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PAR readings through air and water

Jaap

Member
Joined
30 Sep 2011
Messages
1,068
Location
Nicosia
Hello,

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

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

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.
 
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;)

33my0hv.jpg
 
where is clive when you need him
Yep.

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.
 
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.
 
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
 
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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