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Lighting an Aquarium with PAR instead of Watts

436K views 453 replies 170 participants last post by  jeffkrol 
#1 · (Edited)
Before any discussion of aquarium lighting can proceed, first we have to debunk the myth about “watts per gallon” being a measurement of light intensity.

When the only practical source of light for a planted tank was T12 fluorescent tubes, someone decided that the way to pick out the best lighting was to figure out the “watts per gallon” that were needed to grow various types of plants. This would make sense if we could pour a teaspoon of watts of light into a tank, and get a light concentration of X watts per gallon of water, just as we pour a teaspoon of potassium nitrate in the tank to get a nitrate concentration of Y mg per liter of water. But light is nothing like a chemical - you can’t pour it anywhere, you can only shine it on something. That alone should debunk “watts per gallon” as a measure of light intensity.

But, there is more: Let us assume we have two 20 gallon tanks, with 40 watts of T12 Fluorescent light on each one - 2 watts per gallon. One tank is a 20L and one is a 20H. The 20L tank is 12 inches high, and the 20H tank is 16 inches high. If the fluorescent light sits right on top of each of the tanks, the light on the 20H tank is 4 inches farther from the substrate - 33% farther from the substrate. Because light intensity drops approximately proportional to one divided by the distance from the light squared, the intensity at the substrate in the 20H tank has to be about 56% of that at the substrate in the 20L tank. That alone should debunk “watts per gallon” as a measure of light intensity.

But, there is more: Let us assume we have two 20H tanks, one with a 40 watt T12 light sitting on top of the tank, and the other with the same light hanging 12 inches above the top of the tank. Again, because light intensity drops approximately proportional to one divided by the distance from the light squared, the intensity at the substrate for the tank with the light hanging 12 inches above the top of the tank must be about 32% of the light intensity of the tank with the light sitting on top of the tank. And, that alone should debunk “watts per gallon” as a measure of light intensity.

So, that is three strikes against “watts per gallon”.

But, there is more: Let us assume we have two 20H tanks, one with 40 watts of T5HO light, from a Tek light fixture, the other with 40 watts of T12 light. Anyone who has looked at both a T5HO bulb and a T12 bulb, when they are lit up, knows that the T12 bulb can be stared at without distress, but the T5HO bulb causes some temporary blindness if you look at it for more than a few seconds. The T5HO bulb is much, much brighter, and has to give much more light at a given distance than the T12 bulb.

“Watts per gallon” is dead!



PAR

Light intensity can be measured in lux, which is the intensity as perceived by human eyes. Or, it can be measured in PAR units, which is the intensity as perceived by plants. PAR is an acronym for “photosynthetically active radiation” - the radiation (light) that is used by plants for photosynthesis. The units of PAR are micromols of photons per square meter per second. So, a PAR of 1 is one millionth of a mole of photons striking a one square meter area every second.

Human eyes see the yellow green area of the spectrum of light very well - our eyes are very sensitive to yellows and greens, but we see reds and blues much less well. Plants are very sensitive to reds and blues, absorbing most of the light in those colors, but less sensitive to yellows and greens, reflecting a lot of the light in those colors. That is why most plants look green or yellow.



MEASURING PAR

The best way to find out how much light intensity we have in our planted tanks is to measure it. To do that we must use a PAR meter. A few years ago the only PAR meters available cost a few thousand dollars apiece. Now there are much cheaper PAR meters available.

You can buy a Quantum PAR meter, Model MQ, for $329 plus shipping. That is a near laboratory quality meter, with a guaranteed calibration, which can be re-calibrated at the factory when needed. It is the Cadillac of hobbyist PAR meters, usually bought only by clubs, where many members can use it.

A lower priced version of the Quantum PAR meter is just the sensor, Model SQ, for $139 plus shipping. To use this you need to either use a good millivoltmeter, which gives the best accuracy, or a cheap lux meter, like the Mastech LX1010BS, from Amazon.com, at about $20 plus shipping. Used with the lux meter you need to do your own calibration. You can use your digital multimeter, with a millivolt scale and the sensor, to determine the PAR from a light at a fixed distance, then connect the Mastech lux meter to the sensor to see what the meter reads at that PAR. This gives you a calibration constant for that combination of sensor and meter to convert lux to PAR.

