# PAR readings through air and water



## Jaap (31 May 2014)

Hello,

If a light reads 70 micromols through 40cm of air would it read less or more through 40cm of water?

Thanks


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## ian_m (31 May 2014)

I would think it would be the same as I don't see my water "glowing" as is absorbs the light.


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## Martin in Holland (31 May 2014)

It will be less, because there always "dust" particles floating and water is denser than air...just dive in a deeeeeep pool and you won't be able to see clear as at the top


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## pepedopolous (31 May 2014)

In think it would be higher underwater _in an aquarium. _The reason is that you get more reflections from the glass. At the bottom of the page of this article they state that light intensity increased by 100% in a full aquarium compared to an empty one!

I've found the same thing (though I use a SenEye to measure PAR). I made a LED light program based on measurements of PAR when the aquarium was empty. When I filled the aquarium and measured the PAR again it was too high.

P


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## sanj (1 Jun 2014)

I see a substantial decline in PAR from just above the water line to just below. I think there is something in the light reflection in certain areas of an aquarium but I dont find that in a planted tank with substrate in.


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## Mr. Teapot (1 Jun 2014)

pepedopolous said:


> light intensity increased by 100% in a full aquarium compared to an empty one



In corners of my tank there are noticeably lighter areas caused by some reflection from the glass... I’ve been wondering if some of the unexpectedly high power people are coming across with the Grobeam tiles and strips may be due to the nice glitter lines they produce? There’s some reference to natural glitter lines in the sea producing very large magnifications of sunlight in the quick localised flashes. Would a very short, very high PAR level damage the plants sensitive structures?


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## Iain Sutherland (1 Jun 2014)

Water refracts light, so par should always be lower below water line.  There is a refraction equation out there somewhere for fresh water and salt water.


Sent from my iPhone using Tapatalk


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## sciencefiction (1 Jun 2014)

Question now is who is right...I am getting confused as everyone has a different opinion.


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## pepedopolous (1 Jun 2014)

Again, if you missed it, it's last part of the article: http://www.prirodni-akvarium.cz/en/index.php?id=en_measurement



P


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## sciencefiction (1 Jun 2014)

pepedopolous said:


> Again, if you missed it, it's last part of the article: http://www.prirodni-akvarium.cz/en/index.php?id=en_measurement
> 
> 
> 
> P


I took the below quote from the article you posted. That's interesting....I didn't know this.

_Note that the "on the air" at a distance of 50 cm from the light there is approximately 22 µmol PAR, while at the same distance "under water" there is almost 60 µmol PAR, which is almost 2.7-fold difference (increase by 170%) !_


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## Iain Sutherland (1 Jun 2014)

im very sceptical to say the least, everything else ive ever read about par readings over aquariums says it reduces ... and dramatically.  
If you flick through any reef forum where par meters are used daily the levels drop right off from the manufacturer stats (which are always dry numbers).  If the above link were true then im pretty sure the reef light manufacturers would state the in water numbers as we all know reefers are light whores!

That link just makes no sense to me..... given that other info says the opposite and sanj (above post) has a PAR meter and says the opposite im inclined to believe the link is a duck.


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## Randall Postlethwait (1 Jun 2014)

Those are freshwater tests in the link. I'm not saying they are right or wrong; simply that salt water and fresh water are substantially different and mentioning salt water in reference to the link and par value through water is apples and oranges.


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## sciencefiction (1 Jun 2014)

Surely someone with a PAR meter can double check this easily with a glass container of some sort?

I am not sure exactly what Sanj means with his statement as all his says is PAR decreases from just above the water level to just below the water line, but they'd decrease anyhow I'd believe the deeper the light has to travel, but he has not tested a completely empty tank compared to submerged so it's not relevant exactly.
Pepedopolous avove says PAR readings increased from empty to tank to tank filled with water.


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## pepedopolous (1 Jun 2014)

Iain, I'm definitely not denying that light intensity decreases with distance from the light source!

