# Help me understand PAR values in lighting



## junebug

I'm sure this is really simple, but I can't make it make sense in my head. Help me.

Do I absolutely have to have a PAR meter? Or is there a method of calculating a decent estimate on the lighting in my tanks? And if so, could you tell me what that might be? Or better yet, link me to an article on it?

I'm honestly not that concerned about it in my own tanks, as my plants are all doing fine, and if they aren't, it's because my fish have decided they're food (darn those guppies) but I'm putting together a tutorial for another site on setting up a planted tank. And I know people are going to ask me how I know what light I need (the truth is I don't know. I get a decent plant light and let the plants that want to live, live.)

I could tell them one method of estimating light requirements is the watts per gallon method, but it's a bit dated and less accurate than the PAR system, isn't that right?

So yeah, I need help. If anyone could link me to PAR values for dummies, I'd really appreciate it xD.

Anyway, the basics would probably do. How do I calculate PAR without a meter? What PAR values are appropriate for low, medium, and high light tanks? Really just any information would be awesome


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## Seattle_Aquarist

Hi junebug,

Here are a couple to help you get started:
http://www.plantedtank.net/forums/showthread.php?t=184368http://www.plantedtank.net/forums/showthread.php?t=105774


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## junebug

Oh thanks, Seattle. In this case, please ignore the message I sent you lol!

I thought you were AFK.


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## TropTrea

PAR is a rough guide. A true PAR reading is the total light photons between 400 and 700 nm. The problem with this though is that only some of this light is used by either plants or corals. Each photosynthetic chemical has specific wave lengths of light that it absorbs and utilizes. Most have a primary and secondary wave length that they require. Now with plants that appear green the reason they appear green is because the light n the green wave lengths are reflected rather than absorbed. Most of the photosynthetic activating wave lengths are either in the Blue or Red end of the spectrum. Now with most PAR meters they loose there sensitivity to light in the blue end of the spectrum. If you took a blue light at 430nm some PAR meters will not even measure half of the light coming from it. 

It is very possible to have two completely different light sources both reading on meter 100 for there PAR. Light Source 1 can be a special plant bulb with all the light it produces in the red and blue areas of the spectrum and be to powerful for your plants. A second light source can be tuned to look bright with a lot of green light and very little blue or red light and even though it has the same PAR meter reading your plants could up deficient of light under it.

So in reality Spectrum is more important than PAR is. however if you understand the spectrum of your lighting and the requirements of your plants than the PAR meter is good secondary tool to measure changes in the light or comparisons between different tanks.

Some simple thought. All plants need blue light, and most need red light. Red plants do not need a lot of red light, Cyan plants do not need a lot of cyan light. If there is no green light for plants to reflect back at you they wil look considerably darker.


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## junebug

Thanks TropTea!

I have a decent understanding of lighting spectrums, I was more concerned with the "density" (I can't think of a better word) of light in tanks, particularly at or just above the substrate level, for carpeting/foreground plants. I'm hoping a better understanding of lighting PAR values will help me understand why light wattage "X" at "Y" kelvins and full spectrum and length "Z" is or is not a good fit for some of my tanks. 

(you just reminded me of the main reason I need a better light for my bookshelf tank. It's red spectrum and my centerpiece plant is Rotala Wallichii :/ good news is all of the green plants in that tank grow like weeds since they have all the red light they could ever want lol)


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## Newt

PUR is much more meaningful to plant growth. A 'normalized' spectral plot, IMO, is the only way to compare the strength of a bulbs output; from one to the other.

Don't obsess over it too much. Find a good tri-phosphour bulb with a color temp you find pleasing and observe your plant growth.


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## TropTrea

A little about color terms.

Kelvin rating is a system to evaluate the overall total color of a light bulb compared to the temperature of heated black iron. When black iron gets warm it starts to glow a re dish color and as the heat increases the color moves through most of the color spectrum to orange, yellow, green and even blue green. It is brightest around 4,000 K to 6,000 K. Direct sun light in slightly under 4,000K but the light we see outside do to added blue sky is is close to 6,500K which is why daylight bulbs are usually around 6,500 K

Another point of advertising is the CRI of a light bulb which indicates that objects under its light will look what percent like they would under natural balanced light. If the balance of light is equal then it should have a CRI of 100% but in reality few lighting sources get above 90% on this. Many specialty bulbs are much lower that that even as they have a high ratio of either red or blue light compared to the other colors of light.