Still cheaper is to buy one of the DIY PAR meters made by Mistergreen and/or O2surplus, for about $60. These are calibrated, and the meter reads in PAR units, but they may not be available when you want to obtain one.

Cheapest is to buy a Mastech LX1010BS, at $20 plus shipping, and modify it yourself per http://www.plantedtank.net/forums/showthread.php?t=179789 but you have to calibrate this yourself. However, the total cost should be $35 or less. If the Quantum PAR meter is the Cadillac of PAR meters, this is the refurbished Volkswagon bug of PAR meters.

SELECTING A LIGHT

Before we can even start to measure the light intensity, or PAR, that a given light will provide on our tank we first have to obtain the light. It may seem that we have to be working blind when we make this selection, given that knowing the “watts per gallon” won’t tell us anything about the intensity we will get. But, because there are now many PAR meters in hobbyist’s hands, we now have a lot of data on how much PAR we can get from several different lights, made by several different manufacturers. More data becomes available every month.

Today we can chose one of several different types of lights:
T5HO fluorescent lights with 1,2,3,4, etc. bulbs
T5NO fluorescent lights
T8 fluorescent lights with 1,2,3,4, etc. bulbs
PC power compact fluorescent lights with 1 or 2 bulbs
LED lights of many configurations - DIY or ready made
CFL screw-in fluorescent lights - DIY

For each of those types of lights a chart can be made showing the PAR produced by the light versus the distance from the light. These charts show the light intensity as measured without a tank of water being involved - just the intensity as measured in air. This is necessary to avoid the many variations in intensity caused by the tank dimensions and the cleanliness of the tank glass, both of which can have about a 10-20% effect on the intensity.





LOW LIGHT, MEDIUM Light, HIGH LIGHT

I don't believe there is any consensus about the definition of low, medium and high light. But, here is my definiition, subject to, and almost certain to change:
Low light - 15-30 micromols of PAR - CO2 is not needed, but is helpful to the plants
Medium light - 35-50 micromols of PAR - CO2 may be needed to avoid too many nuisance algae problems
High light - more than 50 micromols of PAR - pressurized CO2 is essential to avoid major algae problems

The following charts show the data that I now have for various lights. As I get more data I will keep updating the charts and adding new ones. If you want a light that isn’t included in the charts you can study the reflectors used in the light you want and compare them to the photos following the charts to see which charted light is closest to the one you want, to get a rough guess at what PAR that light will give you.

Fluorescent tube lights produce about the same light intensity for any length of tube, from about 24 inches to at least 60 inches. The longer bulbs are proportionally higher in wattage, so that the bulb wattage is mostly a measure of the bulb length, not the bulb brightness. For bulbs shorter than 24 inches, this may not be true.

CAUTION: Not all lights use a true, full power HO ballast. Some cheaper models use lower power ballasts, and will not produce as much PAR as those with good ballasts. Compare the chart for the FishNeedIt lights to the others for an example.






One layer of window screen over the bottom half of each bulb, right on the bulb, drops the PAR by about 30%






See http://www.plantedtank.net/forums/showthread.php?t=160396 for much more information on LED lights.

Photos of various reflectors used in T5 lights:




Hagen GLO 2 bulb light


ATI 4 bulb T5HO


Home Depot 2 bulb Diamond Plate Shop Light - note the reflections of each bulb.



One bulb T8 light with fairly good reflector


Aquaticlife 4 bulb Light
 
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#198 ·
Great write up Jeff, very interesting opinions.

I would love to get some PAR reading from a narrow reflector for common CFL, I suspect they produce a lot more PAR. Looking at Hoppy's chart (thanks for the effort btw), I'm surprise at the number like everyone else.

Wouldn't these Shop set up be the most cost effective solution? I know they have full UVB or "grow lights" so one can mix their lighting.

Here my experience, take it for what it's worth. -

I picked up some "full spectrum" CFL made with a wider range rather than peaks, it was the same spec as a common CFL. I replace my lighting with these "full spectrum" 23w 6500K and notice for the first time, algae. I switched back to my old 23w 6500k and I kid you not, the algae went away. I'm confused, algae usually doesn't start and go that fast. 1 week algae starts, next week algae dies off. :S

Also, if one were to focus mainly on colour intensity of the plants rather than the growth, would it not be ideal to have both lights peaking in blue and red? To compensate for brightness you can just get a plain 6500k bulb to accompany them. I'm talking about purple/red hues on plants, making plants green is passé.
 