I'm saying that if you measure the PAR from your aquarium light when there is no water in the aquarium, then measure in the same place when the aquarium is filled, you'll often be surprised to get a higher reading with the water. I've seen it myself (admittedly using a SenEye), and using a proper PAR meter, the article shows a similar trend.

I would guess it's down to reflections from the glass being somehow intensified by the water.


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## Iain Sutherland (1 Jun 2014)

Randall Postlethwait said:


> Those are freshwater tests in the link. I'm not saying they are right or wrong; simply that salt water and fresh water are substantially different and mentioning salt water in reference to the link and par value through water is apples and oranges.


not really, the same can be said for fresh water light manufacturers, they aim to get higher par levels from their lights at set distances as technology increases.  I only mention salt water as the manufacturers tend to state the numbers for their lights unlike fresh water units, you have to ask as i had to with my giesemann (all dry stats)

There have been several threads in the past on here where PAR has been tested from a given light in an empty tank and then when scaped, from memory the results were the same, significantly lower when filled.  Ill try and dig some up... 
where is clive when you need him. 



pepedopolous said:


> I'm saying that if you measure the PAR from your aquarium light when there is no water in the aquarium, then measure in the same place when the aquarium is filled, you'll  often be surprised to get a higher reading with the water. I've seen it myself (admittedly using a SenEye), and using a proper PAR meter, the article shows a similar trend.
> I would guess it's down to reflections from the glass being somehow intensified by the water.



I understand what the test is saying, im just not sold.  Not to mention the fact that once you have substrate, rocks, wood, plants all absorbing light the effect that is claimed in an empty tank surely would not work and make it irrelevant to the OPs question..

Light does reflect off the glass and i had an interesting conversation about it prior to buying my new light, but it also happens in an empty tank which doesnt have to penetrate anything??

To be honest, its a conversation of assumptions at the moment but i look forward to the more scientifically minded weighing in or some one with a par meter doing a quick test.


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## Yo-han (1 Jun 2014)

The water surface reflects light, thus between just above the water line and just below the water line there is a drop in PAR. BUT, when going down further, light MIGHT increase. This is due to the fact that the glass reflect light when it is filled with water. In an empty aquarium only the light coming straight down from the light source is measured. In a filled aquarium, you measure the direct light PLUS the light reflected from the glass which was otherwise gone through. I tried finding a decent picture to show it, but couldn't find one, so this is one from my old aquarium. You can see the light going over the glass on the wall. A little lower it is quite dark. When the aquarium is empty, the the dark shade on the wall is almost as bright as the light going over the top. Where is this light going, back inwards, to your PAR meter! That is why an aquarium without a background will always have way higher PAR levels compared to an aquarium with background


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## pepedopolous (1 Jun 2014)

Iain Sutherland said:


> where is clive when you need him


Yep.



Yo-han said:


> You can see the light going over the glass on the wall. A little lower it is quite dark


I hadn't thought of it in that way but I think you hit the nail on the head there. My aquarium doesn't have a background and the light really doesn't show much on the wall just behind it. It appears dark grey, even though it's actually painted white.


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## Marcel G (5 Jun 2014)