Lumns and PAR are both measurements of the amount of total light emitted from a light sources. Lumns measures as what the eye sees and is more sensitive to green light than it is to blue or red light. and PAR measures the range of light that plants may use equally throughout the spectrum from 400nm to 700nm. A light source with a stron Red and or Blue component will give you a higher PAR rating and lower Lumns rating compared to one with a strong green element even thoug the lights may produce the same amount of total energy of light through the full spectrum.

PUR rating are the ratings of what the plant actually absorbs and utilizes. You can put the same light bulb over different plants and or corals and get different PUR values since they may utilize different light frequencies do to difference in there photosynthetic chemical make up. 

I will try to include a link or photo of the photosynthetic spectrum if it works here. If you look at it you will see that photosynthesis occurs much more in the blue and red parts of the spectrum and very little in the green and yellow parts. 

Hope this helps you a little better?
Dennis


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## junebug

I do have a pretty good understanding of how spectrum works with plants... that's really not what I'm concerned with at the moment.

Say I have a very "tall" tank (for me this would be a 20 gallon high but others will likely have 40-75 gallon tanks where the same principle applies). I can have a full spectrum light, but if the light isn't strong enough at the substrate level, carpeting plants won't grow, or at least won't carpet. 

As far as I know, PAR measurement is the best way to determine how much light actually reaches the bottom of the tank. If the light is in the correct spectrum, PUR measurements will correspond with the PAR values on the light at the substrate level. And since PAR meters are now available for 30 bucks and there are charts out there on estimating PAR values (thanks to Seattle_Aquarist I now have one saved in my favorites), PAR readings are the best way for the average home hobbiest to figure out how much light they have, and how much light they need.

Okay, so that's how I understand it. If I'm wrong, please please tell me, but use small words and short sentences lol! I'm not a science person... at least not a mathematical science person. I'm much better with the biological side of things, and where the math and biology get mixed together is where I find my true weak spot in this area. 

Also it should be noted that in theory, the folks I'm trying to explain this to are going to be using fairly shallow tanks. So I have another question for anyone who can help: A single T8 bulb at around 15 watts seems like it's going to grow TONS of algae in a tank that's only 9 or so inches tall. Would a T12 be more appropriate in this instance, without the use of floating plants to dim the light that reaches the substrate?

I ask because I have a bookshelf tank that's about this tall with a T8 bulb (wrong spectrum as mentioned above, it's red spectrum instead of full) and I'm going to be replacing it with a more appropriate bulb. I think I can use a T12 in the fixture for it. I have yet to find a measurement chart for lighting of any kind for a tank this shallow.


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## Michael

PAR measurement may not be the *absolute best* way to measure lighting for plants; for that you need spectral analysis as pointed out by other posters. But a PAR meter is the *most practical* way for hobbyists to know how much light they have in their tanks.

All of the charts suggesting x number of x tubes x inches from the tank are approximations to be used if you don't have a PAR meter. If you do have a PAR meter, then you can experiment with whatever type of light is convenient, and adjust or replace it to obtain the amount of light you need.


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## Newt

Except that if you had a light outputting only in the range of 500nm then you'd have a really high PAR but nothing that most plants can make photosynthesis with.


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## old 97

Find many facts here

http://www.apogeeinstruments.co.uk/aquarium-par-meters/


junebug said:


> I do have a pretty good understanding of how spectrum works with plants... that's really not what I'm concerned with at the moment.


I think the spectrum argument is overwieghted

"buy for the eye" - PAR is PAR
translation -- PAR = photosynthetic ACTIVE radiation
Soon after purchasing a PAR sensor I was surprised to see a "marineGLO" bulb giving equal PAR as a 6500K daylight. Even a 1500Watt infared shop heater will throw a healthy amount of PAR for 30 inches.
Buy whatever lighting is pleasing to look at.



junebug said:


> o
> 
> Say I have a very "tall" tank (for me this would be a 20 gallon high but others will likely have 40-75 gallon tanks where the same principle applies). I can have a full spectrum light, but if the light isn't strong enough at the substrate level, carpeting plants won't grow, or at least won't carpet.
> .


