understanding par

so, I think I sorta am starting to understand light a little. Please help me fill in the blank spots, and correct me if this is way off. I understand par is the light in between 400 and 700nm, but I don't understand how it speaks about power, distance, and space or how to read a signal par number. I sorta understand lumens: a amount of light, over a size of space, at a set distance. I think I understand how to compare a light of equal spectrum (nm range) with lumens, but I'm not really getting par.

 

Photosynthetically active radiation - Wikipedia, the free encyclopedia

 

dam wikipedia won't load all day today?? I'm pretty sure I've read the article or at least I've read stuff about par from wikipedia a few times and did not fully understand it. I'll read it as soon as I stop getting the wikipedia is too slow right now resonance.

 

damon, below are some in depth articles written by Sanjay in 2006 with regards to lighting. Theres a lot of stuff covered, so grab a cup of coffee.

Some of the original links are broken, so instead I used googles webchache links to the articles. The articles themselves are pretty technical - I've only skimmed them awhile ago, but it should be a good read if you're really interested.

Part I: What is Light?
http://www.reefkeeping.com/issues/2006-02/sj/index.php

Part II: Photons
Facts of Light – Part 2: Photons by Sanjay Joshi - Reefkeeping.com

Part III: Making Sense of Light Measures
Facts of Light – Part 3: Making Sense of Light Measures by Sanjay Joshi - Reefkeeping.com

Part IV: Color Temperature
Facts of Light – Part 4: Color Temperature by Sanjay Joshi - Reefkeeping.com

Part V: Everything You Need to Know About Metal Halide Lamps and Ballasts
Facts of Light – Part 5: Everything You Need to Know About Metal Halide Lamps and Ballasts by Sanjay Joshi - Reefkeeping.com

 

thanks guys. I the odd feeling in my head suggests that I might be learning something, but the jury is still out on that one.

 

so this is what I got so far, Par is a measurement of photons moving at rates in between 400 and 700 nm over one square meter for one second.I don't think I said that right. I'm not really getting what's up with nm and hertz. I don't really understand how moles work, and I also am now wondering why people say par is the best means of talking about a reef light. I'm told that blue light is more important than orange, so you could get a orange light with a high par value and it would not be as good as a lesser par of better nm range, correct?

 

A simple, aquarium-specific look at PAR, with links to PUR and CRI info

 

Ok... honestly, I know what im talking about when it comes to lighting so you can believe everything I say with 100% certainty. I am a close friend of Sanjay's and I have had many opportunities to discuss this with him.
ALL light (regardless of its wavelength aka nm) travels at the same speed (3x10^6 meters/second). Now the only light that we usually measure is photosynthetically active radiation (PAR) and it is in the range of 400nm to 700nm... and it is measured in micromoles per meter squared per second (micro moles/m^2/s). A mole of light is simply 6.02x10^23 photons... which may sound like jargon, but it is only used by chemists to standardize a measurement between every type of molecule/particle.

So, PAR only tells us the amount of photons that are released from a source in this given range. It tells us nothing about how much of each nanometer is released. So generally speaking, this can be an adequate measure of a quality light source for our tanks, however it is not exact for determining if a given light source is better than another source.
To determine which light source is better, one will have to look at the actual spectral output of each nanometer from a given bulb (measured as spectral irradiance over wavelength). This data will show exactly what wavelengths are released and in what quantity they are released. When one matched these quantities up to the specific needs of a coral, the coral will respond very positively. Below is an informal article I have written about the matter... And I want to make it clear that I, in no way, am making an argument with Sanjay over this matter because I respect him and I have nowhere near the experience that he does in the hobby:

Over the years I have had many opportunities to observe many different tanks with many different types of lighting. Afterwards, I would take the opportunity to compare the colors of the bulb to Sanjay's spectral output research. I have started to identify quite a few trends that exist between "quality" bulbs and "healthy" corals. This article aims to help fellow hobbyists make educated decisions about lighting choice and to understand what it is exactly that makes a light a quality light. Two key observations that I would like to address here are: the whole PAR myth and how each and every coral requires its own specific wavelength of light to be happy.
All this PAR hype from a few years back has literally put everyone out there in the mindset that PAR is exclusively what dictates the growth. In reality the only thing PAR tells us is how much light is put out, not the quality of the light. So people who make such a massive deal over PAR are not necessarily taking the best path for choosing lighting for corals. The perfect lighting is not PAR at all, but a function of matching the desired photosystems of the specific zoox that a coral holds with the respective wavelength at which it is the most efficient. Dana Riddle has some cutting edge research on this subject and it is not getting close to the recognition that it deserves due to the whole PAR stigma.