I'm the man who did all the measurements on the above stated website, and I have to say that from the time I publish my results some things have changed. So you are better to don't read the english version of the page, and try to read the czech one instead (http://prirodni-akvarium.cz/index.php?id=mereni), which is more accurate and up-to-date. Unfortunatelly I don't have a time to update the english version as often as the czech one. So try to use Google Translate on the czech version. What I can say is that there is surelly a difference between the PAR values 1) "outside the tank" = in the air, 2) "in the empty tank", and 2) "under water". If you measure light intensity "in the air" (outside of a tank) then you'll find out that (in accordance with physical laws) the light intensity decreases with distance ... so the further you are from light source, the less light you get. But this doesn't hold true if you measure the light in a glass tank (wheather empty or full of water). While the light intensity decreases in the tank the same way as "outside the tank", there is an increase taking place in the tank also, and this increase (due to reflections) is much higher than the decrease caused by the physical laws which applies outside the tank. So this is the reason why you measure higher values in the tank than outside the tank. I use Apogee MQ-200 PAR meter for all my measurements which is quite respected device. Also what's quite interesting is the fact, that in cube tanks the reflections are much higher that in rectangular tanks (in cube tank the light intensity can be up to 3-times higher than outside the tank, while in rectangular tank the light intensity can be only a little higher compared with the values measured outside the tank). The same applies to small vs. big tanks (small tanks = higher impact of reflections).  Also the difference just above the water surface vs. just under the water surface are small => about 10 µmol PAR according to my findings. Next week I will do some further measurement with ADA Aquasky also (at least I hope so). So I'm working to expand my measurement so that they will be more and more accurate. Right now I'm testing Cree and Bridgelux LED chips => you can check some measurements here: http://prirodni-akvarium.cz/index.php?id=led2 (I'm working on translation of this article into english right now). I hope it helps.


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## parotet (5 Jun 2014)

ardjuna said:


> I'm the man who did all the measurements on the above stated website, and I have to say that from the time I publish my results some things have changed. So you are better to don't read the english version of the page, and try to read the czech one instead (http://prirodni-akvarium.cz/index.php?id=mereni), which is more accurate and up-to-date. Unfortunatelly I don't have a time to update the english version as often as the czech one. So try to use Google Translate on the czech version. What I can say is that there is surelly a difference between the PAR values 1) "outside the tank" = in the air, 2) "in the empty tank", and 2) "under water". If you measure light intensity "in the air" (outside of a tank) then you'll find out that (in accordance with physical laws) the light intensity decreases with distance ... so the further you are from light source, the less light you get. But this doesn't hold true if you measure the light in a glass tank (wheather empty or full of water). While the light intensity decreases in the tank the same way as "outside the tank", there is an increase taking place in the tank also, and this increase (due to reflections) is much higher than the decrease caused by the physical laws which applies outside the tank. So this is the reason why you measure higher values in the tank than outside the tank. I use Apogee MQ-200 PAR meter for all my measurements which is quite respected device. Also what's quite interesting is the fact, that in cube tanks the reflections are much higher that in rectangular tanks (in cube tank the light intensity can be up to 3-times higher than outside the tank, while in rectangular tank the light intensity can be only a little higher compared with the values measured outside the tank). The same applies to small vs. big tanks (small tanks = higher impact of reflections).  Also the difference just above the water surface vs. just under the water surface are small => about 10 µmol PAR according to my findings. Next week I will do some further measurement with ADA Aquasky also (at least I hope so). So I'm working to expand my measurement so that they will be more and more accurate. Right now I'm testing Cree and Bridgelux LED chips => you can check some measurements here: http://prirodni-akvarium.cz/index.php?id=led2 (I'm working on translation of this article into english right now). I hope it helps.


Ardjuna, are you the author of this website? My most sincere congratulations! I have read and re-read this website for a year or so... IMO one of the most brilliant contributions to planted tanks on the internet. Hopefully it was translated into Spanish because I forward many people to this website from Spanish local forum to read the basics. Planted tank science for the layman... Excellent.

Jordi


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## Marcel G (5 Jun 2014)

I'm sad I don't have a time to do what I want to do ... I have many plans:
1) do some *substrates analyzes* (ADA, akadama, terramol ...)
2) test the growth of selected plants in *ADA vs. gravel* to find out if you really need a nutrient-rich substrate or you will get by with a sand/gravel (complete with comparative photos)
3) test *how much light* (PAR values) a selected species of plants really need to prosper (we all know that there are high-light plants vs. low-light plants, but no one yet seems to test what does this mean, and how much light is enough for them)
4) find out *how much nutrients* a high-tech aquarium really consumes each week/month (I would like to find this out by weighing the dry matter) => there are many suggestions on the internet where you can read about 30 mg/L of NO3, 3 mg/L of PO4 etc., but no one seems to prove it or disprove it.
The only problem is that I'm not a millionaire, and I have two jobs to pay the mortgage. But gradually I hope I'll make it all 
_PS: Thank you very much for your encouragement!_


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## Alastair (8 Jun 2014)

This has baffled me as when I was doing my water change yesterday I turned my lights on full and placed my par meter on a branch of the wood just underwater. As the water was dropping the par values were rising and out of water it was slightly higher than below water. 