Here is a interesting calculator to observe PAR falloff
http://www.bmlcustom.com/custom-led-strip/



junebug said:


> o
> And since PAR meters are now available for 30 bucks


I thought everyone should have one when they were $250.00



junebug said:


> o
> Also it should be noted that in theory, the folks I'm trying to explain this to are going to be using fairly shallow tanks. So I have another question for anyone who can help: A single T8 bulb at around 15 watts seems like it's going to grow TONS of algae in a tank that's only 9 or so inches tall. Would a T12 be more appropriate in this instance, without the use of floating plants to dim the light that reaches the substrate?
> 
> I ask because I have a bookshelf tank that's about this tall with a T8 bulb (wrong spectrum as mentioned above, it's red spectrum instead of full) and I'm going to be replacing it with a more appropriate bulb. I think I can use a T12 in the fixture for it. I have yet to find a measurement chart for lighting of any kind for a tank this shallow.


I have an aquarium with four inches of water. I am just after taking a measurement for you with the F15T8 6500K Ushio bulb 9 inches from the substrate .

The PAR reading is 20.5


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## Michael

Newt, I appreciate your dedication to scientific accuracy. But is this condition likely to occur with the equipment available to the hobby?


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## TropTrea

A lot of good points here.

June lets look at issue on height. When you look at a light source and the tank under it think of the light cumming from the source as a big pyramid. If you have a height and base of the pyrimid of 1 inch then the light would be producing 1,000 units of light on an area 1 square inch. The results would be 1,000 parts of light per square inch. Now we move the base and and Height to 2 " with the same light source producing 1,000 units of light. The base is now 2" X 2" square so the light is now covering an area of of 4 square inches and each square inch is getting only 250 units of light. You can continue these calculations throughout the tank height when calculated from your light source. 

Yes it is very common on some Reef lighting tanks to have a tank with a height of 24" to have 1000 PAR at the surface and only 200 PAR at the substrate. 

When I design lights for reef tanks the main concern is BLUE light for coral growth. So I initially try to get enough blue light at the substrate to meet the needs of the coral. Once this is accomplished then I look at the viewers color taste and add enough full spectrum or white light to make it pleasing to there eye.

With a planted tank the same principle can be applied. The big difference is fresh water plants need both blue and red light in roughly a 2 parts blue to 1 part red ratio. So I try to get enough red and blue light on the substrate to allow the plants to flourish. Then considering the lack of green light simply add enough full spectrum or green light to make there eye happy with they are seeing. 

With fresh water plants this becomes easier than with with a reef. Using a lower K rated bulb provides the red light and a higher K rated bulb provides the blue light. I found that equal mix of 5,000K and 6,500K bulbs make an ideal combination. In most cases.


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## junebug

Wow, thanks for all the info, guys! Great links as well 

old97, a question on the measurement you took in your low water tank. Isn't that rating extremely low at the substrate level? I'm not familiar with that light (I always either DIY/home depot my lights or get special plant ones from the pet stores... I know not the best way but certainly the easiest) but I expected the reading to be higher. I guess that means I'll stick with a T8 in my shallow tanks :/


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## niko

Hm, it looks like we have gone a full circle - from yellowish light to white high Kelvins to measuring PAR and now going back to the view that full-spectrum light is the best for plants. Truth is that if your tank is less than 24" tall it is very easy to provide enough light for the plants with pretty much any light setup. We all get caught up in PAR, LED, Spectrum but none of us can deny that if the light is "strong" the plants will grow. You can in fact use the watts per gallon rule - if you have 4+ wpg the plants will grow. That has to do with their adaptability I guess. The fascination with specific wavelengths that came to this hobby with LEDs is misleading. Spectrum is indeed a better way to judge a bulb. So: power + spectrum.