So, some people bring up PUR then… PUR is just the radiation given off only in the range that is accepted for photosynthesis processes so this is pretty much just cutting out the excess infrared at the high end of the spectrum and a bit of the UV at the low end of the spectrum… therefore its practically the same thing as PAR, just with a shorter area to be measured. Its more true for terrestrial plants and stuff anyways because clams and corals actually utilize some of the UV that is "cut out" by only specifying PUR.
So heres an example to help portray what I am trying to say here about the specific wavelengths for a specific coral's photosystem: say you have an orange M. digitata. It looks orange to our eyes because the orange wavelength is reflected (about 630nm). Now this color can be reflected in one of two ways: it can be reflected from the wavelengths being present and just naturally being reflected off of a very specific "sunscreen" molecule (if you want to get technical its likely a carotenoid type molecule and it is used to not allow any of the light in that the coral doesnt want... in this case, the orange wavelengths). This is what is seen under daylight lights. M. digitata has a preferred optimal absorbance at about 510nm aka yellow light. OR the other photosynthesis process here is from re-emitting absorbed wavelengths after they have gone through a species specific [by this I mean that each species, and sometimes even down to each individual, will have their own "most efficient" zooxanthellae photosynthesis pathway that they use for obtaining energy] photosystem (photosynthesis pathway). In the case of the second technique, the coral will absorb the photon at one wavelength and put it through a photosystem and then completely release (re-emit) the photon at a different wavelength and thus a different color. This process is why when you put some actinic, very blue, or even UV light over a tank your corals light up really really bright aka fluorescence.

Now that we have that out of the way, this orange Montipora digitata will only absorb the wavelengths that it prefers for photosynthesis given the very specific photosystems that it possesses. Now knowing this, you can understand that growth is not a function of PAR or PUR, but more-so a function of the intensity of this very specific desired wavelength. (aka 510nm in this example)
Please note... I am not saying that this is THE ONLY wavelength required... because other biochemical processes involving production of amino acids may or may not involve VERRRY MINIMAL amounts of other wavelengths of light in their reactions. This explanation would be verrry long and likely take me about an hour to write and it can also vary between species and its sooo technical that everyone who reads it and is not an expert on biochemistry will be lost almost immediately. Just know that other wavelengths may be needed in extremely small amounts.

So to recap, I said that the intensity of the photons at a very specific wavelength are what dictates energy acquisition, and thus growth, in a specific coral. Sticking with my example from earlier; 510nm (yellow) is absorbed by the zoox of a coral and then harnessed for energy. All wavelengths near 630nm (orange) are reflected. Additionally any very high energy photons (UV, violet, and blue) can be adsorbed, harnessed for energy, and then re-emitted at a lower frequency wavelength (emitted as orange in an orange M. digitata). For reference the process goes like this; blue light 450nm is absorbed, goes through a photosystem which slows the photons wavelength down to 630nm, photon is then released as 630nm and this is the fluorescence “orange” that we see.

PAR is a measure of all the active radiation that is put out by a bulb at any single point in time. This encompasses ALL of the wavelengths of light put out. The way to actually get a number for PAR is to take the amount of light (this includes every single nanometer emitted at one time) and measure how many micromoles of photons are projected onto an area, then divided by the area (typically meters squared), then finally divided by the time in seconds that the light was projected for.
So PAR is a way to measure everything we can see with our eyes at one point in time. What im saying is that the measure is good for determining a quantity of light entering the tank but not the quality of the light entering the tank.
Ultimately the specific wavelengths that are entering are what is the final determinant of how good/bad the light will be for a specific coral.

I dare to say that ‘within reason’ PAR does not matter. It just assigns a number as to how many photons are going in and says nothing about the actual wavelengths that are going in. The actual wavelengths matter MUCH MUCH more than simply the amount of total photons entering.
...
To enforce my concept, I will supply examples; I would like to point out that you will have the outliers which don't necessarily follow into the "PAR" concept: the lower PAR bulbs (like radium) that have great spectral output + very fast growth and then the super high PAR bulbs (like XM) that only have a barely average spectral output + only average growth.


...now back to what you were specifically asking; lumens are terrible for measuring light output (they have NO direct correlation to the spectrums being put out), PAR is on most occasions an acceptable way of measuring light (as it measures the amount of light in a specific range, however this can still be biased by undesirable wavelengths), and actual examination if spectral irradiance is the most effective way of measuring light output (since you can actually look at the light required by a given coral and the light supplied by a given source).

And finally, to answer your last question: most photosystems in corals use lower wavelength light (below 550nm) so therefore blue light is utilized by more corals in general... There are a few outliers however that use red or yellow light in their primary photosystem and will therefore benefit from this higher wavelength. Either way, just having a higher PAR number will not make your corals perform better if their needed wavelength is not supplied by a light source.



I tried to be pretty through here... but any questions?

 

thank you dingo that helped a lot. I was looking at spectral out put to start with, but people got me thinking par would be a better way. Thank you for clarifying this, and (as confusing as that problem is) it's easier for me to look at the numbers of how a mole works instead of trying to think of how much is in the isotope carbon 12.
So now my question is how do we know what corals are using what? can we assume that if it is that color it's not using that color in light? I have not read up on this too much, but I did read an article (not very close at the time) that suggested that a lot of our coral need more orange/yellow light than most of us run?
Thanks again, this really helped me clarify things I sorta understood, but did not really connect completely.

 

Dingo, nice writeup. Do you have any links to the research mentioned above? I would be interested in reading up on that topic a bit more.