Par V Pur

[salty joe]

Is this the difference between PAR and PUR?

Once photosaturation occurs, PUR cannot increase, while there is no limit to PAR.

Is that about right?

[chrisstie]

I am not sure now I'm going to have to look this up. I was under the impression PAR is from the output of the light but PUR was how much light can be absorbed from say, a coral or a plant. Hrmm..

[salty joe]

Yeah, I'm looking at your wrasse-what of it?:D

[salty joe]

This is strictly assumption.

I would assume that different photosynthetic organisms have different radiation needs. While all these needs would satisfied by the wavelengths in PAR, different organisms would rely more on certain wavelengths in PAR than others. If that's the case, it seems unrealistic to even be concerned with PUR, except to not exceed photosaturation.

Is that about it, does that make sense?

[IslandCrow]

Yeah, I'm looking at your wrasse-what of it?

I have no idea what that has to do with the question, but funny!


Alright, hopefully I've erased my original reply before anyone read it, because it seems I misunderstood exactly what PUR is. Luckily, I have that little voice in the back of my head that says, "remember, you're not nearly as smart as you think you are", so I looked it up. Here's from an Advanced Aquarist article (http://www.advancedaquarist.com/2005/12/aafeature2):

Seriously, the right answer is that the Einstein is a good indicator of the photosynthetic activity of plants. The biological mechanisms in place during the luminous phase of photosynthesis do not depend on the photons' energy, but on their number. This is exactly what the Einstein displays. The PAR (Photosynthetic Available Radiation, unit µE/m2/s) measures the number of photons reaching a surface, all this in the wavelengths of the visible light (between 400 and 700nm). It is indeed in this portion of the spectrum that we can find the different absorption peaks of the photosynthetic pigments. As these pigments do not absorb energy in a equal manner on all that 400-700nm range, but only at certain precise wavelengths, some prefer using the PUR (Photosynthetic Usable Radiation) in order to quantify the number of photons truly used by the photosynthetic cells. The PUR is thus defined by the light source (emitted spectrum, intensity) and by the studied pigments (because of their absorption spectrum). This one is probably better left to specialists...

So, if I understand it correctly, PUR is really just a more specific version of PAR, but it is going to be coral specific, depending on that coral's pigmentation. In other words, I'm guessing you can't run out and get a PUR meter, so unless you have a handy-dandy spectrograph, and some really in-depth information on the particular coral you're interested in, PUR is great for academic discussions, but we hobbiests are pretty much stuck with PAR.

At least, I think that's what it's saying. . .

[salty joe]

<a href=showthread.php?s=&postid=14290272#post14290272 target=_blank>Originally posted</a> by IslandCrow
I have no idea what that has to do with the question, but funny!

Just a goofy reply to Chrisstie's goofy tagline.


so unless you have a handy-dandy spectrograph, and some really in-depth information on the particular coral you're interested in, PUR is great for academic discussions, but we hobbiests are pretty much stuck with PAR.
. .

That is pretty much my conclusion too. We keep a variety of photosynthetic creatures and plants-PAR doesn't seem like such a bad thing to be stuck with. Unless you were to concetrate on a specific organism, it seems the best thing to do with PUR is forget it. Or do I have this all wrong?