A word about PAR: Yesterday I was playing with my PAR meter and I noticed that an area of my house that showed PAR=0 is actually lit well enough to support certain kind of low light terrestrial plants. I wondered how can that be if we hold PAR to be so important. I've no answer to that but what I do know for sure is that a 60-100 Watt Halogen bulb that you can get from any store nowadays shows huge PAR values (an $15 100W Halogen has PAR=150 at 4 feet and another kind of halogen that I bought for $6 at a grocery store has PAR=80 at 2 feet!) These PAR readings are somewhat interesting but what is more relevant to this discussion is the wattage. To properly light up a planted tank you will end up using roughly the same wattage no matter what bulbs you chose - fluorescents, Halogens, or LED. ...Watts per gallon again?


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## old 97

junebug said:


> old97, a question on the measurement you took in your low water tank. Isn't that rating extremely low at the substrate level? I'm not familiar with that light (I always either DIY/home depo,,,,,,,t) but I expected the reading to be higher.


I believe I used the same stlye and wattage as your bulb.
The fixture I was using is nothing special,,, in fact I will call it the most average fixture in the world. It is the same one that comes with every tank around 20 gallons. The reflector is a square trough barely wider than the bulb.

Is the reading/light intensity low ?
Good question. 
Weak - perhaps; although strong enough to grow algae, right?

I have grown algae with less than half that PAR. I have had plants refuse to grow with five times the intensity.:cheer2:


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## TropTrea

niko said:


> To properly light up a planted tank you will end up using roughly the same wattage no matter what bulbs you chose - fluorescents, Halogens, or LED. ...Watts per gallon again?


Here I will strongly disagree with you from experience on a Reef tank. I started with 9.6 Watts per gallon and got good coral growth with metal hides and compact florescent lighting, I moved to a T-5 fixture where I was using 5.6 Watts per gallon and the growth and coloration improved. Now I'[m running several tanks with 1.8 to 2.4 Watts per gallon and growth is still the same while coloration has improved more.

So there is a difference in the efficiency of various light sources. However it not just a simple matter of Watts per Lumns but in how effective the light spectrum of the light source is for your needs. If I ran 5 Watts per gallon of properly balanced LED's on my corals I'm sure I would fry them.

As I'm building a LED fixture for converting a 120 gallon Reef to a planted Aquarium I'm not planning on going anywhere near the 5 watts per gallon range so many people recommend. My present plan is to use 40 LEDs running at 3 watts each that can be boasted to as much as 5 watts each if need be. That basically means I'll be runing someplace between 1 and 1.7 watts per gallon.


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## Newt

All this talk about watts. Watts is input NOT output. Microeinsteins is output.

http://en.wikipedia.org/wiki/Einstein_(unit)


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## niko

TropTrea said:


> ...My present plan is to use 40 LEDs running at 3 watts each that can be boasted to as much as 5 watts each if need be. That basically means I'll be runing someplace between 1 and 1.7 watts per gallon.


So you will have LEDs - 120W over a 120 gal. tank. Which can go up to 200W if needed. If the tank is not 24" tall with 160 watts of T5HO you can grow anything you want in that tank too. Cost - around $160. Add another 80W of T5HO (add $50) to make that 240W and the tank can be 24" tall, no problem. How are the LEDs way better? How much will the whole rig cost? Will they have the wide spectrum of a T5HO? Even spread of the light too?


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## TropTrea

Newt said:


> All this talk about watts. Watts is input NOT output. Microeinsteins is output.
> 
> http://en.wikipedia.org/wiki/Einstein_(unit)


Yes I'm in strong agreement here. Different lighting sources convert watts to Microeinsteins at different effeciencies. Add to that the fact that all Microeinsteins are not the same when it comes to trying to get either plants or corals to grow.


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## TropTrea

niko said:


> So you will have LEDs - 120W over a 120 gal. tank. Which can go up to 200W if needed. If the tank is not 24" tall with 160 watts of T5HO you can grow anything you want in that tank too. Cost - around $160. Add another 80W of T5HO (add $50) to make that 240W and the tank can be 24" tall, no problem. How are the LEDs way better? How much will the whole rig cost? Will they have the wide spectrum of a T5HO? Even spread of the light too?