Sent from my D6503 using Tapatalk 2


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## James O (8 Jun 2014)

Index of refraction is ringing in my ear. The glasses composition and the change of the angle of incidence between air/glass and water/glass need to be taken into consideration.

From the tinterweb:

'One indicator of the optical density of a material is the *index of refraction value* of the material. Index of refraction values (represented by the symbol *n*) are numerical index values that are expressed relative to the speed of light in a vacuum. The index of refraction value of a material is a number that indicates the number of times slower that a light wave would be in that material than it is in a vacuum. A vacuum is given an n value of 1.0000. The *n* values of other materials are found from the following equation:





The table below lists index of refraction values for a variety of medium. The materials listed at the top of the table are those through which light travels fastest; these are the least optically dense materials. The materials listed at the bottom of the table are those through which light travels slowest; these are the most optically dense materials. So as the index of refraction value increases, the optical density increases, and the speed of light in that material decreases.

*MaterialIndex of Refraction*
Vacuum1.0000
<--lowest optical density

Air1.0003
Ice1.31
Water1.333
Ethyl Alcohol1.36
Plexiglas1.51
Crown Glass1.52
Light Flint Glass1.58
Dense Flint Glass1.66
Zircon1.923
Diamond2.417
Rutile2.907
Gallium phosphide3.50
<--highest optical density

This equation for the critical angle can be used to predict the critical angle for any boundary, provided that the indices of refraction of the two materials on each side of the boundary are known. Examples of its use are shown below:
*Example A*Calculate the critical angle for the crown glass-air boundary. Refer to the table of indices of refractionif necessary.
The solution to the problem involves the use of the above equation for the critical angle.

Θcrit = sin-1 (nr/ni) = invsine (nr/ni)
Θcrit= sin-1 (1.000/1.52) = *41.1 degrees'
*
Of course none of the numbers are for planted tanks.  And as each one of those is different.  One of my tanks empty will of course measure lower when planted because it's covered in amazon frogbit


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## Iain Sutherland (8 Jun 2014)

that cleared it right up James 

having reread this i understand it a little differently....
correct me if im wrong but its saying that the light reflecting off the glass increases the par in water against just the light hanging in a room at the same distance due to light spill/ no reflection from the tank.  
So the par DOES drop when filled with water against the light over an empty tank.
Is it not the measurement without a tank that is causing confusion here.... no tank so no reflection etc...??
the OP was asking if his par will drop once he flood his tank... so yes.....



ardjuna said:


> Also the difference just above the water surface vs. just under the water surface are small => about 10 µmol PAR according to my findings


isnt this what we are actually getting at?!  which also increases with depth....?

The air measurement is relevant if you plan to grow an emersed tank, ie dry start though.


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## James O (8 Jun 2014)

So light at an angle greater than 61.04 deg will pass from water thru the glass.  Anything less will be reflected back inside (I love wolframalpha.com )

Air/glass is 41.1 deg so there is a 20 deg increase in angle of incidence between air/glass and water/glass

And I've run out of mental steam (well I'm going to the beach.....) so I'll let someone else drawer a conclusion from this info


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## John P Coates (20 Jul 2014)

Just want to mention the importance of light spectrum. Blue light is attenuated to a lesser extent than red light, for example. Based on my measurements of a full-spectrum light, PAR is less in water and decreases as depth increases.


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