything in intimate contact other than air. Plus, algae or other stuff on the inside of the glass greatly attenuates the reflection, since the light has to pass through that stuff twice. 

I look forward to any data you can get with your 75 gallon tank.:thumbsup: Getting PAR data from a variety of sources has been very difficult.


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## herns (May 6, 2008)

Hoppy said:


> knowledge is all that will keep them in the hobby.
> 
> 
> > +1.
> ...


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## audioaficionado (Apr 19, 2011)

Angles of Incidence and Refraction not only work off the glass sides, but also help bend light hitting the air/water surface at the sides and corners more towards the center.


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## [email protected] (Jul 17, 2008)

One of the fish magazines had an article about backgrounds. Tanks with painted backs had lower PAR readings then the same tank with a black garbage bag taped loosely to the back, for an illusion of depth.


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## Hoppy (Dec 24, 2005)

About 2-3 years ago someone published an article in an on-line magazine that was the first I saw that noted the bad effect of painting any surface of the tank. The explanation is that the reflection of light depends on the ratio of the index of refraction at the interface between two materials. Going from glass to air gives good reflection. Going from glass to paint or any other plastic material joined to the glass gives no reflection. The water to glass interface also gives no reflection. In the article this was used to encourage keeping the glass clean on both inside and outside, and not painting the back glass.


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## Steve001 (Feb 26, 2011)

Hoppy said:


> PAR is *P*hotosynthetically *A*ctive *R*adiation, the total radiant energy that reaches a specific area and is within the spectrum that plants respond to. It is analogous to Lux, which is light intensity weighted to match human eye sensitivity.


Since PAR is the total amount across the visible spectrum that some but not all plants can use and also what any particular light emits why you did not measure the PUR which is specific instead ?


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## WingoAgency (Jan 10, 2006)

Steve001 said:


> Since PAR is the total amount across the visible spectrum that some but not all plants can use and also what any particular light emits why you did not measure the PUR which is specific instead ?


A meter for that could be expensive.


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