First off 80 Watt T-5's are to long for my tank that is only 48" long. If I were to go to a quality 4 bulb T-5 fixture I would have 2016 Watts of T-5 light. cost would be closer to $450.00 <http://www.reefgeek.com/48-4x54w-sunpower-t5-high-output-fixture.html> Then your replacing bulbs every 9 months for an added anual cost of $40 to $90 dependent on the quality of light bulbs you want.

DIY LED's 5 Watt LED's average under $4.00 each. and 200 watts Drives is under under $50.00. Heat sink and framing cost about $40 tops so the fixture is under $200.

Yes there are various spectrum LED's available for covering the wide spectrum needed. With planted tanks it is even easier as your basically interested in balancing the red to the blue ends of the spectrum with full spectrum LED's to meet the needs of the plants. You do not have to worry about an extreme blue spectrum peak like in corals or excess red that hurts corals.


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## junebug

This is what happens when I ask questions of people smarter than me 

Lol!

Thanks for all the help, guys. Keeping in mind the needs of both the plants AND fish (remember the plants are rarely the only thing in the tank) I'll be suggesting a few brands of "plant" lights I've used with good success that keep the water at appropriate temperatures (aka use LED not flourescent if you don't have A/C haha) and a nice tea color...

Sorry if I started an argument O.O I didn't mean to, I swear! But really, thanks everyone for all the info and the newbie-proof graphs for estimating PAR. I just know that one is going to come up and having a nice clean link I can provide is so much easier than me trying to explain it in detail when I barely know what it all means


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## Newt

TropTrea said:


> Yes I'm in strong agreement here. Different lighting sources convert watts to Microeinsteins at different effeciencies. Add to that the fact that all Microeinsteins are not the same when it comes to trying to get either plants or corals to grow.


That's why you need a normalized spectral plot for each bulb.


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## niko

TropTrea said:


> First off 80 Watt T-5's are to long for my tank that is only 48" long. If I were to go to a quality 4 bulb T-5 fixture I would have 2016 Watts of T-5 light. cost would be closer to $450.00 <http://www.reefgeek.com/48-4x54w-sunpower-t5-high-output-fixture.html> ...
> 
> DIY LED's 5 Watt LED's average under $4.00 each. and 200 watts Drives is under under $50.00. Heat sink and framing cost about $40 tops so the fixture is under $200.


And if I DIY the T5HO I can get everything from reefgeek for $300. Two ballasts and waterproof end caps. With 1 ballast and regular end caps cost would be $220.

If you can indeed make a 200W LED fixture for $200 you should consider selling it at The Planted Tank for $300. Plenty of little companies have hyped up their LED fixtures and yours can kill them all at that price. By a far. I want one of those LED fixtures. What would sounds like a reasonable price?


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## TropTrea

niko said:


> And if I DIY the T5HO I can get everything from reefgeek for $300. Two ballasts and waterproof end caps. With 1 ballast and regular end caps cost would be $220.
> 
> If you can indeed make a 200W LED fixture for $200 you should consider selling it at The Planted Tank for $300. Plenty of little companies have hyped up their LED fixtures and yours can kill them all at that price. By a far. I want one of those LED fixtures. What would sounds like a reasonable price?


There is omne thing about selling home built fixtures. And that is Insurance. Unless I could sell well over 100 fixtures per year at $100 over my cost I would not have enough for the insurance. With Electrical fixtures insurance companies want to see a UL approval at a minimum and that is not cheap. Then if I sold them without insurance and someone dropped a fixture into there tank and got a good shock I could end up paying out of my pocket almost everything I own without an insurance company lawyer to back me up. That is the problem with liability laws.

There is an option where you can build it yourself. Required tools are soldering iron, hack saw, wire cutter, drill, and screwdriver. DVM is a nice thing to have as well. Frame work can be purchased from your local metal supplier it is all 1" X 2" channel Aluminum. LED's can be purchased on line usually under $4.00 each for 3 to 10 watt LED's. And depending how fancy you want to go you can the LED drivers for $12.00 and up.

If anyone wants to do it I'll walk you through it through PM's.


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## niko

Sending you a right now.


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