#202 ·
Great write up Jeff, very interesting opinions.

I would love to get some PAR reading from a narrow reflector for common CFL, I suspect they produce a lot more PAR. Looking at Hoppy's chart (thanks for the effort btw), I'm surprise at the number like everyone else.

Wouldn't these Shop set up be the most cost effective solution? I know they have full UVB or "grow lights" so one can mix their lighting.

Here my experience, take it for what it's worth. -

I picked up some "full spectrum" CFL made with a wider range rather than peaks, it was the same spec as a common CFL. I replace my lighting with these "full spectrum" 23w 6500K and notice for the first time, algae. I switched back to my old 23w 6500k and I kid you not, the algae went away. I'm confused, algae usually doesn't start and go that fast. 1 week algae starts, next week algae dies off. :S

Also, if one were to focus mainly on colour intensity of the plants rather than the growth, would it not be ideal to have both lights peaking in blue and red? To compensate for brightness you can just get a plain 6500k bulb to accompany them. I'm talking about purple/red hues on plants, making plants green is passé.
Most cost-effective is LED lights. My Finnex are going to last between 8-13 years before I have to replace. Actually peaking red and blue creates more color.
 
#199 ·
This is an amazing thread. I am sitting here trying to digest that what I thought was a medium-low light tank is actually a high light tank. Please correct me if I'm wrong. I have a typical 20 gallon long tank with non-demanding plants. I have DIY CO2 which is not as prolific as I'd like. The last time I counted it was doing 16 bubbles per minute but it changes.

I have a simple shop light fixture with no reflector holding 2 T8 bulbs (longer than the tank). I figured it out to be around 2 watts per gallon, which I understood to be the upper limit of "low light." But according to the chart, given that my substrate is about 9 and 3/4 inches from my substrate, I have more than 88 PAR. Is this true?

My tank is topped with an old, flat piece of auto glass. (I know that may sound funny, but it was free.) I do not know whether the thickness, or some buildup on the glass has a large effect on the light, but even if it reduced it by 20%, it looks like I'd still be in the high light category.

My anubias pearls wonderfully. I did not know an anubias would do that. It is getting new leaves and has only been in the tank a little more than 2 weeks. The java fern pearls wonderfully too. I do not yet have my ferts going properly, as I'm awaiting their arrival in the mail. However, knowing my true lighting intensity is important to knowing how many weekly EI doses I ought to use. I was planning on just one.

Also I've had a number of different kinds of algae. None of it has become really out of hand, although it's still more than I would like. But it has mostly stayed manageable.

Thank you for any advice. I am still new at this!
 
#200 ·
proof of the pudding is in the eating...
;)

Unless you have old auto glass it most likely has some sort of optical coating to cut out certain detrimental wavelengths.. mostly UV
After the glass is tempered and cleaned, it goes through a laminating process. In this process, two sheets of glass are bonded together with a layer of plastic (the plastic layer goes inside the two glass sheets). The lamination takes place in an autoclave, a special oven that uses both heat and pressure to form a single, strong unit that is resistant to tearing. The plastic interlayer is often tinted to act as an ultraviolet filter.
not that it seems to matter much.
As to light "falloff" it is logrythmic..
Everything one didn't want to know about light in lakes (large aquariums ;) )
http://www.esf.edu/efb/schulz/Limnology/Light.html

 
#204 ·
Thank you Jeffkrol for your reply to my question. It's possible that my glass may have something like that, as I don't know how old it is. I hope that using it will not be a problem. I have little children and I'm afraid they will climb up and reach into the tank, so the glass prevents that. That's why I'm using it.
 
#205 ·
What would we do without Hoppy? :)

Here is a question I have. Over my 55 I have a Catalina 2x54 t5ho. My bulbs, however, are not 7.5 inches apart. I think I just bought the standard fixture - and my bulbs are about an inch apart. Has anyone measured this fixture? Wondering if I need a different fixture to get "high light".
 
#207 · (Edited)
One followup: Added a dimmer/driver to the blue channel. Epoxied WW Led light bar to main 10w heat sinks... Have some odd optical reflections to deal with (like why the red LED lights the front bottom more than any other light source..

