# Quick question about Par and Kelvin



## BlueJack (Apr 15, 2011)

That's a good question. What kind of PAR meter did you use? 

Were the led's the same in both fixture's? By that, I mean, did one have a bunch of .25 watt led's that added up to 10 watts versus one having 5- 1watt led's? I know the 1-2 watt led's have a much higher PAR than the smaller .25 watt led's, even if they draw equivalent wattage.

For kelvin, I don't think PAR is significantly impacted by color temp. I have seen some lower kelvin bulbs give off more PAR because it has more red light than higher Kelvin bulbs, but it's a very small difference, almost insignificant.


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## VeeSe (Apr 16, 2011)

It's possible that you aren't accounting for your nutrient dosing and other things. For example, it might be much better now with lower PAR because you had too much light before in proportion to your dosing, so plant growth was ragged and not good. Now, since you have half as much, you are dosing enough to keep up with the light, and plant growth looks good and things have settled down.


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## FooDog (Jun 28, 2010)

Thanks for responding. 

The meter is a new Apogee I'm having fun with. I don't know the model off the top of my head but it's the cheaper one (if you consider $300 cheap, lol).

And you're right on about the LED size wise. The high PAR fixture had much fewer and larger LEDs equaling 5w. The fixture I had much better results with, the 6000k fixture with much lower PAR, featured a ton of small diodes drawing 10W.


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## FooDog (Jun 28, 2010)

VeeSe said:


> It's possible that you aren't accounting for your nutrient dosing and other things. For example, it might be much better now with lower PAR because you had too much light before in proportion to your dosing, so plant growth was ragged and not good. Now, since you have half as much, you are dosing enough to keep up with the light, and plant growth looks good and things have settled down.



Ah good point. I think that may be the case as I kept dosing the same.

In any case, when we're looking for a lamp, should we be keyed in on PAR values or Kelvin ratings? Isn't PAR what plants can use for photosynthesis? So if I take a bulb high in PAR but at an extremely blue Kelvin rating away from 5000k, shouldn't I get good results regardless of Kelvin?


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## BlueJack (Apr 15, 2011)

FooDog said:


> In any case, when we're looking for a lamp, should we be keyed in on PAR values or Kelvin ratings?


You've got the right idea. PAR is much more important than Kelvin temperature. Plants will do well under most any Kelvin Temp. It's more of an aesthetic choice, really. But plants do need a minimum PAR to grow.



FooDog said:


> Isn't PAR what plants can use for photosynthesis


There are many members on here that are much more knowledgeable than I am, but I'll try to answer best I can. I think PAR just measures the amount of photons hitting a given area per second. Aquatic plants don't need much light at all to do well. I think most species have a lower limit of 10-15 _u_mols needed to survive. Below that and you'll have no growth/slow death. Anything above that and plants will do fine. I think the upper limit is ~200 _u_mols, anything above that, and plants won't photosynthesize any faster proportional to light increase. I've seen that my tanks do very well with 50-75 _u_mols of light. 



FooDog said:


> So if I take a bulb high in PAR but at an extremely blue Kelvin rating away from 5000k, shouldn't I get good results regardless of Kelvin?


yep, it'll just look very blue and reef like.


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## FooDog (Jun 28, 2010)

BlueJack said:


> You've got the right idea. PAR is much more important than Kelvin temperature. Plants will do well under most any Kelvin Temp. It's more of an aesthetic choice, really. But plants do need a minimum PAR to grow.
> 
> 
> 
> ...



Informative input! Thanks man


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## samamorgan (Dec 31, 2011)

Kelvin is just a way to measure the color of a light source. The problem with kelvin is that the numbers are variable, so 5000k is actually a "range" of colors on a non linear axis. This gets even worse as we get very high in kelvin, 10,000k can appear bluish, pinkish, or pure white. Here's how the kelvin range actually works:









Notice how 10,000k can be anything from blue to pink. This is why you can buy two flourescent bulbs from different manufacturers of the same color temperature, and they appear different.

When it comes down to it, kelvin doesn't mean squat. We use kelvin as a general rule of thumb because thats what light manufacturers use to describe the color coming off of a light source. And we know that a 6500k light source is going to have a much higher chance of being in the spectrum range that plants need to photosynthesize.

But if you're trying to relate color temperature K to PAR, don't. They don't relate in any specific way. Two bulbs with wildly different color temperatures can have exactly the same PAR reading.


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## Ibn (Nov 19, 2003)

You can get values above 200 umols and still get plants to grow.


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## Chlorophile (Aug 2, 2011)

FooDog said:


> If you'd be so kind, could you help me clarify the relationship (if any) between Kelvin and PAR values? I always assumed a lower color temp would be great for plant growth. But how does PAR come in to play?
> 
> The reason I ask is because my Fluval wasn't getting the growth (plants were actually dwindling down) I wanted with a 5W LED fixture I put on it. The fixture had a higher K rating (I have no specs but I say that confidently as the color it put off had a white/blue tint) and PAR was near 70 directly under the light. I then switched to another LED fixture, about double the watts but half the PAR with a rating of 6000k and things have settled down.
> 
> ...


They are connected as far as we're concerned, but there isn't a real correlation between kelvin and PAR. 

First you have to realize that white(6000k to 10000k depending on who you ask) light can be made up for varying amounts of all other spectra, but we'll say its just made up of shades of RGB. 

Then you have to realize that PAR is measuring the uMol of light available to the plants. SO it is first measuring the intensity of the light, but it is also factoring in the wavelengths of light the plants use best. 

A light could be very very intense but only in the 550nm wavelength and that isn't going to have very much par, but it would be bright to us!

Now green at 550nm isn't on the color temperature scale because it is based off the color a blackbody produces when it is heated at certain temperatures, (Think Electric Range at full blast is around 1000k) and a blackbody at any temperature will never produce green light, at least not alone...

2 bulbs at 10k at same candela or lumens might have different par, and this is because the intensity of the wavelengths that are making the light appear white may be different. 

Your two different bulbs just didn't produce the same wavelengths. You're looking for these wavelengths, some bulbs have spectral graphs on the back that help you decide if you're getting the right bulbs.


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## BlueJack (Apr 15, 2011)

Ibn said:


> You can get values above 200 umols and still get plants to grow.


Absolutely. I just meant that levels above 200-300 won't increase photosynthesis. So, it'll just be a waste to run 1,000 umol lamps. The plants won't use CO2 + Hydrogen + light to make carbohydrates any faster. 200umols or 2,000umol will generate about the same rate of photosynthesis.


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## FooDog (Jun 28, 2010)

Chlorophile & Samamorgan you words are appreciated! I have a much better understanding now. 

So when purchasing a light, I should be more focused on wavelength (nm) and PAR/PUR values, correct? Kelvin is just a measure of color temperature, and typically people will suggest a 5000k bulb because in most cases, it is suppose to have the proper nm for plant growth. In some cases, that 5000k bulb may not actually omit the proper nm plants are looking for so it is better to shop for nm statistics rather than Kelvin stats, correct? And to add, PAR readings are the intensity of the useable nm that bulb is producing. So an older model of the exact same bulb may be putting off the same nm but par values will be different as the older bulb has lost intensity. Am I finally understanding this correctly?


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## Steve001 (Feb 26, 2011)

FooDog said:


> If you'd be so kind, could you help me clarify the relationship (if any) between Kelvin and PAR values? I always assumed a lower color temp would be great for plant growth. But how does PAR come in to play?
> 
> The reason I ask is because my Fluval wasn't getting the growth (plants were actually dwindling down) I wanted with a 5W LED fixture I put on it. The fixture had a higher K rating (I have no specs but I say that confidently as the color it put off had a white/blue tint) and PAR was near 70 directly under the light. I then switched to another LED fixture, about double the watts but half the PAR with a rating of 6000k and things have settled down.
> 
> ...


A PAR meter measures total photon flux across all photosynthetically active radiation wavelengths ( this includes corals). It can't tell you which specific wavelengths of light any particular source of light produces. To know what wavelengths of light a source of light produces you have to use a Photosyntheticallly Usable Radiation (PUR) meter. 
Now to answer the first question. *Maybe*.
A high or low K rating might indicate a source of light is deficient in redish or blueish light. But there's a catch. A source of light can still have good PUR output but look warm or cool because the predominant light output has a peak in one or the other. So, the only way to know with certainty is to use a PUR meter or call the manufacturer and request a spectral distribution graph of that particular light. 

I can't answer the other question because there are to many variables and that would lead to guessing.

An old thread on this subject http://www.plantedtank.net/forums/lighting/78632-par-vs-pur-measurement-light-efficency.html


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