Anyways an interesting "blend"...


Additional comments: You can see the Olive Nerites (Can't begin to tell you how much I LOVE them for "maintenance") and a "ammonia patch".. cute little spot check device since I was tired of trying to figure out what my API ammonia test results really were... never was "yellow" so I had to "calibrate' using bottled water to compare.. Now I just look.. :).. Next I have to see if I can get more "polish" on my water.. On to get some felt for a "micron filter"..

Oh and I realize it is "messy"... ;)
 
#208 ·
Addendum.. found reason behind color banding (I out-tricked myself) . The 1w "warm whites" were thermal epoxied on an l shaped piece of aluminum. Orig thought was to shield the 10w pucks from your vision. I had hoped the "native" 110 degree angle would compensate for the shading effect. It didn't. It can be somewaht compensated by tilting but that is an issue in itself..

It does give me some interesting effects though..
anyways I'm adding my sunset color and an image of the light construction:

Oh and the 20gal "holding tank" lit w/ a Marineland "puck"

FYI the lights run at about 110F with passive cooling......



 
#209 ·
Hi Hoppy,

I just bought a new 100g tank. The dimensions are: 60" long, 18" wide, 21" high.

I will be growing plants but do not want to have C02. I will use Seachem Flourish. I assume I will want a 60" fixture? Can you recommend a lighting set up? And should I be using more additives than just Seachem Flourish?

Thank you in advance. Brad
 
#214 ·
Hoppy,
Do you remember the brand for the HD cfl bulbs you did the par test on? If I have a glass top on my tank does that change the par value? Thanks for this great write up.
No, I don't remember the brand of those bulbs. While we were doing PAR tests on CFL bulbs, we noticed that our data were not consistent. A lot depended on how long the bulb was - how much of it stuck out of the reflector. And, it depended on the diameter of that reflector. Since then I haven't gone back to that subject to try to get more usable data. It would help if someone did that - very carefully.

The top glass on a tank seems to reduce the PAR by around 10-15%, not very significant, considering that our PAR data is rarely better than +/- 10% accurate.
 
#215 ·
I can't seem to find any information in this thread regarding the Aqueon LED systems. Did I miss it, or does anyone know what kind of output they have?

I currently have 3 20" led strips with 9 LEDs each from Aqueon on my 20 gallon tank. Two are white and one is their "colormax" with alternating white and red LEDs. I have no clue how broad their spectra are or how much of the light they put out is actually useful for my plants. Does anyone know?
 
#217 ·
Hi Hoppy,
Awesome work here! I picked up a 30" ZooMed AquaSun T5HO fixture with dual 24" bulbs. I am using these grow bulbs I found on the cheap http://www.thelashop.com/4x-24w-3000k-2ft-t5-ho-grow-light-fluorescent-tubes.html .

I think it may be overkill directly over my 20 Long but wanted to see if you had any info on the PAR values this fixture puts out at different heights. Any help is appreciated, Thanks!
 
#221 ·
I am doing a 180g. My first planted tank.

I would like the "shimmer" of LED's but dont have alot of money to spend on lights.

Does anyone know the PAR ratings of the beamsworks LED fixtures? I could place one of those in 72", then do a HD shop light for the bright part of the day?

If I just did the HD shop light how many would I need over a 6 ft tank? I cant go higher than like 6" off the surface though.

I could also build an LED light, or try to, but my budget is like $250.
 
#308 ·
I have a similar question. I have the standard tank lights set up on my standard 55G. I have fake plants right now but looking to get into a low light setup. I have a sand substrate so I will supplement with tabs.

I was considering getting the diamond plated shop light from HomeDepot but now I'm concerned that will give me too much light and I just wonder if I should stick to my 2 15w T8 bulb setup (one per ballast). I would rather not get into CO2 but I would like to achieve moderate growth.

My question would be before I purchase this shop light should I stick with the 15w x2 (30w) setup or spring for the shoplight? I think from looking at your chart, my current setup, would give me 20-30 PAR which would achieve a low light setup. That's looking at the black line defined as "one bulb with typical white inside".
 
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#230 ·
I notice there's a note about screen covers reducing PAR by 30%, what about if a plexiglass cover is between the light and the water (with light raised a bit above the cover and water)? How will this effect the PAR (if at all)?
 
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